A Novel Plant Leucine-Rich Repeat Receptor Kinase Regulates the Response ofMedicago truncatulaRoots to Salt Stress

In plants, a diverse group of cell surface receptor-like protein kinases (RLKs) plays a fundamental role in sensing external signals to regulate gene expression. Roots explore the soil environment to optimize their growth via complex signaling cascades, mainly analyzed in Arabidopsis thaliana. However, legume roots have significant physiological differences, notably their capacity to establish symbiotic interactions. These major agricultural crops are affected by environmental stresses such as salinity. Here, we report the identification of a leucine-rich repeat RLK gene, Srlk, from the legume Medicago truncatula. Srlk is rapidly induced by salt stress in roots, and RNA interference (RNAi) assays specifically targeting Srlk yielded transgenic roots whose growth was less inhibited by the presence of salt in the medium. Promoter-beta-glucuronidase fusions indicate that this gene is expressed in epidermal root tissues in response to salt stress. Two Srlk-TILLING mutants also failed to limit root growth in response to salt stress and accumulated fewer sodium ions than controls. Furthermore, early salt-regulated genes are downregulated in Srlk-RNAi roots and in the TILLING mutant lines when submitted to salt stress. We propose a role for Srlk in the regulation of the adaptation of M. truncatula roots to salt stress.