Abscisic Acid Mediates and Co-Ordinates the Developmental and Hydraulic Responses of Plant Roots to Water Deficit

By increasing the amplitude and frequency of drought and flood episodes, climate change typically exacerbates the environmental constraints experienced by plants. More generally, plants have to constantly adjust their water status during development and in response to environmental conditions. By exploring the soil and taking up water, roots play a crucial role in these processes. Water uptake by roots is determined by their architecture, which results from root growth and branching, and the hydraulics of root cells and tissues. Abscisic acid (ABA) is a phytohormone that plays a major role in the adaptation of plants to water deficit. This work addresses the integration at whole root level of mechanisms that determine the root hydraulic architecture of Arabidopsis and the role of ABA in its regulation in response to water stress. For this purpose, root system architecture and root hydraulic conductivity (Lp_r) were analyzed in Arabidopsis plantlets, grown in vitro or under hydroponic conditions under different levels of water stress (PEG8000). Results showed a double response of root growth to water stress: an increase in lateral root primordia initiation and in number and length of lateral roots was observed under mild water stress, whereas these parameters were reduced with higher stress treatments. A similar dual response was observed for Lp_r. Analysis of both root architecture and hydraulics under exogenous-ABA treatments and in ABA-biosynthesis and signaling mutants revealed that ABA acts as an integrator of root responses to water stress. The significance of these effects with respect to water acquisitions strategies will be discussed. A high throughput root phenotyping platform developed in B&PMP is now used to investigate the involvement and interactions of other hormonal signaling pathways in these processes. This platform will also be instrumental for exploring the genetic bases of root adaptive responses to a large array of environmental factors.