Deciphering the Regulation of Abscisic Acid Transport and Homeostasis under Water Deficit in Arabidopsis.

• Applicant: Dr. Rainer Waadt
• Subject Area: Plant Physiology ; Plant Cell and Developmental Biology
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 507930987

One of the most pressing challenges of our time is climate change, which negatively affects the plant water status, leading to yield losses and threatening food security. Plant adaptations to water deficit, caused by drought and hyperosmotic stress, including salt stress, are regulated by sophisticated sensing, and signaling mechanisms, in which the plant hormone abscisic acid (ABA) and the second messenger Ca2+ play central roles. Research using the model plant Arabidopsis thaliana (Arabidopsis) revealed that water deficit in roots triggers long-distance signals to induce ABA biosynthesis in leaf vascular tissues. From there, ABA is transported to guard cells to induce stomatal closure for limiting water transpiration, and to roots to inhibit lateral root development and to mediate hydrotropic root growth towards a water gradient. Although several ABA transporters have been identified, the mechanisms that mediate and regulate ABA transport to root tissues remain to be explored. Also, the coordination of long-distance ABA transport with intracellular ABA storage and release mechanisms remains to be defined.In my previous work I have developed advanced fluorescent biosensors that allow the in vivo monitoring of ABA and Ca2+ dynamics on a cellular-, tissue- and organismic scale. By using these tools, I have discovered ABA concentration gradients in Arabidopsis, and observed ABA transport processes. Preliminary biosensorics analyses provide evidence for shoot-to-root ABA transport, and that Ca2+-signaling can induce the elevation of ABA in roots. Furthermore, simultaneous ABA and Ca2+ imaging analyses revealed rapid cytosolic Ca2+ elevations and a slow accumulation of ABA in roots in response to hyperosmotic stress. How ABA transport and homeostasis is regulated by Ca2+ signaling mechanisms is one of my central questions. To explore this on the cellular level, I have established an ABA biosensor-based assay, that allows the characterization of ABA transporters using a defined heterologous environment. By mining public gene expression data, I have also discovered the ABA transporters that most likely contribute to water deficit responses in roots.My long-term goal is to delineate how plants regulate the transport, biosynthesis, and intracellular release of ABA for mediating cellular-, tissue-, and organismic responses to water deficit. For achieving this goal, I am proposing here the following objectives:1) Define the ABA transport and intracellular release mechanisms that contribute to water deficit responses in roots.2) Define the contributions of Ca2+- and CLE25 peptide signaling mechanisms to the induction of ABA biosynthesis in leaves and downstream ABA responses in roots.By addressing these objectives, I aim to identify new genetic components and discover fundamental mechanistic principles that mediate plant adaptations to water deficit in roots, which will guide future strategies to protect and grow plants in drylands.

• DFG Programme: Research Grants