Rice germplasm for high grain Zn content and tolerance of Zn deficient soils

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Zinc (Zn) is an essential nutrient in micro-quantities for all living organisms. Deficiencies limit crop production in many parts of the world, and Zn is often deficient in the diet of humans subsisting on staple-food crops, causing severe health problems. An important strategy for dealing with this is to breed crops that are efficient in taking up Zn and concentrating it in edible plant parts. Rice is one of the main crops being targeted because of its global importance and the prevalence of Zn deficiency in populations subsisting on rice. However rice is unusual in its Zn relations compared with other cereals in two respects. First, it is mainly grown in submerged soils, and because of the peculiar biogeochemistry of submerged soils, Zn deficiency in the crop is widespread, affecting up to 50% of rice soils globally. Second, as a result of inherent physiological differences, little Zn is remobilized from existing plant reserves to grains during the grain filling growth stages, as in other cereals, so that Zn uptake appears to be one of the main bottlenecks limiting rice grain Zn contents. Research has shown that grain Zn concentrations in rice - already low compared with other cereals or pulses - are further reduced in Zn deficient soils, and large fertilizer additions are needed to overcome this. Dietary and crop Zn deficiency are inevitably linked in areas with low Zn soils, as in most parts of Asia where rice is the staple. Enhancing the Zn uptake capacity of rice varieties will therefore be crucial to increasing grain contents. It will also be important to understand long-term sustainability of growing high grain Zn rice under inherently Zn-limited conditions, and what can be done to avoid problems in the future.Current research at the International Rice Research Institute (IRRI) is using classical plant breeding combined with molecular biological markers for useful plant traits to develop rice varieties with high grain Zn contents and improved yields on Zn-deficient soils. Research is also underway to enhance grain Zn through agronomic means, including fertilizer and water management. However progress in these activities, and in understanding long-term sustainability issues, is constrained by our poor understanding of the mechanisms underlying genotype differences, and of the dynamics of plant-available Zn in the soil within the growing season and longer term.In recent research by members of the project team, we have shown that three key mechanisms enhance growth of rice seedlings in Zn deficient soil: (a) secretion from roots of Zn-chelating compounds called phytosiderophores and subsequent uptake of chelated Zn in the rhizosphere, (b) maintenance of new root growth, and (c) prevention of root damage by oxygen radicals linked to high bicarbonate concentrations. Studies with a limited set of genotypes suggest that Zn loaded into grains mostly comes from Zn uptake during the reproductive stages rather than by re-translocation from vegetative tissue. The mechanisms listed above in relation to seedling growth may also assure adequate Zn uptake during the reproductive phase. However, this has not been systematically investigated so far, nor have any genes related to reproductive-stage Zn uptake been tagged.The proposed research addresses these knowledge gaps with an interdisciplinary approach linking fundamental research on soil biogeochemistry, molecular physiology and genetics with applied work on agronomy and plant breeding, with a conceptual framework provided by mathematical modelling. Our goal is to develop genotypes and management practices for growing high Zn rice in Zn deficient soils, suitable for resource-poor farmers. This will encompass agronomic interventions based on understanding of limiting factors for Zn uptake and translocation, and breeding approaches based on understanding of genetic factors controlling key tolerance mechanisms.