CROPNUT: increasing iron in cereals

铁和锌的缺乏是全球性的健康问题，目前通过补充剂和强化计划来解决。在发展中国家，铁补充剂是援助计划的一个组成部分，防治贫血是世界卫生组织的一个主要优先事项。在离家较近的地方，约5%的11 - 64岁女性普遍存在低血清铁水平（2012年全国饮食和营养调查），这与铁摄入量低有关。为了防治缺铁，在联合王国碾磨的所有面粉都用铁盐或铁粉进行化学强化，最高可达16.5 mg/kg（联合王国面粉条例，1998年）。现代面包小麦中铁的自然变化限制在白色面粉中6 - 13 mg/kg的范围内。施肥实验表明，多余的铁不会被植物吸收。因此，使用遗传方法增加铁和锌的水平，被称为生物强化，是最可持续的方法来增加我们的饮食中的矿物质水平，这是由几种主要作物为主。面包小麦中铁水平的有限变化归因于许多因素。几个世纪以来，作物品种的选择是以产量为代价，以微量营养素含量为代价。此外，小麦等多倍体作物在遗传上是缓冲的：可能改变某种性状的基因变异被同一基因的其他拷贝所掩盖，这使得很难选择新的性状。此外，植物对铁的水平进行严格的调节，以防止这种金属的过度积累，这种金属在其游离离子形式下是有毒的。最后但并非最不重要的一点是，谷物还没有进化到将大量矿物质放入淀粉质胚乳中，这是我们喜欢吃的谷物的一部分。在Balk和Uauy实验室之间非常成功的合作中，我们最近发现，与预期相反，小麦或大麦的白色面粉中的铁和锌含量可以分别增加3倍和2倍。元素分析结果表明，白色面粉铁含量为16 ~ 17 mg/kg，接近法律的强化要求，全麦面粉铁含量更高。这是通过顺式遗传学方法实现的：小麦植物经过遗传修饰，但序列来自小麦本身。我们将胚乳特异性调控序列置于小麦铁转运蛋白前。我们的研究结果表明，原则上，植物可以直接更多的铁和锌的胚乳比他们自然。此外，对增长似乎没有任何重大的负面影响。虽然液泡铁转运蛋白的及时过表达在提高谷物中的铁和锌水平方面效果显著，但我们还不清楚为什么这种策略如此成功。毕竟，上调的基因是一个简单的转运蛋白，而不是一个调节因子。如果我们用交通流量来类比，增加M25上的车道数量本身并不能改善交通流量。通道、路口等都需要扩阔，以增加交通量和防止挤塞。此外，我们发现转运蛋白是铁特异性的，不能转运锌。为什么锌含量会增加？为了利用我们的研究结果进行生物强化，我们将研究高铁小麦品系中矿物质转移增加的分子和细胞生物学变化。我们还将研究铁和锌的来源，例如，植物是否从土壤中吸收更多的铁，或者铁是否更有效地从植物的其他部分重新动员。然后，我们将利用这些信息开发非转基因策略，以增加小麦和其他谷物中的铁和锌。将通过向培养的肠细胞提供消化的白色面粉和面包来测试铁和锌的生物利用度。总之，这些研究将大大提高我们对营养物质运输的认识，为我们提供新的非转基因策略来提高小麦的营养质量，并为我们提供一种评估其对人类营养影响的方法。

Deficiencies in iron and zinc are global health issues, which are currently addressed by supplements and fortification programmes. In the developing world, iron supplements are an integral part of aid programmes and combatting anaemia is a major priority of the World Health Organization. Closer to home, low serum iron levels are prevalent in ~5% of females aged 11 - 64 (National Diet and Nutrition Survey 2012) correlating with low iron intake. To combat iron deficiency, all flours milled in the UK are chemically fortified with iron salts or iron powder up to 16.5 mg/kg (UK Flour Regulations 1998). The natural variation of iron in modern bread wheat is limited within the range of 6 - 13 mg/kg in white flour. Fertilization experiments have shown that extra iron is not taken up by plants. Therefore, increasing iron and zinc levels using genetic methods, known as biofortification, is the most sustainable approach to increase mineral levels in our diet which is dominated by a few staple crops.The limited variation in iron levels in bread wheat has been attributed to a number of factors. Over centuries, crop varieties have been selected for yield, at the cost of micronutrient content. Moreover, polyploid crops such as wheat are genetically buffered: gene variants that could change a certain trait are masked by other copies of the same gene, which makes it hard to select novel traits. In addition, iron levels are tightly regulated by plants to prevent over-accumulation of this metal that is toxic in its free ionic form. And last but not least, cereals have not evolved to put large amounts of minerals into the starchy endosperm, the part of the grain that we prefer to eat.In a very successful collaboration between the Balk and Uauy labs, we have recently found that, against expectations, iron and zinc levels in white flour from wheat or barley can be increased 3- and 2-fold, respectively. Element analysis showed that the iron levels of white flour were 16 - 17 mg/kg, similar to the legal requirement for fortification, and higher still in wholemeal flour. This was achieved by a cis-genics approach: wheat plants were genetically modified but the sequences are from wheat itself. We placed an endosperm-specific regulatory sequence in front of a wheat iron transporter. Our results show that, in principle, plants can direct much more iron and zinc to the endosperm than they do naturally. Moreover, there does not seem to be any major negative effect on growth. While the timely overexpression of the vacuolar iron transporter works remarkably well in boosting iron and zinc levels in grain, we do not yet understand why this strategy is so successful. After all, the up-regulated gene is a simple transporter and not a regulator. If we draw an analogy to traffic flow, increasing the number of lanes on the M25 does not per se improve traffic flow. Access roads, junctions and so on all need to be widened to increase traffic and prevent congestion. Also, we found that the transporter is specific for iron and cannot transport zinc. So why are zinc levels increased?To exploit our findings for biofortification, we will investigate the molecular and cell biological changes that underlie increased mineral transfer in the high-iron wheat line. We will also investigate what the source of iron and zinc is, for example if the plants take up more iron from the soil or whether the iron is more efficiently remobilized from other parts of the plant. We will then use this information to develop non-GM strategies to increase iron and zinc in wheat and other cereals. The bioavailability of the iron and zinc will be tested by offering digested white flour and bread to cultured intestinal cells. Taken together, these studies will greatly enhance our knowledge on nutrient transport, provide us with novel and non-GM strategies to increase the nutritional quality of wheat and give us a way to assess their impact on human nutrition.