CLE peptides regulate a wide variety of processes during plant development, including in the root where they can suppress root growth through inhibition of protophloem formation, for instance in response to environmental clues. However, the receptors and co-receptors for CLEs in the root have remained unclear, as have the relationships between different CLEs and different (co-)receptors. Now, Christian Hardtke and colleagues describe CLERK, a receptor kinase involved in sensing CLEs in the root. CLERK mutant alleles were identified in a genetic screen for seedlings with CLE26-resistant root growth. clerk mutants are defective in the sensing of various CLE peptides; this promiscuity suggests that CLERK may act as a co-receptor for various CLE receptors. CLERK is functionally equivalent to the related SARK and NIK kinases at the protein level, and genetic interaction experiments show that CLERK acts independently of CLV2-CRN signalling, so likely forms part of an independent CLE-sensing pathway in the root. Using a fusion reporter, the authors find that CLERK is expressed at a low level in the developing protophloem sieve elements, and that mutants show precocious protophloem differentiation. The work thus identifies CLERK as a novel and crucial factor in the sensing of CLE peptides during root development.
