ABA-dependent control of plant hydraulics in acclimation to water deficit

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Water plays a crucial role in terrestrial ecosystems with plants acting as a major factor in water cycling. By dominating the water flux from the soil to atmosphere, plant transpiration is key for terrestrial climate and carbon cycling. Besides its general ecological importance, water availability is the major constraint for higher plant productivity in the field. When subjected to water limitation, plants homeostase their water status by coordinating transpiration with root-mediated water uptake. This leads to altered gas exchange and can result in improved leaf water use efficiency. How this is achieved is only rudimentarily understood. It appears that the plant response to water limitation involves a rapid hydraulic dialog between roots and shoots which itself relies on fundamental plant physiological mechanisms. Water flux within the plant is facilitated by water-conducting aquaporin channels and water use efficiency as well as plant’s gas exchange are governed by the stress hormone abscisic acid (ABA) which is the key factor in adjusting plant responses to drought. Different ABA sensitivities and distinct ABA signalling responses of cells and plant organs are required for coordinated water homeostasis.Understanding the long distance-coordinated water homeostasis of plants beyond these principles has been a challenge. Progress in this field has been very slow over the last decade because of the experimental difficulties in defining molecular mechanisms and to test them in plants. Initiated by recent advances in the field of ABA signaling and the control of plant hydraulics, the joint French-German research team feels to be in an excellent position to achieve a breakthrough on this enigma. The proposed ABAqua project will use and generate frontline understanding of ABA signalling pathways and aquaporin regulation to elucidate how water homeostasis is achieved through communication between roots and leaves. An emphasis of the project is to answer how the different hormone sensitivities and actions of ABA on root and guard cell hydraulics are realized and relayed to water-conducting aquaporins. Detailed molecular analysis of prototypic ABA signaling modules targeting aquaporins will be employed in combination with phosphoproteomics and expression of deregulated signaling components in Arabidopsis. Analyses at the whole-plant level will include grafting experiments and tissue-specific complementation of ABA signaling and aquaporin mutants. These analyses will reveal the key hydraulic regulatory mechanisms and their impact on gas exchange in plants under well-watered condition and drought. The emerging principles of plant’s water homeostasis, affecting gas exchange and concomitant changes in water use, will provide a mechanistic basis for improved prediction of carbon cycling and plant productivity under water limitation.