ABA transport at the nexus of nutrient deficiency and water stress in plants

农业粮食生产严重依赖矿物质肥料。迄今为止，全球仅磷肥的年使用量就达4300万吨。欧盟委员会已将磷列为“关键原材料”，而英国则100%依赖进口。再加上对环境和人类健康的负面影响，目前的磷使用率显然是不可持续的。因此，开发种植磷投入较少的作物的方法对国家和全球粮食安全至关重要。由于它们久坐不动的生活方式，植物是风险规避的，当它们感知到环境挑战时，它们会为最坏的情况做准备。它们有检测到磷供应减少并立即做出反应的机制。对高亲和力运输和根分枝的上调加强了土壤的“开采”，而下调生长和能源消耗则保护了内部资源。如果我们能够推迟后者，优化前者，我们就有机会缩小表观收益率和潜在收益率之间的差距。然而，为了通过作物育种或基因组编辑将这些想法付诸实践，我们需要对支撑植物对磷缺乏的早期反应的分子信号通路有一个精确的了解。考虑到缺水和气候变化，我们还需要知道这些途径是否与那些介导干旱或盐入侵造成的渗透胁迫反应的途径相互作用。我们最近发现，在拟南芥中，一种名为NPF4.2的基因被敲除，在低磷条件下完全取消了早期对主根的抑制。NPF4.2编码脱落酸(ABA)的转运体，位于根的中央维管系统的细胞的液泡膜上。虽然npf4.2突变体的根在低磷条件下继续生长，但它们仍然可以受到其他营养缺乏或盐的抑制。这些发现不仅突出了“应激激素”ABA在磷缺乏反应中的全新作用，也指出了ABA转运蛋白在赋予该途径特异性方面的新作用。这个项目的目的是精确地定位抑制根生长的信号通路在低磷和渗透胁迫下的差异和趋同，并定位NPF4.2在这个网络中的位置。为此，我们将利用A.thaliana可用的先进工具。我们将利用最近开发的体内ABA成像和单细胞转录技术，以及反向遗传学和蛋白质生物化学。拟议的工作方案包括三个部分。工作包1将提供根中应激诱导的ABA信号的空间地图，其中将覆盖相关信号的响应模式，如钙离子、ROS和pH，以及空间根转录。工作包2将告诉我们NP4.2如何与其他ABA转运体和动员ABA储存形式的酶合作形成信号特征。这一工作包还将确定NPF4.2与先前已确定的低磷信号通路成分之间的关系，如铁氧合酶和CLE多肽。最后一个工作包将提供有关NPF4.2蛋白如何调控的信息。候选靶标将从转录组学研究中选择，特别强调与低磷诱导成员CIPK和CBL基因家族的相互作用。已知CBL/CIPK调节子以钙离子依赖的方式激活膜转运蛋白，从而实现对营养和盐胁迫的反应。然而，在调节ABA运输方面的作用将是全新的。预期的结果将为开发将改善资源利用与抵御非生物胁迫的稳健性相结合的“智能”作物提供基础科学基础。我们将确定信号通路中的关键点，使我们能够分离或连接不同的信号输入和响应输出。因此，这项研究将为不同环境情景下的精准农业提供新的机遇。

Agricultural food production heavily relies on mineral fertilization. To date, the global annual use of phosphorous (P) fertilizer alone stands at 43 Mio tons. The European Commission has classified P as a 'critical raw material' and the UK is 100% reliant on import. Adding the negative impacts on environment and human health, the current rate of P use is clearly not sustainable. Developing ways to grow crops with less P input is therefore paramount for national and global food security. Due to their sedentary lifestyle, plants are risk-adverse and prepare for the worst-case scenario when they perceive environmental challenges. They have mechanisms to detect a decrease in P supply and immediately react. Up-regulation of high-affinity transport and root branching enhances soil 'mining' while down-regulation of growth and energy consumption safeguards internal resources.. If we can delay the latter and optimize the former, we have an opportunity to close the gap between apparent and potential yield. However, to put these ideas into practice through crop breeding or genome editing, we need a precise understanding of molecular signalling pathways that underpin early responses of plants to P deficiency. Considering water shortage and climate change, we also need to know whether these pathways interact with those mediating responses to osmotic stress imposed by drought or salt intrusion.We recently discovered that knockout of a gene called NPF4.2 in Arabidopsis thaliana completely abolishes early main root inhibition in low P. NPF4.2 encodes a transporter for abscisic acid (ABA) and is located on the vacuolar membrane of cells located in the central vasculature of the root. While the roots of npf4.2 mutants continue to grow in low P they can still be inhibited by other nutrient deficiencies or by salt. These findings not only highlight an entirely new role of the 'stress hormone' ABA for P-deficiency responses but also point to new role of ABA transporters for endowing the pathway with specificity. The aim of this project is to precisely map differences and convergence of the signalling pathways that inhibit root growth in response to low-P and osmotic stress and to position NPF4.2 in this network. To this end we will take advantage of the advanced tools available for A. thaliana. We will employ recently developed technology for in-vivo ABA-imaging and single-cell transcriptomics alongside reverse genetics and protein biochemistry. The proposed work programme has three parts. Work package 1 will deliver spatial maps of stress-evoked ABA signatures in roots, which will be overlaid with response patterns of related signals such as Ca2+, ROS and pH, and with spatial root transcriptomes. Work package 2 will tell us how NP4.2 shapes the signal signatures, in collaboration with other ABA-transporters and with enzymes that mobilize ABA-storage forms. This work package will also identify the relationship between NPF4.2 and previously identified components of the low-P signalling pathway such as ferroxidases and CLE peptides. The last work package will produce information on how the NPF4.2 protein is regulated. Candidate targets will be selected from the transcriptomics studies with particular emphasis on interactions with low-P induced members CIPK and CBL gene families. CBL/CIPK regulons are already known for activating membrane transporters in a Ca2+-dependent manner thereby effectuating responses nutrient and salt stress. However, a role in regulating ABA transport would be entirely novel.The expected outcomes will provide a fundamental science base for the development of 'smart' crops combining improved resource use with robustness against abiotic stress. We will identify key points in the signalling pathways that will allow us to de-couple or connect different signal inputs and response outputs. This research will therefore open offers new opportunities for precision agriculture in different environment scenarios.