Keywords

1 Introduction

Plants are the best example of autotrophic organisms that use a prime energy source via photosynthesis, providing food to nearly all living organisms. Photosynthesis requires water, essential nutrients, sunlight, and air to give plants the energy to grow, survive, and reproduce (Wiedenhoeft 2006). Food, whether it is bread, cereal, or cornflakes, is the major source of energy for poor and rich people and is derived from crops. Plants extract a considerable amount of essential nutrients from the soil as they cannot synthesize the required nutrients. The nutrients are taken up by the plants and then transferred to other organisms through the food chain. Several biotic and abiotic stressors affect the growth and productivity of crop plants. Among these stresses, nutrient stress is of prime importance as it affects not only crop yield but also the primary end-users, that is, humans or livestock. Subsistence agriculture provides a living for most of the people inhabiting semi-arid tropics (SAT). Low fertility and water scarcity are the characteristic indicators of these areas due to which their agricultural soils are referred as hungry and thirsty soils. Moreover, crop productivity in these areas is also lower as compared to irrigated agricultural areas. In order to feed rapidly increasing human population, such poor soils will have to be brought under cultivation in the future (Rego et al. 2006). Therefore, in order to upgrade the fertility status of these soils, micronutrients can play a central role. Hence, we have to develop a model strategy and create massive awareness, in order to get the maximum benefits of micronutrients on crop yield and quality, that can also be readily accepted by farmers, consumers, and scientific community.

2 Legumes

Legumes are considered as “poor man’s meat,” and the claim appears to be accurate based on their global consumption patterns (Messina 1999). After cereals, legumes are regarded as the second most important nutritionally valuable food source (Kouris-Blazos and Belski 2016) because they provide essential amino acids, proteins, complex carbohydrates, dietary fiber, unsaturated fats, vitamins, and minerals to humans (Rebello et al. 2014). Besides nutritional supremacy, legumes also produce certain beneficial bioactive compounds and possess cultural, physiological, and medicinal roles (Philips 1993). Various health benefits are linked with consuming legumes (Messina 1999) as these crops possess antiatherogenic, hypocholesterolemia, anticarcinogenic, and hypoglycemic properties. They contain a high amount of vitamin B-group such as folate, thiamine, and riboflavin and essential minerals such as zinc, iron, calcium, selenium, phosphorus, copper, potassium, magnesium, and chromium but low amounts of fat-soluble vitamins and vitamin C (Brigide et al. 2014; Kouris-Blazos and Belski 2016). These micronutrients are essential for bone health (calcium), protein synthesis, hemoglobin synthesis (iron), antioxidant activity, enzyme activity, carbohydrate, lipid (chromium and zinc), and iron metabolism (copper), as well as plasma membrane stabilization (zinc). Hence, it is necessary to escalate the legume yield keeping in view their nutritional quality traits (Naeem et al. 2017).

3 Role of Micronutrients in Crop Plants

Numerous factors influence crop production that has an impact on yield either directly or indirectly. Among these, certain soil factors, such as pH, soil texture, organic matter, soil–water relationships, and balanced nutrients, are typically highlighted in recent studies. The long-term yield and quality of legume plants, as well as the fertility status of soils, depend critically on the proper nutrition of crop plants. Mineral nutrition plays a crucial role in plant growth. Most crop plants exhibit a linear relationship between the amount of fertilizers taken up and the resulting harvest. Plants can successfully thrive to their full genetic potential with the right and steady supply of required minerals (Naeem et al. 2017).

The desire for greater crop productivity without balanced mineral nutrition created severe problems such as soil fertility depletion and plant nutrient imbalance of primary and secondary nutrients including micronutrients (Patel and Singh 2009). Plants cannot survive without micronutrients, even though they are required in a relatively low concentrations (Prasad et al. 2005). Although micronutrients are needed in trace amounts in a living system, they play a crucial role in maximizing the effectiveness of macronutrients and promoting plant growth and development (Shukla et al. 2009).

The group of micronutrients is comprised of eight essential elements, iron (Fe), sodium (Na), chlorine (Cl), boron (B), manganese (Mn), zinc (Zn), copper (Cu), and molybdenum (Mo). Among the scientific community, silicon (Si) has also been considered as a potential micronutrient. Although plants accumulate and use Si in relatively high concentrations even though it is not considered an essential nutrient. Some plant species with root nodules require cobalt (Co) as an essential micronutrient. Furthermore, nickel (Ni) is a micronutrient that is rarely in short supply or deficient in the natural world (Shukla et al. 2009).

4 Causes of Micronutrient Deficiency in Soils

Certain edaphic and ecological factors such as soil organic matter, pH, cation exchange capacity, and clay content affect the availability of micronutrients to crop plants. Moreover, water-retentive soils and calcareous/peat soils can also hinder the bioavailability of some micronutrients (Lindsay 1984; Ibrahim et al. 2011; Ramzan et al. 2014). In fertile soils, the critical limit of DTPA extractable Zn and Fe is 0.6 and 4.5 mg kg−1, respectively (Alloway 2009). The soil’s critical limit is the lowest possible soil test value that guarantees a maximum crop yield. The threshold below which insufficiency symptoms appear is the concentration at the bottom of the sufficiency range (Sillanpää 1982).

Soil contains a large amount of Fe, but only a small percentage is biologically available to crop plants. Most of the Fe on earth exists in Fe3+ form, which is readily not available to plants, making up a significant portion of the crust. The more soluble form of iron is Fe2+; however, it is easily oxidized to a ferric (Fe3+) form, which precipitates in the soil as oxide/hydroxide, phosphate, carbonate, and other unavailable complex forms (Lindsay and Schwab 1982). High soil pH reduces the zinc (Zn) bioavailability as it precipitates or adsorbs onto the surface of CaCO3 and Fe oxides (Harter 1983; Chirwa and Yerokun 2012). The soil’s cation exchange capacity (CEC) is negatively correlated with availability of Zn or Fe (Yoo and James 2002). The availability of Zn also decreases with increased clay content of soils. Similarly, a greater P content in soil and electrical conductivity (EC) also affects the bioavailability of Zn (Gao et al. 2011).

Soil micronutrient content is affected by various factors, including geochemical composition, soil type, macronutrient availability, micronutrient interactions, and vegetation type. Intrinsic properties of soil, such as pH, redox potential, soluble salt concentration, quantity and quality of soil organic matter, and trace element inputs, also play a role in determining the micronutrient content of soil (Fageria et al. 2002; Alloway 2008; Shukla et al. 2016). Additionally, agricultural intensification without sufficient micronutrient replenishment through fertilization is also a contributing factor toward low levels of micronutrient bioavailability. Furthermore, leaching, liming of agricultural soils, scarce manuring, and frequent application of micronutrient-deficient chemical fertilizers have also accelerated the depletion of available micronutrients in soils. Furthermore, as per the GPS-aided analysis of soil samples, element-wise deficiency is as follows: Zn 36.5%, B 23.4%, Fe 12.8%, Mn 7.1%, Cu 4.2% (Shukla et al. 2018).

Among these micronutrients, iron (Fe) and zinc (Zn) are essential plant cofactors which play important roles in different plant processes such as respiration, photosynthesis, and stress tolerance (Sharma et al. 2013; Rout and Sahoo 2015; Tripathi et al. 2018). Furthermore, in legume nodules, Fe- and Zn-dependent processes are crucial for establishing endosymbiotic associations between arbuscular mycorrhiza and soil rhizobia (González-Guerrero et al. 2016; Day and Smith 2021). Iron deficiency causes reduced vegetative development and chlorosis resulting in poor crop productivity. Although plants have a low affinity toward Zn but it is an important component of Cu and Zn superoxide dismutase enzymes actively involved in chlorophyll synthesis and auxin metabolism (Sharma et al. 2013). The detailed information regarding the significance of these two micronutrients is given below.

5 Importance of Iron (Fe)

Iron is a life-preserving micronutrient that is required for the survival of all living organisms including microorganisms, plants, and humans. The iron content of crops is a significant factor in determining human health in addition to its crucial role in promoting plant growth (Briat et al. 2015). Iron is instrumental in various metabolic processes including photosynthesis, respiratory functioning, and synthesis of DNA (Schmidt et al. 2020). Additionally, it triggers the activation of numerous metabolic pathways and is a crucial component in electron chains and a vital cofactor for many enzymes (Zuo and Zhang 2011). Plants require iron for various biological events, as it is the fourth plentiful element found in soil. Among all plants, legumes that participate in nitrogen-fixing symbiosis have a particularly high demand for iron (Brear et al. 2013). Iron is an indispensable micronutrient for plants being part of some antioxidant enzymes that protect chloroplasts from harmful free radicals. It also serves as an integral component of the heme group, which is a precursor of chlorophyll (Barker and Pilbeam 2015). Moreover, iron has a strong impact on phytohormonal regulation, specifically with regards to carbohydrate metabolic activities, auxin production, regulation of stress responses, and protein synthesis (Al-Amri et al. 2020). Iron also has a central role in formation of hemoglobin, body homeostasis, brain functioning and development, catecholamine metabolism, and muscle activity. Moreover, iron is also fundamental for the cellular respiration and transportation of oxygen (Sarkar et al. 2018). According to an estimate, adults need 14 mg of iron daily to maintain good health (Carvalho et al. 2012).

Approximately 80% of iron is found in photosynthetic cells which is crucial for the creation of various heme molecules such as cytochromes, chlorophyll, Fe-S clusters, and functioning of the electron transport system (Briat et al. 2007). Plants rely on iron for the production of chlorophyll and the preservation of chloroplast structure and its proper functioning. The porphyrin structure of chlorophyll is dependent on iron which makes it an intrinsic component of chloroplasts. In the photosynthetic process, iron atoms can be found in varying amounts in different components: 2–3 iron atoms in molecules linked to photosystem II, 12 atoms in photosystem I, 5 in the cytochrome complex, and 2 in the ferredoxin molecule. The distribution of iron in various components of the photosynthetic process demonstrates its direct involvement in plant photosynthesis and its impact on crop productivity (Varotto et al. 2002; Briat et al. 2007).

Despite the limited presence of iron in living organisms (50–100 μg per gram of dry matter), it is still a basic element for plant life. It plays a pivotal role in various metabolic processes through its involvement in key proteins and enzyme synthesis. Iron, being an integral component of proteins and enzymes, holds a prominent place in different fundamental biological procedures such as photosynthesis, DNA synthesis, chlorophyll synthesis, respiration and nitrogen fixation through the action of the ribonucleotide reductase (Asad and Rafique 2000; Rout and Sahoo 2015).

Legumes with an active symbiotic mechanism usually have a high demand for iron, as numerous symbiotic proteins need iron. The nitrogen-fixing enzyme, nitrogenase, as well as cytochromes, ferredoxin, and hydrogenase, which are synthesized by the abundant bacteroids, require iron (Dixon and Kahn 2004; Peters and Szilagyi 2006). The importance of iron in the symbiosis is emphasized by the high concentration of iron in the nodule relative to other plant tissues (Dixon and Kahn 2004; Peters and Szilagyi 2006; Brear et al. 2013).

6 Importance of Zinc (Zn)

Zinc is a life-sustaining micronutrient required for proper functioning of humans, crops, and livestock (Hussain et al. 2015). Although zinc is needed in limited quantities for the optimal growth of plants, it plays a significant role in plant metabolism. Its presence in the right concentration is necessary for various plant physiological activities such as photosynthesis, sugar production, seed and fertility formation, growth regulation, and resistance to diseases. Zinc is also considered indispensable for the proper functioning of cells in all living organisms and plays an important role in boosting the human immune system (Solanki et al. 2016).

Zinc is also involved in the synthesis of plant proteins, being an important constituent of ribosomes. Zinc deficiency causes significant decline in protein synthesis, which can be seen by the increased buildup of amino acids in plant tissues (Mousavi 2011). The synthesis of proteins usually occurs in pollen tubes and is disrupted by Zn deficiency, leading to negative effects on the pollination process (Outten and O’halloran 2001; Pandey et al. 2006). The protein synthesis process during transcription and translation is also impacted by a Zn deficiency. Moreover, Zn also safeguard rRNA from ribonuclease and helps in normal functioning of RNA polymerase (Kryvoruchko 2017). The decline in protein synthesis in Zn-deficient plants is due to the involvement of Zinc in nitrogen metabolism (Fageria 2002; Rehman et al. 2019). In addition, zinc plays a supporting role in the function of various enzymes such as metalloproteases, copper–zinc superoxide dismutase (SOD), nucleases, and aminopeptidases (Hänsch and Mendel 2009). Moreover, the transcriptionally regulated, MucR regulator of reactive oxygen species (ROS), responsible for maintaining cell integrity, balancing iron levels, synthesizing polysaccharides, regulating transcription, and promoting genome plasticity in numerous legume species is also maintained by specific concentration of Zn (Caswell et al. 2013). Zinc is also a major component of proteins that regulate gene expression by interacting with DNA (Liu et al. 2005).

Moreover, zinc plays its part in controlling the activity of enzymes involved in carbohydrate metabolism, such as aldolase, carbonic anhydrase, and fructose-1,6-bisphosphate aldolase, which operate in both the cytoplasm and chloroplasts. These enzymes aid in the transfer of sugar molecules during photosynthesis. Fructose-1,6-bisphosphate aldolase divides 6-carbon sugars between the cytoplasm and chloroplasts, and aldolase moves 3-carbon sugars from the cytoplasm to chloroplasts. Zinc deficiency inhibits the activity of these enzymes due to which carbohydrates starts to accumulate in plant foliage (Cakmak et al. 1989; Mousavi 2011). Legumes are susceptible to a variety of abiotic stressors, such as drought, salinity, high temperature, and chilling, leading to significant reductions in yield. This impact is further exacerbated when the plants are deficient in zinc. Being widely planted under rain-fed conditions or marginal soils, legumes are often exposed to these environmental stresses at various stages of their growth. Under water scarce conditions, Zn deficiency results in stunted root development and hindered nutrient uptake (Broadley et al. 2007; Rehman et al. 2018; Ullah et al. 2019). The deficiency of zinc leads to a decrease in the functioning of multiple enzymes involved in plant metabolic and physiological processes (Salama et al. 2006; Ullah et al. 2019). Additionally, the active involvement of Zn in several enzymes helps plant to develop resistance against different abiotic stresses (Rehman et al. 2019). The Cu-Zn-SOD enzyme has zinc attached to copper and performs a catalytic function. When there is a shortage of zinc, the activity of Cu-Zn-SOD decreases and the production of free radicals increases, resulting in damage to plant cells (Marschner 1995; Rehman et al. 2019). Moreover, when plant is prone to a stressful condition, an excessive production of reactive oxygen species (ROS) can harm proteins, nucleic acids, and lipids, resulting in cell damage and eventual death (Gill and Tuteja 2010). By providing plants with zinc during periods of stress, the negative effects can be reduced through regulation of superoxide dismutase activity. This helps to counter the damaging impact of reactive oxygen species by detoxifying them (Cakmak 2000; Ullah et al. 2019). Studies have shown that zinc finger proteins also participate in the regulation of reactive oxygen species, conferring tolerance to various abiotic stresses (Davletova et al. 2005; Mittler et al. 2006; Ciftci-Yilmaz and Mittler 2008).

Zinc is deemed as a vital nutrient for plants and is involved in multiple plant regulatory, metabolic, and developmental processes (Broadley et al. 2007). Legumes rely heavily on zinc for various aspects of plant reproduction, including initiation of flowers, development of blooms, formation of male and female reproductive cells, fertilization, and seed growth (Pathak et al. 2012). Zinc scarcity leads to alterations in the shape, size of stigma, and secretions of black gram and interferes with the interaction between pollen and stigma (Pandey et al. 2009). Furthermore, Zn deficiency also triggers flower failure and infertility of pollen and ovules, leading to a decrease in yield due to limited seed formation, resulting in lower crop productivity (Pathak et al. 2012).

7 Biofortification

The deficiency of micronutrient mainly Zn and Fe remains one of the most serious agricultural and public health issues. Scientists have proposed numerous ecologically compatible approaches to combat micronutrient deficiencies in soils and to enhance the nutritional status of our food products (Marques et al. 2021). Among the approaches, biofortification is the most practical, eco-friendly, and cost-effective method in which nutritional profile of food products is upgraded using different agronomic practices, conventional breeding approaches, and modern biotechnological approaches (Sarkar et al. 2018). The agronomic procedures mainly enhance mineral uptake from soil to crop either through increased nutrient mobility or solubilization (White and Broadley 2009). The other two approaches improve the mineral content in edible portion of crop plants and increase their bioavailability for end-users (Carvalho and Vasconcelos 2013).

Biofortification aims to tackle the issues related to micronutrients deficiencies by enhancing the micronutrient levels in the edible portions of crops and enhancing their digestibility and absorption in the human body following ingestion (Carvalho and Vasconcelos 2013; Ramzani et al. 2016; Vasconcelos et al. 2017). In developing nations, over 20 million people are eating crops that have been biofortified (Rubiales and Mikić 2015; Soares et al. 2019). The United Nations emphasized the significance of biofortifying legumes in 2014, recognizing them as a crucial aspect in the battle against micronutrient deficiencies. Despite this recognition, progress in this area has been limited and the biofortification of legumes remains an underdeveloped area of focus (Foyer et al. 2019; Ghosh et al. 2019; Rehman et al. 2019).

7.1 Organic Agronomic Biofortification Approaches

Biofortification via agronomic methods is widely adopted globally due to its simplicity and efficiency. These methods refer to preharvest agricultural techniques that increase the nutritional value of food (Sarkar et al. 2018). A limitation of these agronomic biofortification methods is that they must take place before harvest in order for the food to be classified as biofortified. If the process is carried out after harvest, the food is considered fortified instead (Sarkar et al. 2018).

The green revolution saw a rise in technological advancements that led to an increase in crop production and an adequate supply of food to fulfill the caloric needs of the huge population worldwide (Smil 2000; Tilman et al. 2002). As a result, using organic methods instead of conventional synthetic fertilizers may be desirable. One commonly observed reason is to restore depleted soil organic matter, thereby taking advantage of the physical, chemical, and biological benefits that are associated with the soils high in organic matter (Loveland and Webb 2003; Diacono and Montemurro 2011; Murphy 2015). Moreover, by using organic methods, the uptake of plant nutrients can be enhanced, the efficiency of nutrient utilization can be maximized, and the environmental impact can be reduced when compared to the use of inorganic pesticides (Edmeades 2003; Quilty and Cattle 2011). In agriculture, particularly in small-scale subsistence farming, animal waste is utilized as fertilizer because it is the most accessible and economically viable source of nutrients for plants in many situations (Onduru et al. 2008). Adopting organic methods can raise the level of carbon in the soil and, through a series of interconnected processes, enhance the soil’s biological activity, structure, ability to retain cations, water retention capacity, and other related factors (Lal 2006). As a result, these alterations can result in a rise in agricultural production (Diacono and Montemurro 2011). Moreover, organic methods can also supply substantial mineral-based nutrients to plants, which directly enhance crop yields through fertilization.

7.1.1 Organic Manures

The integrated application of different organic manures primarily gliricidia green leaf manure, composts, and vermicompost not only increase crop yield but also improve soil physical health (Babalad et al. 2009). Moreover, the nutrient requirement of crops has been fulfilled using liquid organic manures that ensure better nutrient use efficiency and counter the deficiency symptoms observed under an organic production system (Shwetha et al. 2009). As a legume crop, chickpeas obtain their larger nitrogen needs through biological nitrogen fixation, which can be aided by providing better soil chemical and physical conditions. Under the organic production system, the edaphic environment will be more promising for crop growth, and regular organic applications sustain its optimal level for greater time period. Studies have demonstrated that the organic production system can increase and sustain the productivity of the legume crop (Shwetha et al. 2009).

Foliar spray of liquid biofertilizers and soil application of organic manures had a better effect on growth, seed yield, and yield attributes of the targeted crop. The foliar application of panchagavya resulted in significantly increased plant height. The presence of sufficient amounts of N, P, and K, as well as additional amounts of the micronutrients Zn and Fe in vermicompost and FYM, provides favorable conditions for cell division, tissue growth, and improved plant growth through the development of a stronger and vigorous root system, thereby empowering plants to derive sufficient amounts of the nutrients available in the soil (Nekar et al. 2009; Deotale et al. 2011).

7.1.2 Plant Growth-Promoting Rhizobacteria

Actinomycetes, diazotrophic bacteria, mycorrhizal fungi, and rhizobia are useful soil microbes that live in symbiotic relationships with plant roots and guard plants from micronutrient deficiencies using different ways such as by producing certain plant growth hormones and promoting nutrient mineralization (Mekouar 2019). These microorganisms are naturally found in the soil; however, certain agricultural management practices or exogenous inoculation can increase their populations. A variety of different plant growth-promoting (PGP) soil microbes such as Bacillus, Enterobacter, and Pseudomonas can be used to enhance the bioavailability of micronutrients for crop plants. These microorganisms are mostly used in the form of seed inoculants, which promote growth of the targeted plants and induce resistance in them by producing different growth hormones, siderophores, chitinases, and antibiotics (Jha and Warkentin 2020). By producing siderophore compounds, plant growth-promoting (PGP) microorganisms avert the growth of pathogens, solubilize phosphorus, and chelate iron (Panhwar et al. 2012; Sreevidya et al. 2016), therefore playing an important role in iron fortification and fertility of the soil. PGP microorganisms are generally present in decomposing organic matter, compost, and soil and provide an ecologically compatible and economical method for improving environmental and soil health as well as increase crop production (Gopalakrishnan et al. 2016).

Several studies revealed that these microorganism-based inoculants via mycorrhizal associations increased the concentrations of Zn and Fe in different crop plants (Cavagnaro 2008; Brear et al. 2013; Jha and Warkentin 2020). Furthermore, the Acinetobacter, Bacillus, Enterobacter, and Pseudomonas species have been reported to ameliorate nitrogen fixation, plant growth, and yield of the grains in legumes including soybeans, peas, and chickpeas (Tokala et al. 2002; Valverde et al. 2007; Minorsky 2008; Soe et al. 2010; Gopalakrishnan et al. 2015). In chickpeas, PGP actinobacteria inoculation offered improved concentrations of seed minerals, that is, Zn (13%–30%) and Fe (10%–38%) as compared with uninoculated plants (Sathya et al. 2013). Likewise, field inoculation of these microorganisms and arbuscular mycorrhizal fungi enhanced the nutritional profile in chickpea grains along with protein content and yield by increasing the Zn and Fe concentrations (Pellegrino and Bedini 2014; Khalid et al. 2015; Gopalakrishnan et al. 2016).

Additionally, endophytic microorganisms can indirectly affect the regulation of metal transporters and contemplated as more promising agents to increase Zn and Fe uptake as well as their translocation within plant tissues (Reiter et al. 2002; Weyens et al. 2013). Rice and wheat grains have been biofortified with Zn and Fe using different fungal and bacterial endophytes (Ramesh et al. 2014; Abaid-Ullah et al. 2015).

7.1.3 Intercropping

Intercropping is an effective, economical, and environmentally friendly method, mostly adopted by small-scale farmers to overcome micronutrient deficiencies in crop plants (Gunes et al. 2007; Singh et al. 2016; Szerement et al. 2022). Achieving food security usually involves confirmation that everyone has access to adequate and nutritious food at all times, while also minimizing the environmental impact through the use of sustainable agriculture practices and maintaining a balance in the agroecosystem (Maitra and Ray 2019). Previous studies have demonstrated the benefits of intercropping in terms of enhanced crop quality, higher grain yield, increased protein content, and more efficient use of resources in a sustainable manner (Akhtar et al. 2013). Intercropping has also been shown to improve the iron and zinc content in grains, which can address the devastating consequences of micronutrient deficiencies on human health (Palmgren et al. 2008; Singh et al. 2016; De Valença et al. 2017). Moreover, Gunes et al. (2007) also found that intercropping of wheat and chickpea leads to higher levels of N, P, K, and Fe in wheat seeds and N, P, K, Fe, Zn, and Mn in chickpea seeds. Additionally, implementing multiple cropping systems can decrease the levels of anthropogenic disturbances associated with N and P contents while preserving soil fertility and lowering CO2 emissions from the cropping system (Soares et al. 2019), potentially reducing the costs associated with their mitigation efforts. This approach is considered a key component of climate-smart agriculture and provides a comparatively cost-effective and comprehensive solution for mitigating micronutrient deficiencies particularly in those regions that are susceptible to climate change (Bouis et al. 2017; Maqbool et al. 2020). Therefore, intercropping represents a natural solution that can address malnutrition and simultaneously achieve sustainable crop production with reduced inputs, lower cultivation costs, preservation of land for nature, and a more holistic management of ecosystems (Hu et al. 2018; Kiwia et al. 2019). Similarly, Zuo and Zhang (2009) also reported that intercropping dicot and monocot plants leads to increased Fe and Zn content in seeds through interspecific root interaction and changes in the rhizosphere.

The principles of complementarity and facilitation are the key ecological concepts mostly seen in different intercropping systems and often results in improved resource utilization efficiencies (Li et al. 2020). The complementarity technique usually involves the division of resources and reduced competition between species, while the facilitation system refers to a positive interaction in which one species positively impact the growth, reproduction, or survival of another species by altering the biotic or abiotic environment (Duchene et al. 2017; Li et al. 2020).

7.1.4 Biofertilizers

Biofertilizers are eco-friendly and cost-effective components of organic farming which play a vital role in sustaining soil health and improving crop productivity. Biofertilizers are made up of certain living cells belonging to different microorganisms and are applied either directly to the soil, plant parts, or crop seeds. The applied nutrients usually colonize interior section of the crop plants or rhizosphere and enhance plant growth by converting unavailable nutrients forms into their available forms. This is usually done through activation of various biological processes such as solubilization of fixed soil phosphate, nitrogen fixation, and synthesis of growth-promoting substances (Vessey 2003). The microorganisms also play their part in building up soil organic matter and restoring natural nutrient cycle. Majority of the biofertilizers are specific to major nutrients. However, the information regarding the role of biofertilizers on the availability of micronutrients is still limited in literature. Azolla, a floating fern usually fixes atmospheric N and enhances the availability of certain secondary micronutrients (Ca, Fe, Zn, Mn, Cu, B, Co, and Ni) in rice crop. It has been reported that presence of few microorganisms significantly improved the bioavailability of secondary micronutrients such as Fe—Thiobacillus ferroxidans and Ferrobacillus ferroxidans; Zn—Bacillus spp.; and S—Thiobacillus sulfoxidans and Beggiota (Kc et al. 2016).

7.1.5 Soil Amendments

The application of different soil amendments mainly alters soil pH, which ultimately improves soil chemical and physical properties and crop productivity. Crop productivity in acidic soils can be improved with application of limestone (Foy 1992; Kochian 1995). Moreover, sodic and saline-sodic soils can be amended to a great extent with application of sulphur and gypsum. These elements also lower the pH of alkaline soils and ultimately increase contents of plant-available Mn, Fe, Zn, Cu, and Co. Gypsum is also used to mitigate bicarbonates from soil solution and to exchange Ca for Na on the soil cation-exchange complex. The decline in soil pH and increased bioavailability of micronutrients can be achieved with the removal of excessive bicarbonates from soil solution (Singh et al. 1989). Application of gypsum not only improves soil physical properties but also lowers soil pH and ultimately enhances the mobility of micronutrients from soil to crop plants (Singh et al. 1989).

7.1.6 Plant Residues

In many arid and semi-arid regions across the world, most crop residue is either utilized as animal feed or burned as fuel, leaving little to no residue in the field (Timsina and Connor 2001). It is believed that developing countries produce an estimated yearly total of over 1000 million tons of cereal residue. In 1998, the major crops that globally generated residues, including soybean, rice, corn, wheat, potato, barley, and rapeseed produced a total of 2956 million tons of residues (Mekouar 2019).

Crop residues are seen as significant providers of a variety of micronutrients. For instance, every ton of rice and wheat produces removals of Zn, Fe, Mn, Cu, B, and Mo of 96, 777, 745, 42, 55, and 4 gha − 1, respectively. Based on a total crop residue production estimate of 105 million tons in India, and the micronutrient content of the residues, it is estimated that the potential of these residues is to provide approximately 35,400 tons amounts of micronutrients (Prasad 1999). The recycling of crop residues has the potential to enhance soil availability of micronutrients, as it is estimated that 50%–80% of the Zn, Cu, and Mn was taken up by rice and wheat crops can be recovered through the incorporation of the residues (Prasad and Sinha 1995).

The addition of crop residues to flooded soils enhances microbial metabolism, leading to an increase in soil solution Fe and Mn concentrations as a result of a significant change in redox potential (Katyal 1977; Yodkeaw and De Datta 1989; Atta et al. 1996). In soils rich in calcium, the breakdown of crop residue produces organic acids that can enhance plant zinc absorption by loosening zinc from its solid form in the soil and making it more soluble in the soil solution (Prasad and Sinha 1995). The chelating agents released during the decay of crop residues boost the total concentration of zinc that is available for diffusion and enhance the diffusion rate of zinc. For instance, the use of rice straw has been discovered to enhance the zinc content of rice plants, which may be due to the improvement of soil pH and reduction of exchangeable sodium levels (Singh et al. 2005). Despite this, the successful implementation of crop residue management techniques in agricultural systems necessitates a thorough comprehension of the influence of crop residues on the movement of nutrients from the soil and fertilizers, as well as their effects on soil properties, both chemical and physical, and the production of crops.

7.2 Phytohormones

The role of phytohormones, such as gibberellic acid and cytokinin, in mitigating metal stress is crucial (Al-Hakimi 2007; Gangwar et al. 2010; Masood et al. 2016). Studies have shown that certain plant hormones have an impact on the expression of Fe uptake genes, namely, IRT1 and FRO2. Research has shown that auxin has a positive effect on FRO2 gene expression under iron-deficient conditions (Chen et al. 2010), while ethylene positively regulates both IRT1 and FRO2 in Arabidopsis and cucumber plants (Lucena et al. 2006). Strengthening Fe-deficiency-inducible responses in plants can improve their ability to acquire more iron from soil with limited iron contents.

In recent years, plant physiologists have been trying to discover the signals that trigger root responses to iron deficiency and have identified various hormonal compounds as signaling agents (Mori and Nishizawa 1987; Hindt and Guerinot 2012; Ivanov et al. 2012). Among these signaling agents, nitric oxide (NO) (Graziano and Lamattina 2007), auxins (Chen et al. 2010), cytokinin (Séguéla et al. 2008), brassinosteroids (Wang et al. 2012), and ethylene are of prime importance (García et al. 2011). Furthermore, nitric oxide, auxins, and ethylene are particularly noteworthy agents as they can be produced by various soil microorganisms. This highlights the important and potential connections between soil microorganisms and plant ability to uptake Fe. Research has shown that auxins play a key role as a chemical signal, boosting the response to iron deficiency. The exogenous application of synthetic auxin either in the form of α-naphthaleneacetic acid or of IAA has significantly enhanced expression of FRO2 and IRT1 gene and Fe-deficiency-induced reduction of ferric Fe. Moreover, auxins are also involved in stimulating the growth of root hairs and expansion of lateral roots for uptake of iron (Chen et al. 2010; Wu et al. 2012). Soil microorganisms that produce auxin-like compounds can positively impact plant iron uptake in situations where limited iron is available (Jin et al. 2006).

7.3 Genetic Approaches

Classical breeding techniques, gene discovery, and marker-assisted breeding are utilized in biofortification of different crops to increase their mineral contents (Grusak 2002). According to Hindu et al. (2018), genome-wide association studies (GWAS) were employed to identify various genomic regions in maize plants that are associated with Zn and Fe biofortification in kernels. It was suggested that the genomic selection (GS) could be a promising breeding approach for biofortifying wheat with Fe and Zn (Velu et al. 2016). Additionally, researchers have highlighted that crop breeding with a focus on nutrition has several advantages in terms of sustainability. While breeding typically involves a long and repetitive process of hybridization and selection, modern advancements have enabled us to control signaling pathways. However, this process can be time-consuming and labor-intensive. In recent years, the use of modern molecular tools such as DNA markers and marker-assisted selection (MAS) has accelerated the development of nutrient-rich varieties. According to the studies of Kumar et al. (2018), QTLs for Fe and Zn biofortification in pearl millet were identified using a combination of DArT and SSR markers. Literature has shown that numerous rice, wheat, and maize varieties enriched in Fe and/or Zn have been released in different parts of the globe. Ramesh et al. (2004) innovated a new method for enhancing seed zinc and iron content by overexpressing a zinc transporter in Hordeum vulgare cv. Golden Promise using a ubiquitin promoter. A threefold increase was achieved in iron content in rice grains through the Agrobacterium-mediated transfer of the complete coding sequence of the ferritin gene from soybean plants (Goto et al. 1999). Similarly, Lucca et al. (2002) created transgenic rice plants with elevated iron content, enhanced in phytase and cysteine-peptides, for improved iron intake and bioavailability. Similarly, Vasconcelos et al. (2003) engineered the expression of the soybean ferritin gene using the glutelin promoter in an elite Indica rice line with desirable agronomic traits. This led to an improvement in the nutritional levels of both brown and polished rice grains. Liu et al. (2004) created rice varieties that contained ferritin and had 64% more iron content after milling. The ferritin gene was expressed specifically and at a high level in the endosperm of the transgenic rice.

7.4 Transgenic Approaches

In addition to plant breeding and genetic approaches, several transgenic interventions have proven successful in biofortification of food crops. With transgenic techniques, genes can be transferred between completely different species or new genes can be introduced into food or cash crops. This approach can be an effective solution for developing biofortified crops when there is limited or no variation in nutrient content among plant varieties (Brinch-Pedersen et al. 2007; Zhu et al. 2007). The transgenic approach for biofortification takes advantage of the unlimited genetic pool for transferring and expressing desirable genes between plant species, regardless of their evolutionary or taxonomic relationship. When a specific micronutrient is not naturally present in crops, transgenic techniques are the only practical option for fortifying the crops with that nutrient (Pérez-Massot et al. 2013). The development of transgenic crops becomes possible with identification and understanding of gene function and use of these genes to manipulate plant metabolism (Christou and Twyman 2004). Additionally, alternative metabolic pathways from bacteria and other organisms can also be introduced into crops through transgenic techniques (Newell-Mcgloughlin 2008). Developing biofortified varieties through genetic approaches can be challenging for breeders in soils that are naturally low in iron and zinc micronutrients. To fully reap the benefits of these varieties, it is also important to consider other factors such as soil pH and organic matter, which can impact root exudation and enzyme activity in the rhizosphere and therefore affect micronutrient uptake and its accumulation in crop plants (Cakmak 2008).

8 Conclusion

Hidden hunger or malnutrition is a global phenomenon affecting lives of millions of people in both developing and developed nations. The deficiencies in our food system need to be addressed to make human immune system stronger in fight against global pandemics such as COVID-19. One way to combat malnutrition is the adoption of diverse organic biofortification approaches. Biofortification is a cost-effective method of producing nutrient-rich food crops which are readily available for humans in their natural form. Organic biofortification involves use of different organic manures, plant growth-promoting rhizobacteria, intercropping, phytohormones, biofertilizers, soil amendments, crop residues, genetic approaches, and transgenic approaches. However, synergism of different organic biofortification techniques and public acceptance of biofortified crops will remain a major challenge. In this regard, an integrated approach from farmers, extension workers, politicians, dietitians, food and genetic engineers, and educators will be needed to successfully implement biofortification techniques at the global level.