Root SAT-NAV: uncovering the molecular mechanisms guiding root angle in soil

粮食安全是一个重大的全球性问题。到2050年，农作物产量必须翻一番，才能跟上全球人口增长到90亿的步伐。考虑到气候变化对水资源供应的影响，以及减少化肥投入以使农业在环境上更具可持续性的目标，这一目标更具挑战性。在这两种情况下，开发提高水分和养分吸收效率的作物将对解决这一问题作出重大贡献。根系结构对养分和水分吸收效率有重要影响。例如，通过控制根系生长角度来更好地探索磷素积累的表层土壤，可以显著提高磷素吸收效率。尽管有这些知识，调控作物根角的基因仍有待鉴定。根角主要受向地性响应调节。然而，当遇到干燥的土壤时，水梯度等其他定向信号会诱导根系的应力激活取向导航（SAT-NAV）系统。尽管它们的重要性显而易见，但目前尚不清楚亲水性如何与向地性相互作用，使根系能够觅食水分。因此，了解根角的遗传和环境调控对作物改良具有重要意义。最近，我们与我们的合作者一起发现，根尖分别通过形成植物激素、生长素和ABA的梯度来响应重力或湿度的变化。重力刺激后，生长素在根的下侧积累，抑制细胞生长，导致根向重力方向弯曲。我们怀疑ABA以类似的方式引导根向水分生长。这些激素是如何引起这些变化的？我们已经确定了大约570个由生长素调控的导致根弯曲的基因。因此，生长素对根弯曲的影响是非常复杂的。为了帮助我们处理这种复杂性，我们将采用一种称为系统生物学的新方法，它汇集了生物学和数学的精华。这包括生成大量关于这些不同基因及其控制过程的实验信息，然后将这些信息整合到数学模型中。生长素通过诱导许多不同细胞和组织的反应来调节根弯曲。因此，我们的模型不仅要包括基因列表的信息，还要考虑它们在许多不同的根细胞和组织中的行为。然后，我们需要通过设计实验来测试其准确预测实际结果的能力，从而确定我们的根模型有多现实。然后，该模型可以用于测试想法，并提供关于生长素（或ABA）如何在基因，细胞和组织水平上控制根弯曲的新见解。检验我们在实验室的发现是否与土壤有关将是非常重要的。我们将利用无创成像技术的新进展，利用x射线微计算机断层扫描（CT）监测土壤中根系的生长，以此来解决这个重要问题。利用这种新的成像能力，我们将测试改变水稻根系向地向性和向水性对根系水分和养分吸收效率的影响。从这项研究中获得的知识将帮助科学家了解如何最好地操纵根角和提高作物产量。

Food security represents a major global issue. Crop production has to double by 2050 to keep pace with global population increasing to 9 billion. This target is even more challenging given the impact of climate change on water availability and the aim to reduce fertilizer inputs to make agriculture become more environmentally sustainable. In both cases, developing crops with improved water and nutrient uptake efficiency would contribute significantly to the solution. Root architecture critically influences nutrient and water uptake efficiency. For example, phosphate uptake efficiency could be significantly improved by manipulating root growth angle to better explore the topsoil where it accumulates. Despite this knowledge, the genes that regulate root angle in crops remain to be identified. Root angle is primarily regulated by the gravitropic response. However, other directional signals like water gradients induce roots' stress-activated-tropisms navigation (SAT-NAV) system when encountering drying soil. Despite their obvious importance, it is currently unclear how hydrotropism interacts with gravitropism to enable roots to forage for water. Understanding the genetic and environmental regulation of root angle is therefore of vital importance to crop improvement.Together with our collaborators, we have recently shown that root tips respond to changes in gravity or moisture by forming gradients of the plant hormones, auxin and ABA, respectively. After a gravity stimulus, auxin accumulates on the lower side of roots, inhibiting cell growth and causing roots to bend in the direction of gravity. We suspect that ABA functions in a similar way to direct root growth towards water. How do these hormones cause these changes? We have identified ~570 genes that auxin regulates to cause root bending. The effect of auxin on root bending is therefore very complicated. To help us deal with this complexity, we will employ a new approach termed Systems Biology that brings together the best of Biology and Maths. This involves generating a large body of experimental information about these different genes and the processes they control that is then integrated into mathematical models. Auxin regulates root bending by inducing responses in many different cells and tissues. Our model therefore has to include information not just about a list of genes but also consider their behaviour in many different root cells and tissues. We then need to determine how realistic our root model is, by designing experiments to test its ability to accurately predict real results. The model can then be used to test ideas and provide new insight about how auxin (or ABA) controls root bending at the gene, cell and tissue level. It will be very important to test whether our findings in the lab are relevant to soil. We will address this important question by exploiting new advances in non-invasive imaging to monitor root growth in soils using X-ray micro Computed Tomography (CT). Using this new imaging capability we will test the impact of altering root gravitropism and hydrotropism on root water and nutrient uptake efficiency in rice. The knowledge gained from this study will help scientists understand how best to manipulate root angle and enhance crop yield.