Sounding the all clear: investigating how and why plant cells deplete the stress hormone ABA

干旱胁迫是影响全球作物生产的主要环境胁迫，但对干旱和其他胁迫的耐受性是一种植物性状，在不同品种和作物物种之间差异很大。植物对胁迫条件的耐受性是许多过程协调作用的结果，因此合理地提高作物的抗逆性需要对植物胁迫生物学有详细而复杂的了解。植物激素脱落酸（ABA）在控制植物对干旱等环境胁迫的反应中起着关键作用。ABA也是植物生长发育的重要调节剂，调控种子休眠、根系生长等过程。为了适应不断变化的环境条件，植物必须不断调整ABA水平。众所周知，在应激条件下，ABA在细胞中积累是一个暂时的事件，随后是ABA的消耗，保持ABA反应的严格时间调节。虽然已知三种生化活动——分解、转化为储存形式和从细胞中输出——会降低细胞中的ABA水平，但对这些活动如何联合决定植物细胞中ABA耗竭率的定量理解仍然难以捉摸。更深入地了解这些机制以及它们是如何被控制的，对于提高作物调节其发育以适应其环境的能力非常重要。提出的研究项目旨在揭示根细胞中的动态ABA模式，并了解这些动态模式如何与ABA依赖的根发育相关。该知识将扩展到确定决定根系发育如何响应环境硝酸盐可用性的机制。生物传感器通过直接与细胞内的ABA结合来报告ABA的浓度，已被用于测量分子生物学研究参考植物拟南芥的动态ABA模式。利用表达ABA浓度和摄取传感器（ABACUS）的拟南芥根系生长的时间过程显微成像，我们已经观察到ABA耗耗率在空间和时间上的变化。我们现在的目标是了解几种生化活动如何结合起来阐明ABA水平，形成适合特定环境条件的动态模式。我们将利用受ABA消耗活动影响的拟南芥突变体详细研究ABA的时空消耗，从而揭示每种活动对ABA耗损率的影响。这些突变体的根生长表型将与ABA水平同时检测。将ABA耗竭率与相应的根生长表型联系起来，将为ABA如何影响植物发育提供详细的假设。例如，ABA被认为在调节根结构以适应环境中的硝酸盐水平方面发挥作用。研究人员将对根细胞中对硝酸盐反应的ABACUS进行成像，以确定控制根硝酸盐反应的ABA积累的特定细胞类型和时间。这种类型的详细知识可以指导对作物进行有针对性的干预，以提高农业对环境压力的适应能力。该项目的另一个主要目标是设计下一代ABACUS传感器，以提高根和其他植物组织中ABA动态模式的高灵敏度可视化。此外，一种新的生物传感器-脱落酸-葡萄糖酯传感器（SAGE）将被开发出来，作为解决ABA-葡萄糖酯池（ABA的非活性“储存”形式）在发育和环境反应中何时何地重要的问题的有力工具。扩大ABA时空格局的知识对于理解环境胁迫下生理和发育调节的机制至关重要，这些机制可以防止重大作物损失。

Drought stress is a major environmental stress that impairs crop production worldwide, but tolerance to drought and other stresses is a plant trait that varies greatly among cultivars and crop species. Tolerance of a plant to a stress condition is the result of coordinated action of many processes and thus rational improvement of crop stress tolerance will require a detailed and sophisticated understanding of plant stress biology. The plant hormone abscisic acid (ABA) plays a key role in controlling responses to environmental stress conditions like drought stress. ABA is also an important regulator in plant growth and development regulating processes such as seed dormancy and root growth. Plants have to adjust ABA levels constantly in order to match physiology and development to ever changing environmental conditions. It is known that ABA accumulated in cells during a stress condition is a temporary event that is followed by ABA depletion, maintaining a tight temporal regulation of ABA responses. Although three biochemical activities - breakdown, conversion to a storage form, and export from the cell - are known to lower cellular ABA levels, a quantitative understanding of how these combine to determine ABA depletion rates in plant cells remains elusive. A deeper understanding of these mechanisms and how they are controlled is important in improving the ability of crop plants to tune their development to suit their environment.The proposed research project aims to uncover dynamic ABA patterns in root cells and to understand how these dynamic patterns relate to ABA-dependent root development. The knowledge will be expanded to identify the mechanisms determining how root development responds to environmental nitrate availability. Biosensors that report concentrations of ABA by directly binding to ABA in cells have been used to measure dynamic ABA patterns in Arabidopsis thaliana, a reference plant for molecular biology research. Using time-course, microscopic imaging of growing Arabidopsis roots expressing ABA concentration and uptake sensors (ABACUS), we have already observed that ABA depletion rates vary in space and time. We now aim to understand how several biochemical activities combine to articulate ABA levels into dynamic patterns appropriate for a given environmental condition. Spatial and temporal depletion of ABA will be studied in detail using Arabidopsis mutants that are affected in ABA depletion activities, thus revealing the impact of each activity on ABA depletion rates. Root growth phenotypes of these mutants will be examined concurrently with ABA levels. Linking the maps of ABA depletion rates to corresponding root growth phenotypes will provide detailed hypotheses regarding how ABA impacts plant development. For example, ABA is thought to play a role in attuning root architecture to the levels of nitrate in the environment. Imaging ABACUS in root cells responding to nitrate availability will be carried out to pinpoint the specific cell-type and timing of ABA accumulations that control root nitrate responses. This type of detailed knowledge can then guide targeted interventions into crop plants to improve agricultural resilience to environmental stress. Another main objective of the project is to engineer next generation ABACUS sensors for improved high sensitivity visualisation of ABA dynamic patterning in roots and other plant tissues. In addition, a new biosensor - Sensor of Abscisic Acid-Glucose Ester (SAGE) - will be developed as a powerful tool to address the question of where and when ABA-glucose ester pools, inactive 'storage' forms of ABA, are important in development and environmental responses. Broadening the knowledge of spatio-temporal patterning of ABA will be important in understanding the mechanisms underlying physiological and developmental adjustments during environmental stresses that can prevent significant crop losses.