key: cord-0059631-kcc8y6am
authors: Freire, Bruna Moreira; Pereira, Rodrigo Mendes; Lange, Camila Neves; Batista, Bruno Lemos
title: Biofortification of Crop Plants: A Practical Solution to Tackle Elemental Deficiency
date: 2020-08-29
journal: Sustainable Solutions for Elemental Deficiency and Excess in Crop Plants
DOI: 10.1007/978-981-15-8636-1_7
sha: d9fdd3eba670ed4f11e464018ef24cd0b522db83
doc_id: 59631
cord_uid: kcc8y6am

Malnutrition englobes overnutrition and undernutrition. One in four children suffer from chronic undernutrition and approximately 820 million people have a caloric deficit. The effects of malnutrition are transgenerational and they have an impact from the individual to the national level. Although globally there is sufficient food for all, several countries have inadequately domestic food production. Moreover, the deficit in micronutrient achieves about 3 billion people worldwide due to the lower levels in food or availability of these micronutrients for absorption by the intestines. Therefore, agronomic sciences have an important role in providing nutritious food (quality) rather than adequate calories (quantity). In this scenario, biofortification is a notable tool to improve individual nutritional status. Biofortification is the use of the most appropriate biotechnological or traditional breeding practices for micronutrient enrichment (such as vitamins and chemical elements) of staple crops. From the chemical elements considered essential, the deficiencies of calcium, copper, iodine, iron, magnesium, selenium, and zinc are the most common. Several studies for biofortification were conducted focusing the use of agronomic approaches (use of fertilizers in soils, irrigation water, and hydroponic cultivation systems, or by the foliar application during plant growth), conventional breeding, and genetic approaches (the ancient breeding to the modern genetic engineering employing synthetic genes), and the plant growth-promoting microorganisms (PGPM) approaches (use of microorganisms in soil/plant rhizosphere during plant growth). These biofortification approaches have disadvantages and advantages and are dependent on important variables such as farming practices and soil properties. Moreover, biofortification must be associated with the plant-resistance to stress during cultivation, yield improvement, food color/palatability, and the bioavailability of the nutrients after human ingestion. The highest number of publications on biofortification are from countries that are among the main food producers in the world (the USA, India, China, Australia, and Brazil), evidencing the importance of this technique in contributing to more nutritious food, especially for the poverty population.

cultivation, yield improvement, food color/palatability, and the bioavailability of the nutrients after human ingestion. The highest number of publications on biofortification are from countries that are among the main food producers in the world (the USA, India, China, Australia, and Brazil), evidencing the importance of this technique in contributing to more nutritious food, especially for the poverty population.

Zinc · Iodine · Breeding · Genetic · Fertilizers · Bioavailability · Hidden hungry · Farming practices 7.1 Why Crop Biofortification Is Necessary?

According to the Food and Agriculture Organization of the United Nations (FAO 2019) after continuously declining for over a decade, global hunger is on the rise again, especially in regions where economic slowdowns occurred. It is also worth noting that this tendency would be more pronounced in the next following years as a consequence of the coronavirus (COVID-19) pandemic and its economic impacts (Hafiz et al. 2020) . Malnutrition englobes both undernutrition (wasting, stunting, underweight, and mineral-and vitamin-related malnutrition) and overnutrition (overweight, obesity, and diet-related noncommunicable diseases) (Dukhi 2020) . Around 820 million are in a state of caloric deficit (FAO 2019); nearly one in four children suffer from chronic malnutrition; 52 million children suffer from acute malnutrition; and two billion adults are overweight (Meybeck et al. 2018) . Malnutrition is a root cause of many health disorders and it is an imbalance of needs and intake of nutrients and calories. The main reasons for this problem are unavailability or lack of access of food; poor diet (due to a person's inability to select, take-in, and absorb the nutrients in the food); vulnerability of an individual (i.e., increased micronutrient needs during certain life stages, including pregnancy; and health problems such as diseases, infections, or parasites that can spread in unhealthy environments with poor water, sanitation, and hygiene conditions) and finally, decrease in the micronutrient content of common crops due to productivity demands and climate change (Stein 2010; Von Grebmer et al. 2014; Nelson et al. 2018 ). According to Nelson et al. (2018) , the challenge in 2050 will be providing nutritious diets rather than adequate calories and future studies and policies should emphasize nutritional quality by increasing the availability and affordability of nutrient-dense foods and improving dietary diversity (Nelson et al. 2018) . However, in the past, the main focus of the professionals from the agriculture field was on yield increasing without balancing the nutritional qualities of staple crops (Stein 2010) . Production systems must, therefore, align with nutritional and health goals (Geyik et al. 2020) .

Chronic malnutrition effects are transgenerational and they have an impact at the individual, community, and national levels in the short-and long term (Reinhardt and Fanzo 2014) . Thus, dietary solutions that could manage to balance nutritional, economic, environmental, and health pillars are a great challenge for a sustainable future and will require the efforts of agriculturists, public health professionals, educators, nutritionists, policy-makers, and food industries (Tilman and Clark 2014) .

Minerals and vitamins malnutrition are defined as hidden hunger and its effects hold significant and immediate negative consequences for the cognitive and physical development of children, however in long term may cause profound consequences in health, on productivity and economic potential in later adulthood (Ruel-Bergeron et al. 2015; Biesalski 2013) . In diets from nutritionally vulnerable groups, the co-occurrence of deficiencies from more than one micronutrient is common (Ruel-Bergeron et al. 2015) . The most affected continents are Africa and South Asia, nevertheless, it occurs globally, especially to underprivileged people (FAO 2019). In a recent study, Geyik et al. (2020) have explored the spatiotemporal trends in dietary nutrient content and adequacy of primary production based on the production of 174 commodities covering a period of 1995-2015 for 177 countries. The authors highlighted that while total production can adequately provide the global human population with all nutrients except vitamin A, more than 120 countries have inadequate domestic production (Geyik et al. 2020) .

Mineral nutrients are fundamentally metals and other inorganic compounds (Gupta and Gupta 2014) . Adequate mineral intake is needed for the maintenance of normal organism functions. However, about 3 billion people worldwide have a micronutrient deficient diet (Khush et al. 2012) . Factors contributing to this scenario are low concentrations or low bioavailability of these nutrients in food (El-Ramady et al. 2014) .

Considering the hidden hungry is a serious public health concern, many strategies have been developed to overcome this problem (Khush et al. 2012) . No single intervention will offer a "silver bullet" to micronutrient deficiencies, but there are some strategies commonly employed, such as supplementation, dietary diversification, food fortification, and biofortification (Bouis and Saltzman 2017; Khush et al. 2012; White and Broadley 2009 ). Although been defended by nutritionists, dietary diversification is a contradictory strategy since people tend to return to their old habits (Khush et al. 2012) . The World Health Organization (WHO) highlights food fortification and nutrient supplementation as strategies to combat malnutrition (WHO 2019). The Consultative Group on International Agricultural Research (CGIAR) emphasizes the importance of biofortification through breeding and biotechnological approaches (Khush et al. 2012) . By making staple foods more nutritious, people can overcome malnutrition without changing their habits.

According to Nestel et al. (2006) , the definition of biofortification is the process of development of micronutrient-rich staple crops using the most suitable traditional breeding practices and recent biotechnology to develop staple crops (Nestel et al. 2006) . It is important to biofortified staple foods even if they accumulate micronutrients in a relatively low rate since they are consumed regularly in larger quantities for many vulnerable populations in the way that they can enhance the micronutrient status of these populations (Junqueira-Franco et al. 2018 ). However, the feasibility of biofortification depends on: (1) nutrients bioavailability for plants and humans; (2) nutrients stability after harvesting of the crop (not degrade during processing, storage, and preparation); (3) the acceptance of the crop sensory qualities by producers and consumers in the target regions; (4) provide high yielding and profitability to the producers (Sharma et al. 2017) . This process should be comparatively cost-effective, sustainable, and long-terms of delivering more micronutrients (Saltzman et al. 2013) .

There is an estimative that by the end of 2018, 7.6 million farming households were growing biofortified planting material, benefiting around 38 million people (HarvestPlus 2019). According to Herrington et al. (2019) , the selection of the regions, crops, and micronutrients to prioritize biofortification should be based on production, consumption, and micronutrient deficiency using country-level data (Herrington et al. 2019) . Also, the continuous search for new techniques, or the improvement of the existing biofortification techniques, is essential to continue this positive scenario and expand the food biofortification around the world. In this chapter, an overview of the biofortification of crop plants will be described, and several studies showing a wide variety of biofortification approaches will be discussed to demonstrate the main challenges and trends.

Considering dietary minerals, there are more than twenty elements considered essential for human body maintenance (Williams 2005) . Adequate mineral intake is needed for the maintenance of normal organism functions. However, about 3 billion people worldwide have a micronutrient deficient diet (Khush et al. 2012) . Factors contributing to this scenario are low concentrations or low bioavailability of these nutrients in food (El-Ramady et al. 2014 ). The hidden hunger or micronutrient deficiencies resulting from unbalanced diets is a high priority issue that impedes human and economic development (Khush et al. 2012; Valença et al. 2017) . Nowadays, the big challenge is increasing the productivity and the concentration of micronutrients in food crops (El-Ramady et al. 2014) . Around the world, the most common and devastating mineral deficiencies involve calcium, copper, iodine, iron, magnesium, selenium, and zinc. The main functions, as well as the problems related to deficiency and/or excess, are presented in Table 7 .1 (White and Broadley 2009; Khush et al. 2012) . It is worth mentioning that worldwide, starchy food crops such as rice, maize, wheat, cassava, and legumes are the main focus of biofortification programs. It occurs because these foods are prevalent in the diet of the majority world population, especially for the most vulnerable populations who do not have access to supplements, diverse diets, and commercially fortified foods (Saltzman et al. 2013) . Studies have shown that some crops such as Table 7 .1 Main functions of minerals in the human organism and some problems related to inadequate intake of calcium, copper, iodine, iron, magnesium, selenium, and zinc in the human diet Element Description Calcium (Ca) It is the most abundant mineral in the human body, and it is present mainly in the skeleton. It plays many essential functions, such as supporting the structure and hardness of bones and teeth, being also vital for muscle movement, enzymes, hormones release, and blood movement through blood vessels (Weaver 2012; NIH 2020a; Gharibzahedi and Jafari 2017). Besides, nerves need Ca to transmit messages between different parts of the body. The average recommended intake for this element in adults is 1000 mg day À1 (NIH 2020a). In general, Ca ingestion around the world is below the recommended intake, which increases the risk of many diseases (Weaver 2012). Insufficient Ca intake leads the body to take it from bone to keep healthy levels in the blood. Calcium deficiency can cause osteoporosis and fractures due to the decrease in bone mass. Other possible consequences are convulsions, numbness and tingling in the fingers, and abnormal heart rhythm (NIH 2020a).

The essentiality of Cu is linked to brain development, maintenance of immune and nervous systems, and gene activation (NIH 2020b; Gharibzahedi and Jafari 2017). Copper is a constituent of various enzymes, which take part in many metabolic reactions. These cuproenzymes are involved in energy production and utilization, synthesis of proteins of blood vessels, and connective tissues (NIH 2020b). The recommended intake of Cu for adults is 900 μg day À1 (IOM 2000; NIH 2020b) . Some effects of Cu deficiency are extreme tiredness, high cholesterol levels, weak and brittle bones. Connective tissue disorders, loss of balance, and coordination can also occur. People with a diet deficient in Cu are at increased risk of infection (NIH 2020b).

It is an essential mineral for thyroid function, being constituent of the hormones T3 and T4 (Gonzali et al. 2017 ). These hormones are relevant for the body's metabolism, growth, development, reproduction, nerve and muscle function, production of blood cells, among others (Gharibzahedi and Jafari 2017) . During infancy and pregnancy, thyroid hormones are essential for proper brain and bone development (IOM 2000; NIH 2019a) . The recommended intake of I is 150 μg day À1 for adults (NIH 2019a) . Iodine deficiency is a widespread problem, affecting both developing and developed countries (Gonzali et al. 2017) . In children, cognitive development and mental health can be compromised. Among pregnant women, some possible consequences are spontaneous abortion, stillbirth, and congenital abnormalities (NIH 2019a). According to the World Health Organization (WHO), iodine deficiency is the most prevalent cause of brain damage in the world (WHO 2013).

It is responsible for oxygen transport, antioxidant activity, hormone synthesis, neurodevelopment, connective tissues synthesis, and energy metabolism (Aggett 2012; Gharibzahedi and Jafari 2017) . Iron deficiency is the most common and widespread nutritional disorder in the world, leading to severe anemia. It is estimated that iron deficiency anemia (IDA) affects 2 billion people around the world, mainly in developing countries. Meantime, it is the only nutrient deficiency that is also prevalent in industrialized countries (WHO 2019). The recommended intake of Fe varies between 8 and 18 mg day À1 for adults, depending on gender (NIH 2020c). Among the consequences of IDA in adults are weakness, irritability, and reduced work productivity. In children, IDA can lead to susceptibility to disease, impaired physical and mental development, and increased mortality risk. In developing countries, IDA affects around 40% of preschool children (Khush et al. 2012 ; WHO 2019).

(continued) 

Biofortification approaches are usually used to increase the bioavailable mineral content of food crops, and some techniques have been developed and applied for this purpose (White and Broadley 2009; Khush et al. 2012) . Despite this, biofortification techniques can be employed with different goals. Crops production in mineraldeficient soils may compromise the growth and yield, for example, and biofortification is also useful to solve these issues (Chugh and Dhaliwal 2013) . This fact is because these elements are also essential for the proper development of plants. Thus, biofortification strategies have been also studied aiming yield improvement, resistance to stress, and food palatability (Valença et al. 2017; Gonzali et al. 2017; White and Broadley 2009; Navarro-Alarcon and Cabrera-Vique 2008) . It is important to note that there is a path followed by the minerals from the soil to the human body, passing through the crop and the food, and biofortification strategies should be carefully selected considering each application (Valença et al. 2017) . In this path, many factors can influence elemental bioavailability, as shown in Fig. 7 .1. In this way, some challenges must be overcome to be successful in biofortification (Valença et al. 2017; White and Broadley 2009 ). The first challenge is related to the presence and bioavailability of elements in the soil (Valença et al. 2017) . It is necessary to be aware of the chemical forms of elements that plant roots can acquire, for example. The biological and physicochemical properties of the soil influence the chemical forms of the elements that will be present in the rhizosphere solution. In this way, the phytoavailability may be affected, limiting the accumulation of these species by crops (White and Broadley 2009) . Another critical issue is that different plant varieties can accumulate mineral elements in a wide concentration range, therefore the crop variety needs to be carefully selected for effective Fig. 7 .1 Schematic representation of nutrients pathway from the soil to the human body, highlighting the main factors that have an impact on elemental bioavailability in each step. Adapted from Valença et al. (2017) and Mayer et al. (2011) biofortification (White and Broadley 2009; Valença et al. 2017) . After absorbed by roots, nutrients are translocated to the edible tissues of the crop. Some factors that can influence this process are crop variety and processing methods. Finally, the human ability to absorb nutrients is influenced by individuals' health, dietary intake, and cooking methods (Valença et al. 2017) .

Although fertilizers are often applied when the soil is deficient in mineral elements, there are biofortification strategies based on increasing element uptake from soils. These techniques focus on improving the uptake of nutrients by the roots and their redistribution to edible tissues (White and Broadley 2009; Durán et al. 2013 ). On the other hand, there are agronomic approaches based on fertilizer application in leaves, seeds, as well as in irrigation water and hydroponic cultivation systems. These strategies emerged to circumvent the limitations related to the complex reactions of minerals in the soil and enhance the plant biofortification process. Some minerals have low mobility in the soil depending on the chemical conditions of the soil (pH, composition, etc.) and end up becoming unavailable to plants. In brief, several processes have been used to promote the biofortification of crop plants such as conventional and mutational breeding, genetic engineering, agronomic approaches, among others (Garg et al. 2018; Bouis and Welch 2010; Hirschi 2009; Saltzman et al. 2013) . In this chapter, biofortification strategies will be classified into three categories: agronomic, conventional breeding and genetic, and plant growth-promoting microorganisms (PGPM) approaches.

It is worth mentioning that the development of studies to evaluate the strategies for the biofortification of foods has grown significantly in the last 20 years, and approximately 1918 documents were published from January 2000 to March 2020 as can be seen in Fig. 7 .2. By the end of March 2020, more than 100 documents had already been published, which shows that this upward trend is expected to continue given the relevance of crop plant biofortification today.

Regarding the percentage of publications by country or territory, the USA (15%), India (10%), China (6%), Australia (5%), and Brazil (4%), stand out as they account for about 40% of the publications presented in Fig. 7 .2. Indeed, these countries with a higher number of publications on crop plant biofortification, have a large fertile territory and are the main food producers in the world. In Fig. 7 .3, a choropleth map showing the percentage of publications for the biofortification of foods by country or territory is presented.

The agronomic approaches are based on the application of chemical substances containing minerals (fertilizers) during plant growth aiming to increase micronutrient concentrations in edible tissues (Valença et al. 2017; White and Broadley 2009 ). The most common agronomic approach used for crop plant biofortification is the application of fertilizers in the soil. Solutions of inorganic salts are the most widespread for this purpose. However, micronutrients delivered by using these solutions usually have relatively low availability in the soil since they may be fixed as insoluble forms, or still, be easily released and leached down the soil profile ). Data were obtained on the Scopus database using the following search equation: [TITLE-ABS-KEY ("biofortification") AND TITLE-ABS-KEY ("food" OR "cereal" OR "legume" OR "crop" OR "fruit" OR "staple food")] (El-Ramady et al. 2014 ). On the other hand, the soil application of algal-based iodized organic fertilizer has proved to be an interesting choice for I biofortification in crop plants (Weng et al. 2013 (Weng et al. , 2008b Hong et al. 2008 Hong et al. , 2009 . Also, some studies have shown that the application of organic amendments such as biosolids biochar and hyperaccumulator plants can also be efficient and advantageous for Fe, Se, and Zn biofortification in crop plants (Gartler et al. 2013; Bañuelos et al. 2015; Ramzani et al. 2016) . Nevertheless, the efficiency of soil fertilizer application is dependent on several factors, especially those related to management practices and soil factors, which affect the mobility of elements in soil and their bioavailability for plants.

In this way, the natural process in which plants absorb nutrients through the leaves has been extensively explored in agriculture for crop plant biofortification by foliar application of fertilizers. The foliar application consists of the foliar spray or application of nutrients on aboveground plant parts to supply traditional soil applications of fertilizers. It may be considered one of the most important approaches used for delivering nutrients in suitable concentrations to plants, improving their nutritional status, the crop yield as well as their quality (Alshaal and El-Ramady 2017) . This type of application is much less influenced by external factors than soil fertilizer application and, therefore, it has been the target of several studies aiming at the biofortification of crop plants. The use of nanoparticles containing elements that are intended to increase the concentration in the crop plants should be emphasized among the fertilizers used for foliar application. Recent studies have shown that, in addition to biofortification with essential elements, nanoparticles application has promoted the mitigation of toxic elements, such as Cd and Pb, present in the cultivation soil (Hussain et al. 2018 (Hussain et al. , 2020 .

Other strategies that have also been successfully used for crop plant biofortification are the application of fertilizers in irrigation water, hydroponic systems, and seeds before cultivation (De Figueiredo et al. 2017; Smoleń et al. 2014 Smoleń et al. , 2015 Smoleń et al. , 2018 Smoleń and Sady 2012; Trolove et al. 2018; Rizwan et al. 2019 ). In general, agronomic biofortification is simpler and less expensive in the short term when compared with genetic approaches. On the other hand, fertilizer application must be done regularly and may cause damages to the environment, beyond increasing labor and cost in the long term. Some studies showing the application of common agronomic strategies are shown in Table 7 .2. In short, these studies demonstrate the main challenges and trends of agronomic approaches for crop plant biofortification.

There is scientific evidence that the nutritional quality of staple crops can be improved by using agronomic biofortification. Valença et al. (2017) stated that these techniques are useful tools for enhancing micronutrient content in edible parts of food crops. Some factors that can influence the success of these approaches are the soil composition, application method, plant species, which can affect mineral mobility and accumulation, and the nutrient accumulation on plant tissues. Thus, some strategies may be limited by geographical locations and crop types, so they may not be applied universally. The efficiency of micronutrient fertilization can be optimized by using integrated soil fertility management, such as combination with organic and NPK fertilizers and selection of improved crop varieties, which can more effectively capture nutrients and accumulate them in consumed parts (Valença et al. 2017 ). (2017) Chinese cabbage I NaI and NaIO 3 solutions containing I concentrations of up to 5.0 mg L À1 and an organic iodine fertilizer (seaweed composite) were evaluated for I biofortification of lettuce. Results show that I uptake by cabbage was more effective using NaIO 3 when low I concentration (<0.5 mg L À1 ) was applied. On the other hand, I uptake was also useful using NaI when I concentration of 0.5 mg L À1 or higher was applied. NaI and NaIO 3 provided a quicker supply for Weng et al. (2008a) (continued) 

Lettuce I I concentrations of up to 129 μg L À1 , applied as iodate (IO 3 À ) or iodide (I À ), was evaluated for I biofortification of lettuce in a winter and summer trial. I application did not affect plant biomass, produce quality, or water uptake. Nevertheless, increases in I concentration significantly enhanced I biofortification of the plant, and I concentrations in plant tissue were up to fivefold higher with I À application. The outer Voogt et al. (2010) (continued) 

(continued) The elements most targeted for crop plant biofortification are micronutrients, such as I, Se, and Zn. This may be associated with the fact that these elements are probably more efficiently absorbed by plants. Moreover, they are extremely important for the human organism and are usually found in very low concentrations in crop Souza et al. (2014) plants. According to Gonzali et al. (2017) , I biofortification of food crops can be a cost-effective approach to control I deficiency with a bioavailable source. In many plant species, such as potato and lettuce, the agronomic approach is sufficient to increase I content. The most common administration ways are in the soil, as a foliar spray or in hydroponic solutions. The chemical form varies since there are studies with the application of organic and inorganic species. Doses and timing of application must be evaluated for each specie (Gonzali et al. 2017) .

In turn, Se biofortification by agronomic strategies such as fertilizer application is an efficient way to produce Se-enriched food products (Wan et al. 2018) . However, attention is needed since the levels that characterize deficiency, essentiality, and toxicity of this element are very close (Navarro-Alarcon and Cabrera-Vique 2008). The chemical form of Se influences its bioaccessibility. Fortunately, agricultural methods to improve Se bioaccessibility in food products can be used (Wan et al. 2018 ). Moreover, the major forms of Se in the diet are highly bioavailable (IOM 2000) . According to Wan et al. (2018) , agronomic strategies may help supply the daily needs of this element, mainly in Se deficiency regions. Other studies have shown that agronomic Se biofortification of cereals is effective to increase Se intake in animals and humans (Valença et al. 2017 ). On the other hand, processing methods such as heating and milling may decrease Se content in food due to volatilization and solubilization (Wan et al. 2018; Navarro-Alarcon and Cabrera-Vique 2008) .

Zinc biofortification of edible crops has been identified as a strategy to improve the intake of this element. For this purpose, agronomic strategies namely Zn-fertilizers application have been employed and showed to increase Zn content in roots, stems, and leaves without compromising yield. Zinc fertilizers showed promising results when applied either in the soil or in leaves and also in combination with nitrogen fertilizers (White and Broadley 2011) . There is evidence that nitrogen availability is a key component of Zn biofortification (Hefferon 2015) .

Nevertheless, some studies have shown that other nutrients, such as B, Ca, Cu, Fe, K, Mg, Mn, among others, have also been evaluated for plant biofortification (Aziz et al. 2019; Rizwan et al. 2019; Adu et al. 2018) . It is important to mention that in addition to being essential elements for the human organism, they are also very important for the proper development of plants. For this reason, in most cases, the agronomic approaches for crop plant biofortification also improve yield and/or food quality, as described in some studies in Table 7 .2.

Breeding and genetic engineering are the main tools employed in this type of biofortification (Gonzali et al. 2017) . Genetic engineering can employ synthetic genes (Khush et al. 2012) . In general, these approaches are more complex and laborious than agronomic ones (Gonzali et al. 2017 ), but are sometimes needed when conventional methods are insufficient to obtain substantial enhancement of the target element (De Steur et al. 2017) . The two methods aim to achieve plant lines carrying genes that result in the most efficient accumulation of bioavailable minerals. However, plant breeding achieves this by crossing the best performing plants and selecting those with favorable traits over many generations, whereas genetic engineering accesses genes from any source and introduces them directly into the crop (Gómez-Galera et al. 2010) .

Plant breeding started more than 10,000 years ago with the selection of seeds to domestication, as occurred to the crops of maize (Zea mays L.), wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), among others (Dudley 1997) . With Mendel's laws, genetic principles began to be applied to plant breeding, ranging from the introduction, phenotypic selection on natural variants, selection with controlled mating, to marker-assisted selection for desirable genes (Allard 1999) . In the beginning, plant breeding was performed unconsciously and deliberately by farmers when they kept some plant of the harvest for planting or sowing their next crop. Besides that, natural selection occurred during the genetic diversity of the crop in new environments, during domestication and subsequent dispersion. Then, the hybridization and genetic-based process were added (Bradshaw 2016) . Conventional breeding is possible only between closely related (sexually compatible) individuals, thus relies on natural variation of the target compound within parental lines (De Steur et al. 2015) . To increase mineral content by breeding is challenging since numerous genes may be involved in elemental uptake by the roots, translocation throughout the plant, and deposition in edible tissues. Moreover, other factors such as environmental conditions and cultural practices can modify gene expression and alter mineral accumulation by plants (Bouis and Welch 2010) .

According to Saltzman et al. (2017) more than 30 countries have officially released biofortified varieties developed using the conventional plant breeding approach, and at least an additional 20 countries have commenced the testing of these varieties and they provide considerable amounts of bioavailable micronutrients, and consumption of these varieties may help to reverse the micronutrient deficiency status among target populations .

Genetic approaches refer to developing crops with improved abilities to acquire and accumulate minerals in edible parts. Modified varieties can also present increased concentrations of "promoter" substances, which stimulate mineral absorption and reduced concentrations of "antinutrients", substances that negatively interfere with nutrient absorption. However, food's taste and color may be affected by changes in the concentration of promoters and antinutrients, so these strategies must be cautiously evaluated (White and Broadley 2009 ). Research and development phases and the regulatory approval process for genetically modified (GM) crops are often time-consuming and expensive. However, after establishment, enhanced crops become sustainable (Khush et al. 2012) . Then, in the long term, these strategies can be cost-effective (Gonzali et al. 2017) , besides that, they can increase micronutrient concentrations in the desired tissue, such as cereal endosperm, to reduce post milling losses through the outer layers (De Steur et al. 2017) . Various genomic approaches, such as quantitative trait loci (QTL) mapping, markerassisted selection (MAS), marked-assisted recurrent selection (MARS), genome-wide selection (GS), and next-generation sequencing (NGS) have been widely employed for the biofortification.

Multiples genetic approaches are commonly employed to achieve the best results on mineral biofortification. For instance, Masuda et al. (2012) combined three transgenic approaches to produce Fe-biofortified rice: (1) enhancement of Fe storage in grains via expression of the Fe storage protein ferritin using endosperm-specific promoters;

(2) enhancement of Fe translocation through overproduction of the natural metal chelator nicotianamine and (3) enhancement of Fe flux into the endosperm through the expression of the Fe(II)-nicotianamine transporter OsYSL2 expression under the control of an endosperm-specific promoter and sucrose transporter promoter. The authors reported that the Fe concentration of polished seeds increased up to sixfold in greenhouse cultivation and 4.4-fold in paddy field cultivation (Masuda et al. 2012) . Johnson et al. (2011) reported that Fe concentrations were increased, reaching 14 mg kg À1 , in rice grains by GM. Besides that, Fe was unlikely to be bound by phytic acid and therefore likely to be more bioavailable in human diets (Johnson et al. 2011 ). Conventional breeding is also an option since there is a natural genetic variation in Zn concentrations of edible crops. Other approaches use genetic engineering to develop modified plants with increased abilities to acquire and accumulate Zn. Still, higher Zn concentrations in edible plant parts can be reached with the development of crops with more tolerance to high Zn levels in tissues. There are already genetically modified plants that have higher concentrations of Zn in the edible parts compared to traditional varieties (White and Broadley 2011) . It was noted that plants modified to increase Fe accumulation have also presented increased Zn concentrations. It may indicate a cross-talk between Fe and Zn transport pathways (Hirschi 2009 ). Connorton and Balk (2019) reviewed several GM crops for iron biofortification, including cassava, maize, wheat, rice, soybean, and sweet potato. The authors also mentioned that several quantitative trait loci and transgenes increase both iron and zinc, due to overlap in transporters and chelators for these two mineral micronutrients (Connorton and Balk 2019) .

Considering I biofortification, in some cases, there is a need for genetic engineering strategies to guarantee an effective result. It occurs mainly in cereals because the amount that reaches grains is insufficient to supply human needs. Genetic approaches focusing on reducing I volatilization from leaves or aiming to control the uptake and mobilization of this element through the phloem are promising, but still very scarce. There is a need for reliable protocols for I biofortification of staple crops to enable the dissemination of these practices (Gonzali et al. 2017) .

Therefore, the development of genetic biofortification methods must consider the impact that these modifications may have on the accumulation of other elements that are not necessarily the object of the study. Other questions that must be considered are the impact of biofortification on plant metabolism, growth, productivity, environment, and conservation of genetic resources (Garcia-Casal et al. 2017) . For instance, enzyme activities may be modified by metal content. Finally, possible alterations of plant stress, interactions with other nutrients, and allergic reactions in humans must be evaluated (Hirschi 2009 ). The main limitations of genetically modifying crops included consumers acceptance and to fulfill the regulatory requirements for labeling and approving commercialization of these crops.

Some recent strategies do not fit the previous definitions since they do not include the application of fertilizers during plant growth or even conventional breeding and genetic strategies. The use of plant growth-promoting microorganisms (PGPM), especially the plant growth-promoting rhizobacteria (PGPR), is one of the strategies that has grown significantly in the last years aiming at the crop plant biofortification. The PGPM approaches consist of the application of beneficial microorganisms (bacteria, fungi, among others) in cultivation soil. The soil application of these microorganisms increases mineral bioavailability contributing to crop plant biofortification and improve the soil fertility and crop yield (Khan et al. 2019; Rana et al. 2012) . In turn, PGPR consists of a varied group of beneficial bacteria that colonize the rhizosphere and plant roots (Glick 1995) . In short, the PGPR is the soil bacteria that stimulate the growth of the host through increasing mobility, uptake, and enrichment of nutrients in the plant (Prasanna et al. 2016) . Moreover, they contribute to plant growth development by fixing biological nitrogen, enhancing root function, suppressing disease, among other benefits (Glick 1995; Vessey 2003; Hafeez et al. 2006) .

The application of PGPR in agriculture is an attractive way to minimize the use of fertilizers and related agrochemicals (Rana et al. 2012 ). According to De Santiago et al. (2011) , agronomic and genetic approaches have a higher cost than PGPR application, present ethical problems, and are non-environmental friendly. In this way, the use of PGPR agents could be an interesting alternative to agronomic and genetic approaches aiming to promote the crop plant growth as well as enhance the uptake of micronutrients by plants (De Santiago et al. 2011; Mora et al. 2015) . Vessey (2003) defined PGPR as biofertilizers, i.e., substances that contain living microorganisms and, once applied to plant or soil, colonizes the rhizosphere or the interior of the plants promoting the increase of supply or availability of primary nutrients to the host plant (Vessey 2003) . However, some authors consider that the use of PGPR for crop plant biofortification should be carried out as a possible supplementary measure, along with other approaches Blanchfield 2004) . In addition to the use of bacteria, other organisms such as fungi have also been used for this purpose (Durán et al. 2013) . In Table 7 .3 are presented some studies demonstrating the application of microorganism strains to the soil for crop plant biofortification.

According to the studies described in Table 7 .3, it is possible to verify that the application of microorganism strains to the cultivation soils, especially for cereals and legumes, is a promising approach for mineral biofortification. Although the combination of agronomic and PGPR approaches can be an advantageous alternative for crop plant biofortification, in some cases only the application of microorganism strains to the soil may promote the same benefits. The application of strains of bacteria in soil, for example, was effective in increasing the Ca, Cu, Fe, Mg, Mn, and Zn concentration of chickpeas and wheat without the need to add fertilizers avoiding problems related to environmental pollution (Rana et al. 2012 ). Moreover, the use of fungi strains has also been promising. Durán et al. (2013) , for example, observed an increase of 24% in Se concentration in wheat co-inoculate with a mixture of rhizobacteria and arbuscular mycorrhizal fungi. It is important to note that, in both cases, the application of microorganism strains to soil would not have the same success if were performed in mineral-deficient soils. Even so, in these cases, the use of PGPM is an environmentally friendly and low-cost alternative that, associated with agronomic approaches, may provide savings regarding the use of fertilizers.

It is known that hidden hunger or micronutrient deficiency is a worldwide concern, leading to about two billion people who do not have access to supplements or a diversified diet to consequences such as anemia and even death (HarvestPlus 2020). It is also known that biofortification strategies are sustainable and effective tools to improve the nutritional status of staple crops (Díaz-Gómez et al. 2017 ). In the last Durán et al. (2013) years, it was possible to observe significant progress in research and development of biofortified foods, with a variety of new strategies emerging for several nutrients/ crops (Hefferon 2015) . However, biofortification efficiency to tackle elemental deficiency in humans is not yet a fully clarified subject, generating controversies among researches. There is a lack of nutritional assessment regarding biofortified foods and their impact on global human health. Valença et al. (2017) highlighted that, despite the potential of biofortification to increase nutritional content and yield of food crops, more evidence is necessary to prove its influence in human health and its efficacy to alleviate micronutrient deficiencies. Another point is that biofortification strategies must be adapted for different staple crops that are commonly harvested in each region. Moreover, the success of biofortification is related to the correct choice of food preparation and cooking methods that can impact on nutrient bioavailability (Díaz-Gómez et al. 2017) . Another challenge that must be overcome is the public perception of biofortification, which may influence the regulation and implementation of genetically modified crops (Hefferon 2015) . Thus, before these techniques are widely applied, its influence on nutrient bioavailability must be confirmed (Díaz-Gómez et al. 2017) . Beyond that, systematic research and comprehensive feeding trials are needed to clarify the benefits that they can have on human health in the long term. Finally, the impacts of these foods must be assessed in the fields of nutrition, health, environment, and agriculture (Hirschi 2009 ).

On the other hand, a review conducted by White and Broadley (2009) concluded that biofortification of crop plants has a great potential to improve the nutritional status of humans, without compromising crop yield. Khush et al. (2012) and Díaz-Gómez et al. (2017) agreed that biofortification is a promising tool to alleviate malnutrition in vulnerable populations.

Biofortification is one of the tools to combat hidden hunger by increasing the micronutrient content of staple foods (HarvestPlus 2020). Both Food and Agriculture Organization of the United Nations (FAO) and HarvestPlus, which is part of the CGIAR Research Program on Agriculture for Nutrition and Health (A4NH) and is led by the International Food Policy Research Institute (IFPRI), have been working together in the development, production, and implementation of biofortified staple crops aiming to improve nutrition and health of vulnerable populations. Iron, zinc, and vitamin A are the main focus of these programs where biofortification is carried out through conventional crop breeding. In general, the target foods are stapling crops such as rice, maize, wheat, cassava, beans, and sweet potato. The adoption and expansion of biofortification programs are highly encouraged and supported by the aforementioned agencies (HarvestPlus 2020).

For the success of a biofortified crop, tests must be carried out to scientifically prove that it will indeed contribute to the increase in micronutrient intake. Only after this stage, the biofortified crop can be disseminated and consumed as a safe and effective nutrient source. One of the advantages of these crops is that they can be continuously improved after implementation, since varieties with superior qualities, such as the higher concentration of micronutrients, can be always selected (HarvestPlus 2020).

According to HarvestPlus (2020), biofortified crops of 200 varieties are already officially present in 30 countries (HarvestPlus 2020). For example, in 2019 there were 39 varieties of iron-biofortified beans released in Africa and 21 in Latin America and the Caribbean. These beans, when consumed as a staple, would supply 80% of the estimated average requirement (EAR) for Fe. Also, a total of ten varieties of pearl millet and eight of cowpea biofortified with Fe were released, supplying 80% and 25% of the iron EAR, respectively. In 2019, Zn biofortified crops (11 varieties of wheat, 10 of rice, and 7 of maize) were legalized, providing respectively 50%, 40%, and 70% of the EAR of Zn. The overall climateadaptiveness and higher yields of biofortified crops contributed to its acceptance by farmers (HarvestPlus 2018). By 2018, biofortified crops such as iron beans and zinc rice were grown by about 7.6 million farmers (HarvestPlus 2018, 2019) . Consumers usually have a good acceptance of biofortified crops, enjoying its taste, appearance, odor, and texture (HarvestPlus 2020). A total of 38 million people were growing and consuming biofortified crops in 2018 (HarvestPlus 2018).

Many studies have shown the nutritional and health benefits of biofortified crops, mainly to people who consume then as staple foods. These studies found that nutrients in biofortified crops are as bioavailable as those of traditional varieties. The consumption of these crops can improve micronutrient status, cognitive function, and reduce morbidity, as well as supply 80% of the daily average requirement of Fe and 70% of Zn (HarvestPlus 2020).

A study conducted with Rwandan women suggested that the consumption of Fe-biofortified beans contributed to the improvement of iron status and to prevent and reverse iron deficiency among those women (Haas et al. 2016) . Scott et al. (2018) performed an intervention study in 140 Indian boys and girls, aged 12-16 years old, concluding that the consumption of iron-biofortified pearl millet improved Fe status as well as some measures of cognitive performance (memory and attention) (Scott et al. 2018) . Brnić et al. (2015) have compared the zinc absorption from a rice variety fortified with Zn and the same rice variety biofortified with zinc. The results showed that rice biofortification was as good as the postharvest fortification to combat zinc deficiency and biofortified rice presented more bioavailable zinc than conventional rice (Brnić et al. 2015) . A study conducted with 6005 participants suggested that the consumption of zinc-biofortified wheat reduces maternal and child morbidity (Sazawal et al. 2018) .

In conclusion, scientific research, development, and application studies have suggested that biofortification can contribute to more people having access to a healthy and diverse diet by making staple crops more nutritious. It contributes to the improvement of the nutritional status of vulnerable populations and helps fight hidden hunger. Moreover, there is evidence that farmers and consumers have accepted biofortified foods well (HarvestPlus 2020). Then, biofortification together with other approaches namely supplementation, dietary diversity, and food fortification are complementary strategies to tackle elemental deficiency.

Agronomic biofortification of selected underutilised solanaceae vegetables for improved dietary intake of potassium (K) in Ghana

Iron. In: Macdonald IA (ed) Present knowledge in nutrition

Biofortification of zinc in onions (Allium cepa L.) and soil Zn status by the application of different organic Zn complexes

Foliar application: from plant nutrition to biofortification

Foliar application of micronutrients enhances crop stand, yield and the biofortification essential for human health of different wheat cultivars

Selenium biofortification of broccoli and carrots grown in soil amended with Se-enriched hyperaccumulator Stanleya pinnata

Hidden hunger in the developed world

Iodine biofortification and antioxidant capacity of lettuce: potential benefits for cultivation and human health

Improving nutrition through biofortification: a review of evidence from HarvestPlus

Biofortification-a sustainable agricultural strategy for reducing micronutrient malnutrition in the global south

Genetically modified food crops and their contribution to human nutrition and food quality

Zinc absorption by adults is similar from intrinsically labeled zinc-biofortified rice and from rice fortified with labeled zinc sulfate

Selenium biofortification of high-yielding winter wheat (Triticum aestivum L.) by liquid or granular Se fertilisation

Se-enrichment of cucumber (Cucumis sativus L.), lettuce (Lactuca sativa L.) and tomato (Solanum lycopersicum L. Karst) through fortification in pre-transplanting

Iodine fortification plant screening process and accumulation in tomato fruits and potato tubers

Iodine uptake and distribution in horticultural and fruit tree species

Chapter 9 -biofortification of staple crops

Iron biofortification of staple crops: lessons and challenges in plant genetics

Selecting iodine-enriched vegetables and the residual effect of iodate application to soil

Availability of iodide and iodate to spinach (Spinacia oleracea L.) in relation to total iodine in soil solution

Zinc and selenium accumulation and their effect on iron bioavailability in common bean seeds

Effect of Trichoderma asperellum strain T34 on iron, copper, manganese, and zinc uptake by wheat grown on a calcareous medium

Status and market potential of transgenic biofortified crops

The socioeconomics of genetically modified biofortified crops: a systematic review and meta-analysis

Biofortification of crops with nutrients: factors affecting utilization and storage

Biofortification of green bean (Phaseolus vulgaris L.) and lettuce (Lactuca sativa L.) with iodine in a plant-calcareous sandy soil system irrigated with water containing KI

Comparison study of zinc nanoparticles and zinc sulphate on wheat growth: from toxicity and zinc biofortification

Quantitative genetics and plant breeding

Global prevalence of malnutrition: evidence from literature

Enhanced selenium content in wheat grain by co-inoculation of selenobacteria and arbuscular mycorrhizal fungi: a preliminary study as a potential Se biofortification strategy

Selenium enriched vegetables as biofortification alternative for alleviating micronutrient malnutrition

The State of Food Security and Nutrition in the World: Safeguarding against economic slowdowns and downturns

Staple crops biofortified with increased vitamins and minerals: considerations for a public health strategy

Biofortified crops generated by breeding, agronomy, and transgenic approaches are improving lives of millions of people around the world

Carbonaceous soil amendments to biofortify crop plants with zinc

Spatiotemporal trends in adequacy of dietary nutrient production and food sources

The importance of minerals in human nutrition: Bioavailability, food fortification, processing effects and nanoencapsulation

The enhancement of plant growth by free-living bacteria

Critical evaluation of strategies for mineral fortification of staple food crops

Iodine biofortification of crops: agronomic biofortification, metabolic engineering and iodine bioavailability

Plant growth-promotion and biofortification of chickpea and pigeonpea through inoculation of biocontrol potential bacteria, isolated from organic soils

Sources and deficiency diseases of mineral nutrients in human health and nutrition: a review

Consuming iron biofortified beans increases iron status in Rwandan women after 128 days in a randomized controlled feeding trial

Plant growthpromoting bacteria as biofertilizer

Regulating in pandemic: evaluating economic and financial policy responses to the coronavirus crisis

Iodine biofortification through expression of HMT, SAMT and S3H genes in Solanum lycopersicum L

Catalyzing Biofortified Food Systems

Biofortification: the evidence

Biofortification: a food-systems solution to help end hidden hunger

Nutritionally enhanced food crops; progress and perspectives

HarvestPlus https:// www.harvestplus.org/content/prioritizing-countries-biofortification-interventionsbiofortification-priority-index-second

Nutrient biofortification of food crops

Transfer of iodine from soil to vegetables by applying exogenous iodine

The fate of exogenous iodine in pot soil cultivated with vegetables

Zinc oxide nanoparticles alter the wheat physiological response and reduce the cadmium uptake by plants

Foliage application of selenium and silicon nanoparticles alleviates Cd and Pb toxicity in rice

Dietary reference intakes for vitamin C, vitamin E, selenium, and carotenoids

Constitutive overexpression of the OsNAS gene family reveals single-gene strategies for effective iron-and zinc-biofortification of rice endosperm

Selenium deficiency risk predicted to increase under future climate change

Iron absorption from beans with different contents of iron, evaluated by stable isotopes

The effects of selenium biofortification on mercury bioavailability and toxicity in the lettuce-slug food chain

Biofortification of iron in chickpea by plant growth promoting rhizobacteria

Microbial biofortification: a green technology through plant growth promoting microorganisms

Biofortification of crops for reducing malnutrition

Tomato fruits: a good target for iodine biofortification

Iodine biofortification in tomato

Selenium biofortification of wheat grain via foliar application and its effect on plant metabolism

Soil versus foliar iodine fertilization as a biofortification strategy for field-grown vegetables

Enhancing iodine content and fruit quality of pepper (Capsicum annuum L.) through biofortification

Iodate and iodide effects on iodine uptake and partitioning in rice (Oryza sativa L.) grown in solution culture

Importância do zinco na nutrição humana

Simultaneous Zinc and selenium biofortification in rice. Accumulation, localization and implications on the overall mineral content of the flour

Zinc fertilization increases productivity and grain nutritional quality of cowpea (Vigna unguiculata [L.] Walp.) under integrated soil fertility management

Iron biofortification in rice by the introduction of multiple genes involved in iron nutrition

A food systems approach to increase dietary zinc intake in Bangladesh based on an analysis of diet, rice production and processing

Food security and nutrition in the age of climate change

Improving selenium status in plant nutrition and quality

Iodine: fact sheet for health professionals. Office of Dietary Supplements

National Institutes of Health (NIH) (2020d) Zinc: fact sheet for health professionals. Office of dietary supplements

Selenium in food and the human body: a review

Income growth and climate change effects on global nutrition security to mid-century

Biofortification of staple food crops

Iron biofortification in mung bean using siderophore producing plant growth promoting bacteria

Biofortification of rice grain with zinc through zinc fertilization in different countries

Soil and foliar zinc biofortification in field pea (Pisum sativum L.): Grain accumulation and bioavailability in raw and cooked grains

Biofortification with microorganisms: present status and future challenges

Inoculation of zinc solubilizing Bacillus aryabhattai strains for improved growth, mobilization and biofortification of zinc in soybean and wheat cultivated in Vertisols of central India

Iron biofortification of wheat grains through integrated use of organic and chemical fertilizers in pH affected calcareous soil

Biofortification of wheat through inoculation of plant growth promoting rhizobacteria and cyanobacteria

Addressing chronic malnutrition through multi-sectoral, sustainable approaches: a review of the causes and consequences. Front Nutr 1:13

Biofortification of Se and induction of the antioxidant capacity in lettuce plants

Zinc and iron oxide nanoparticles improved the plant growth and reduced the oxidative stress and cadmium concentration in wheat

Global update and trends of hidden hunger, 1995-2011: the hidden hunger index

Biofortification: progress toward a more nourishing future

Availability, production, and consumption of crops biofortified by plant breeding: current evidence and future potential

Plant growth-promoting actinobacteria on chickpea seed mineral density: an upcoming complementary tool for sustainable biofortification strategy

Efficacy of high zinc biofortified wheat in improvement of micronutrient status, and prevention of morbidity among preschool children and women -a double masked, randomized, controlled trial

Cognitive performance in Indian school-going adolescents is positively affected by consumption of ironbiofortified pearl millet: a 6-month randomized controlled efficacy trial

Biofortification of Triticum aestivum through the inoculation of zinc solubilizing plant growth promoting rhizobacteria in field experiment

Enhancing grain iron content of rice by the application of plant growth promoting rhizobacteria

Biofortification: a new approach to eradicate hidden hunger

Selenium nanoparticles as a nutritional supplement

Influence of iodine form and application method on the effectiveness of iodine biofortification, nitrogen metabolism as well as the content of mineral nutrients and heavy metals in spinach plants (Spinacia oleracea L.)

Assessment of biofortification with iodine and selenium of lettuce cultivated in the NFT hydroponic system

Iodine biofortification with additional application of salicylic acid affects yield and selected parameters of chemical composition of tomato fruits (Solanum lycopersicum L.)

The absorption of iodine from 5-iodosalicylic acid by hydroponically grown lettuce

The effect of salicylic acid on biofortification with iodine and selenium and the quality of potato cultivated in the NFT system

Combined biofortification of carrot with iodine and selenium

Genotypic variation of zinc and selenium concentration in grains of Brazilian wheat lines

Global impacts of human mineral malnutrition

Global diets link environmental sustainability and human health

Development of a method for producing selenium-enriched radish sprouts

Agronomic biofortification of crops to fight hidden hunger in sub-Saharan Africa

Plant growth promoting rhizobacteria as biofertilizers

Magnesium. In: Macdonald IA (ed) Present knowledge in nutrition

Synopsis of 2014 Global hunger index: The challenge of hidden hunger

Biofortification of lettuce (Lactuca sativa L.) with iodine: the effect of iodine form and concentration in the nutrient solution on growth, development and iodine uptake of lettuce grown in water culture

Advances in selenium-enriched foods: from the farm to the fork

Different increases in maize and wheat grain zinc concentrations caused by soil and foliar applications of zinc in Loess Plateau

Weaver CM (2012) Calcium. In: Macdonald IA (ed) Present knowledge in nutrition. Wiley online books

Mechanism of iodine uptake by cabbage: effects of iodine species and where it is stored

Increment of iodine content in vegetable plants by applying iodized fertilizer and the residual characteristics of iodine in soil

Iodine biofortification of vegetable plants-An innovative method for iodine supplementation

Biofortification of crops with seven mineral elements often lacking in human diets -iron, zinc, copper, calcium, magnesium, selenium and iodine

Physiological limits to zinc biofortification of edible crops

Dietary supplements and sports performance: minerals

World Health Organization (WHO) (2013) Micronutrient deficiencies: iodine deficiency disorders

World Health Organization (WHO) (2019) Micronutrient deficiencies: iron deficiency anaemia

Microbial-enhanced selenium and iron biofortification of wheat (Triticum aestivum L.) -applications in phytoremediation and biofortification

Engineering glutathione transferase to a novel glutathione peroxidase mimic with high catalytic efficiency Incorporation of selenocysteine into a glutathione-binding scaffold using an auxotrophic expression system

Effects of foliar application of selenate and selenite at different growth stages on selenium accumulation and speciation in potato (Solanum tuberosum L.)

Iodine uptake by spinach (Spinacia oleracea L.) plants grown in solution culture: effects of iodine species and solution concentrations