Quantitative Genetics Dissection of Root Hydraulics Uncovers Novel Pathways for Plant Adaptation to Stresses

Soil water uptake by roots is a key component of plant performance and adaptation to adverse environments. It is contributed by both the root system architecture and its hydraulic properties. Environmental constraints such as drought or flooding, nutrient deprivation or oxidative stress exert deep effects on root functions by altering the root hydraulic conductivity (Lp_r). Here, we have investigated the genetic bases of root hydraulics in Arabidopsis thaliana, using quantitative genetics approaches, including linkage mapping and genome-wide association (GWA) mapping. Several genes controlling Lp_r were identified and will be discussed. A RAF-like MAP3 kinase named Hydraulic Conductivity of Root 1 (HCR1) was found to delineate a combinatorial signaling pathway integrating two soil signals, K⁺ and O₂ availability. HCR1 regulates root hydraulics and hypoxia responsive genes, thereby supporting plant adaptation to various flooding scenarios. GWA studies pointed to XND1, a transcription factor which counteracts xylem differentiation, thereby acting as a negative regulator of Lp_r. As a consequence, XND1 diminishes drought stress tolerance and counteracts plant infection by the prototypal root pathogen Ralstonia solanacearum. Thus, genetic variation at XND1 and xylem differentiation contribute to resolving the major trade-off between abiotic and biotic stress resistance in Arabidopsis. In all these studies, natural allelic variants provide useful genetic resources to understand and possibly manipulate the modes of plant adaptation to combined or opposing environmental stresses.