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

锌（Zn）是所有生物体微量必需的营养素。在世界上许多地方，缺锌限制了作物产量，而以主食作物为生的人类的饮食中往往缺乏锌，导致严重的健康问题。处理这一问题的一个重要策略是培育能够有效吸收锌并将其集中在可食用植物部位的作物。由于水稻的全球重要性和以水稻为生的人群中锌缺乏症的普遍性，水稻是主要的目标作物之一。然而，与其他谷物相比，水稻在两个方面的锌关系不寻常。首先，它主要生长在沉水土壤中，由于沉水土壤特有的土壤地球化学，作物中的锌缺乏是普遍的，影响全球高达50%的水稻土壤。第二，由于内在的生理差异，很少有锌从现有的植物储备在灌浆生长阶段，在籽粒中，在其他谷物，使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已经很低，但在缺锌土壤中会进一步降低，需要大量的肥料添加来克服这一点。在低锌土壤的地区，如亚洲大部分地区以大米为主食，饮食和作物缺锌是不可避免的。因此，提高水稻品种的锌吸收能力将是提高籽粒锌含量的关键。这也将是重要的，以了解长期可持续性的高籽粒锌水稻在固有的锌有限的条件下，可以做些什么，以避免在未来的problems.Current的研究在国际水稻研究所（IRRI）是使用经典的植物育种结合分子生物学标记有用的植物性状，以开发水稻品种与高籽粒锌含量和提高产量的锌缺乏的土壤。通过农艺手段，包括肥料和水管理，提高粮食锌含量的研究也在进行中。然而，这些活动的进展，以及对长期可持续性问题的理解，受到我们对基因型差异的机制以及生长季节和长期土壤中植物有效锌动态的理解不足的限制。在项目团队成员最近的研究中，我们已经表明，三种关键机制可以促进缺锌土壤中水稻幼苗的生长：（a）从根分泌称为植物铁载体的锌螯合化合物，随后在根际吸收螯合的锌，（B）维持新根生长，和（c）防止与高碳酸氢盐浓度相关的氧自由基对根的损害。有限的一组基因型的研究表明，锌加载到籽粒中主要来自锌的吸收在生殖阶段，而不是从营养组织的再转运。上述与幼苗生长有关的机制也可以确保生殖阶段有足够的锌吸收。然而，这还没有被系统地调查到目前为止，也没有任何基因相关的生殖阶段锌的吸收被tagged.The拟议的研究解决这些知识差距与跨学科的方法连接土壤地球化学，分子生理学和遗传学的基础研究与应用工作农艺学和植物育种，与数学建模提供的概念框架。我们的目标是发展基因型和管理措施，在缺锌土壤中种植高锌水稻，适合资源贫乏的农民。这将包括基于对锌吸收和转运的限制因素的理解的农艺干预，以及基于对控制关键耐受机制的遗传因素的理解的育种方法。

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.