Receptor-like cytoplasmic kinase MARIS functions downstream ofCrRLK1L-dependent signaling during tip growth

Growing plant cells need to rigorously coordinate external signals with internal processes. For instance, the maintenance of cell wall (CW) integrity requires the coordination of CW sensing with CW remodeling and biosynthesis to avoid growth arrest or integrity loss. Despite the involvement of receptor-like kinases (RLKs) of theCatharanthus roseusRLK1-like (CrRLK1L) subfamily and the reactive oxygen species-producing NADPH oxidases, it remains largely unknown how this coordination is achieved. ANXUR1 (ANX1) and ANX2, two redundant members of theCrRLK1L subfamily, are required for tip growth of the pollen tube (PT), and their closest homolog, FERONIA, controls root-hair tip growth. Previously, we showed that ANX1 overexpressionmildly inhibits PT growth by oversecretion of CW material, whereas pollen tubes ofanx1 anx2double mutants burst spontaneously after germination. Here, we report the identification of suppressor mutants with improved fertility caused by the rescue ofanx1 anx2pollen tube bursting. Mapping of one these mutants revealed an R240C nonsynonymous substitution in the activation loop of a receptor-like cytoplasmic kinase (RLCK), which we named MARIS (MRI). We show that MRI is a plasma membranelocalized member of the RLCK-VIII subfamily and is preferentially expressed in both PTs and root hairs. Interestingly, mri-knockout mutants display spontaneous PT and root-hair bursting. Moreover, expression of the MRIR240Cmutant, but not its wild-type form, partially rescues the bursting phenotypes ofanx1 anx2PTs and fer root hairs but strongly inhibits wild-type tip growth. Thus, our findings identify a novel positive component of theCrRLK1L-dependent signaling cascade that coordinates CW integrity and tip growth.