Zinc biofortification of maize (Zea mays L.): Status and challenges

Abstract

Zn deficiency is one of the leading health problems in children and women of developing countries. Different interventions could be used to overcome malnutrition, but biofortification is most impactful, convenient, sustainable and acceptable intervention. Maize is one of the major crops grown and consumed in the regions with prevalent Zn malnutrition; therefore, this is suitable target for Zn biofortification. Zn biofortification of maize could be achieved through agronomic and genetic approaches. Discussion of agronomic approaches with genetic approaches is prerequisite because soils in developing countries are deficit of Zn and availability of Zn in soils is mandatory for estimating the genetic responses of maize genotypes through genetic approaches. Seed priming, foliar and soil applications are agronomic tools for biofortification, but solo and combined applications of these treatments have different effects on Zn enrichment. Genetic approaches include the increase of Zn bioavailability or increase of kernel Zn concentration. Zn bioavailability could be increased by reducing the anti‐nutritional factors or by increasing the bioavailability enhancers. Kernel Zn concentration could be improved through hybridization and selections, whereas genetically engineered attempts for improving Zn uptake from soil, loading in xylem, remobilization in grains and sequestration in endosperm can further improve the kernel Zn concentration. Key challenges associated with dissemination of Zn biofortified maize are also under discussion in this draft. Current review emphasized all of above‐mentioned contents to provide roadmap for the development of Zn biofortified maize genotypes to curb the global Zn malnutrition.

Foliar Zn Spraying Simultaneously Improved Concentrations and Bioavailability of Zn and Fe in Maize Grains Irrespective of Foliar Sucrose Supply

“…However, these studies did not clearly distinguish between the effects of soil Zn application and foliar Zn spraying. Soil and foliar applications have been reported to have differential effects on Zn concentrations of maize kernels [60]. Our study revealed that effects of various foliar treatments on maize grain Fe accumulation and bioavailability were similar to those on Zn.…”

“…Therefore, foliar Zn spraying should be adopted as an effective way to biofortify maize with Zn. However, to overcome Zn malnutrition, a Zn concentration of more than 37 mg/kg in the whole grain [60] and an increase of 30 mg/kg in the endosperm (www.harvestplus.org) of maize (because the major proportion of the maize grain is comprised of endosperm) are recommended. In terms of Zn bioavailability, the critical molar ratio of PA/Zn is 15-20; a value of 5-15 represents about 30-35% Zn availability, and ≥15 represents about 15% Zn availability [42].…”

“…In terms of Zn bioavailability, the critical molar ratio of PA/Zn is 15-20; a value of 5-15 represents about 30-35% Zn availability, and ≥15 represents about 15% Zn availability [42]. The effectiveness of foliar Zn spraying on grain Zn concentrations is related to the spray timing, location, rates, type of fertilizers (e.g., nano-particles), maize genotypes, and environmental conditions [60]. Based on the low maize grain Zn concentrations (<37 mg/kg) and high PA/Zn values (> 15) achieved by foliar spraying of 0.2% or 0.3% ZnSO 4 ·7H 2 O (w/v) in our present study (Tables 3 and 5), there is still a need to further enhance Zn absorption by maize, its translocation to grains, and its bioavailability, possibly by appropriately increasing the spraying frequency and Zn concentration and spraying with other beneficial fertilizers (e.g., urea) or stimulators [51].…”

“…This result suggests that this process can successfully achieve simultaneous Zn and Fe biofortification. In case of foliar Zn applications, genetic mechanisms regulating Zn absorption by maize leaves and translocation to grains are still unidentified [60]. It has also been reported that the foliar application of Zn not only increases the transport of shoot Fe to grains but also reduces cadmium toxicity in maize and other cereals [69].…”

Current Status and Future Perspectives of Biofortification in Wheat

“…Analyses of multiple grain samples from maize and wheat that have been collected by CIMMYT across environments and genotypes suggest that the levels of Fe and Zn are higher in wheat than in maize. However, we are not aware of any specific studies comparing wheat and maize mineral concentrations in the same experiment and in different grain tissues [43,44,45,20].…”