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

锌是所有生物必需的微量营养物质。锌的缺乏限制了世界许多地区的农作物产量，而以主食作物为生的人的饮食中往往缺乏锌，导致严重的健康问题。解决这一问题的一个重要战略是培育能够有效吸收锌并将其集中在可食用植物部分的作物。水稻是主要的目标作物之一，因为它具有全球重要性，而且在以水稻为生的人口中普遍存在缺锌。然而，与其他谷物相比，大米与锌的关系在两个方面都是不寻常的。首先，它主要生长在沉水土壤中，由于沉水土壤特殊的生物地球化学特性，作物中普遍存在缺锌现象，全球高达50%的水稻土壤受到影响。其次，由于内在的生理差异，与其他谷物一样，在灌浆生长阶段，很少有锌从现有的植物储备转移到籽粒中，因此锌的吸收似乎是限制水稻籽粒锌含量的主要瓶颈之一。研究表明，与其他谷物或豆类相比，水稻中的谷物锌浓度本来就很低，但在缺锌土壤中，这种浓度会进一步降低，需要大量增加化肥来克服这一点。在低锌土壤的地区，膳食缺锌和作物缺锌不可避免地联系在一起，就像在以大米为主食的亚洲大多数地区一样。因此，提高水稻品种对锌的吸收能力将是提高籽粒含锌量的关键。了解在内在缺锌条件下种植高粒锌水稻的长期可持续性以及如何避免未来出现的问题也是非常重要的。目前，国际水稻研究所(IRRI)的研究正在利用经典植物育种和有用植株性状的分子生物学标记来培育在缺锌土壤上具有高籽粒锌含量和提高产量的水稻品种。通过包括肥料和水管理在内的农艺手段来提高粮食锌的研究也在进行中。然而，这些活动的进展以及在理解长期可持续性问题方面的进展，受到了我们对潜在的基因型差异的机制以及生长季和长期内土壤中植物有效锌的动态的了解的限制。在项目组成员最近的研究中，我们表明，在缺锌土壤中，促进水稻幼苗生长的三个关键机制：(A)称为植物铁载体的锌螯合化合物的分泌和随后在根际吸收螯合锌，(B)维持新的根生长，以及(C)防止与高碳酸氢盐浓度有关的氧自由基对根的损害。对一组有限的基因类型的研究表明，籽粒中的锌主要来自生殖阶段的锌吸收，而不是营养组织中的重新运输。上述与幼苗生长有关的机制也可能确保生殖阶段对锌的充分吸收。然而，到目前为止，这方面的研究还没有得到系统的研究，也没有任何与生殖阶段锌吸收有关的基因被标记。拟议的研究通过一种跨学科的方法来解决这些知识差距，该方法将土壤生物地球化学、分子生理学和遗传学的基础研究与农学和植物育种的应用工作结合起来，并通过数学模型提供了一个概念框架。我们的目标是开发适合资源贫乏的农民在缺锌土壤上种植高锌水稻的基因型和管理实践。这将包括基于对锌吸收和转运限制因素的了解的农艺干预，以及基于对控制关键耐性机制的遗传因素的了解的育种方法。

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.