The function of the Arabidopsis receptor kinase THESEUS1 in plant cell wall integrity maintenance: from evolutionary origin to future perspectives.

Plants actively monitor the state of their cell walls and adapt their structure and composition as needed. THESEUS1 (THE1), a receptor kinase from the Catharanthus roseus RECEPTOR-LIKE KINASE 1-LIKE (CrRLK1L) family, was the first receptor kinase described to be involved in this maintenance of cell wall integrity in Arabidopsis thaliana. It contributes to the regulation of cell wall stiffness, participates in lateral root development, and modulates production of the stress hormones abscisic acid and jasmonic acid. Besides, it is required for responses to cellulose biosynthesis inhibition, such as growth inhibition and ectopic lignification. The THE1 ligand, RAPID ALKALINIZATION FACTOR 34, and one intracellular interaction partner, GUANINE EXCHANGE FACTOR 4, have been identified. However, relatively little is known about other interaction partners and mechanisms by which THE1 influences downstream responses. Given that FERONIA, a related member of the CrRLK1L family, has dozens of interaction partners, it is likely that there is a wealth of interactors waiting to be described for THE1. Here, we shed light on the evolutionary origin of THE1 and describe the many open questions surrounding THE1-dependent signaling processes. We expect that many of these questions will be answered in the coming years and that these answers will provide more insight into the molecular mechanism of cell wall integrity maintenance mediated by THE1.