The cell wall is a rigid network being the first barrier between a plant cell and its environment, but at the same time has to be a dynamic network whose cell growth and shape is given by deposition and remodeling of the cell wall. Maintaining cell wall integrity (CWI) is essential for correct plant development and stress response. Members of the family of Catharanthus roseus receptor-like kinase 1-like (CrRLK1L) proteins have been shown to play a role in cell wall homeostasis, mechanoperception, CWI maintenance and growth control. One of the 17 members in Arabidopsis, THESEUS1 (THE1), was identified in a suppressor screen of a cellulose deficient mutant, revealing that the reduction in growth is part of a THE1-mediated compensatory response to cell wall perturbation (Hématy et al., 2007).
Interestingly, several CrRLK1L members have been shown to be receptors for Rapid Alkalinisation Factor (RALF) peptides. RALFs are on average 50 amino acids highly basic, cysteine-rich peptides, most of which are predicted to be cleaved from a highly acidic prodomain by a subtilisin protease. Recently, THE1 has been identified as a receptor for RALF34 (Gonneau et al., 2018). However, this peptide might not be the only THE1 ligand, since ralf34 loss-of-function mutants do not phenocopy all aspects of the1 mutants. RALF24 and RALF31 clustered together with RALF34, based on expression values across different tissues. We generated CRISPR/Cas9 mutants on RALF24, RALF31 and RALF34 with the aim to study which THE1 responses depend on these peptides. Our data suggest that RALF34 could not be the ligand for CWI response of THE1. The presence of RALF34 is acting negatively through THE1, inhibiting its response to cell wall damage.
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