Zinc deficiency is a well-documented problem in food crops, causing decreased crop yields and nutritional quality. Generally, the regions in the world with Zn-deficient soils are also characterized by widespread Zn deficiency in humans. Recent estimates indicate that nearly half of world population suffers from Zn deficiency. Cereal crops play an important role in satisfying daily calorie intake in developing world, but they are inherently very low in Zn concentrations in grain, particularly when grown on Zn-deficient soils. The reliance on cereal-based diets may induce Zn deficiency-related health problems in humans, such as impairments in physical development, immune system and brain function. Among the strategies being discussed as major solution to Zn deficiency, plant breeding strategy (e.g., genetic biofortification) appears to be a most sustainable and cost-effective approach useful in improving Zn concentrations in grain. The breeding approach is, however, a long-term process requiring a substantial effort and resources. A successful breeding program for biofortifying food crops with Zn is very much dependent on the size of plant-available Zn pools in soil. In most parts of the cereal-growing areas, soils have, however, a variety of chemical and physical problems that significantly reduce availability of Zn to plant roots. Hence, the genetic capacity of the newly developed (biofortified) cultivars to absorb sufficient amount of Zn from soil and accumulate it in the grain may not be expressed to the full extent. It is, therefore, essential to have a short-term approach to improve Zn concentration in cereal grains. Application of Zn fertilizers or Zn-enriched NPK fertilizers (e.g., agronomic biofortification) offers a rapid solution to the problem, and represents useful complementary approach to on-going breeding programs. There is increasing evidence showing that foliar or combined soil+foliar application of Zn fertilizers under field conditions are highly effective and very practical way to maximize uptake and accumulation of Zn in whole wheat grain, raising concentration up to 60 mg Zn kg−1. Zinc-enriched grains are also of great importance for crop productivity resulting in better seedling vigor, denser stands and higher stress tolerance on potentially Zn-deficient soils. Agronomic biofortification strategy appears to be essential in keeping sufficient amount of available Zn in soil solution and maintaining adequate Zn transport to the seeds during reproductive growth stage. Finally, agronomic biofortification is required for optimizing and ensuring the success of genetic biofortification of cereal grains with Zn. In case of greater bioavailability of the grain Zn derived from foliar applications than from soil, agronomic biofortification would be a very attractive and useful strategy in solving Zn deficiency-related health problems globally and effectively.

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Enrichment of cereal grains with zinc: Agronomic or genetic biofortification?

Summary
The diets of over two-thirds of the world's population lack one or more essential mineral elements. This can be remedied through dietary diversification, mineral supplementation, food fortification, or increasing the concentrations and/or bioavailability of mineral elements in produce (biofortification). This article reviews aspects of soil science, plant physiology and genetics underpinning crop biofortification strategies, as well as agronomic and genetic approaches currently taken to biofortify food crops with the mineral elements most commonly lacking in human diets: iron (Fe), zinc (Zn), copper (Cu), calcium (Ca), magnesium (Mg), iodine (I) and selenium (Se). Two complementary approaches have been successfully adopted to increase the concentrations of bioavailable mineral elements in food crops. First, agronomic approaches optimizing the application of mineral fertilizers and/or improving the solubilization and mobilization of mineral elements in the soil have been implemented. Secondly, crops have been developed with: increased abilities to acquire mineral elements and accumulate them in edible tissues; increased concentrations of ‘promoter’ substances, such as ascorbate, β-carotene and cysteine-rich polypeptides which stimulate the absorption of essential mineral elements by the gut; and reduced concentrations of ‘antinutrients’, such as oxalate, polyphenolics or phytate, which interfere with their absorption. These approaches are addressing mineral malnutrition in humans globally.

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

Nutritional iron deficiency

https://www.itqb.unl.pt/~saibo/projectos2012/AssuncaoRef5.pdf

The ZIP family of metal transporters.

The OsNAC6 gene is a member of the NAC transcription factor gene family in rice. Expression of OsNAC6 is induced by abiotic stresses, including cold, drought and high salinity. OsNAC6 gene expression is also induced by wounding and blast disease. A transactivation assay using a yeast system demonstrated that OsNAC6 functions as a transcriptional activator, and transient localization studies with OsNAC6-sGFP fusion protein revealed its nuclear localization. Transgenic rice plants over-expressing OsNAC6 constitutively exhibited growth retardation and low reproductive yields. These transgenic rice plants showed an improved tolerance to dehydration and high-salt stresses, and also exhibited increased tolerance to blast disease. By utilizing stress-inducible promoters, such as the OsNAC6 promoter, it is hoped that stress-inducible over-expression of OsNAC6 in rice can improve stress tolerance by suppressing the negative effects of OsNAC6 on growth under normal growth conditions. The results of microarray analysis revealed that many genes that are inducible by abiotic and biotic stresses were upregulated in rice plants over-expressing OsNAC6. A transient transactivation assay showed that OsNAC6 activates the expression of at least two genes, including a gene encoding peroxidase. Collectively, these results indicate that OsNAC6 functions as a transcriptional activator in response to abiotic and biotic stresses in plants. We conclude that OsNAC6 may serve as a useful biotechnological tool for the improvement of stress tolerance in various kinds of plants.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-313X.2007.03168.x

Functional analysis of a NAC-type transcription factor OsNAC6 involved in abiotic and biotic stress-responsive gene expression in rice

To improve the iron content of rice, we have transferred the entire coding sequence of the soybean ferritin gene into Oryza sativa (L. cv. Kita-ake) by Agrobacterium-mediated transformation. The rice seed-storage protein glutelin promoter, GluB-1, was used to drive expression of the soybean gene specifically in developing, self-pollinated seeds (T1 seeds) of transgenic plants, as confirmed by reverse transcription PCR analysis. Stable accumulation of the ferritin subunit in the rice seed was demonstrated by western blot analysis, and its specific accumulation in the endosperm by immunologic tissue printing. The iron content of T1 seeds was as much as threefold greater than that of their untransformed counterparts.

Iron fortification of rice seed by the soybean ferritin gene.

Sapovirus (SaV) infections are a public health problem because they cause acute gastroenteritis in humans of all ages both as outbreaks and in sporadic cases. However, publicly-available SaV sequence information, especially whole genome sequences for all the SaV genotypes, is still limited. Therefore, in this study, we determined the full/near-full-length genomic sequences of 138 SaVs from the 2001-2015 seasons in 13 prefectures across Japan. The GI genogroup was predominant (67%, n = 92), followed by GII (18%, n = 25), GIV (9%, n = 12), and GV (6%, n = 9). Within the GI genogroup, four different genotypes were identified, as follows: GI.1 (n=44), GI.2 (n=40), GI.3 (n=7), and GI.5 (n=1). We then compared these Japanese SaV sequences with a total of 3119 public human SaV sequences from 49 countries over the last 46 years. The results indicated that GI.1 and GI.2 have been the predominant genotypes in Japan as well as in other countries over at least four decades. Together with public SaV sequences, the 138 newly determined Japanese SaV sequences could facilitate a better understanding of the evolutionary patterns of SaV genotypes.

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Naoki Shigemoto

Papers

Iron fortification of rice seed by the soybean ferritin gene.

Distribution of Human Sapovirus Strain Genotypes over the last four Decades in Japan: a Global Perspective.