OCTOPUS Acts in Molecular Dampening of CLE Signalling Promoting Phloem Differentiation

The CLE peptide ligands are ancient, land plants-specific hormones, playing central roles in the meristem maintenance, in the development of vascular tissues and the adaptation of the root system to certain nutritional conditions. The CLE peptides are perceived by the subfamily XI of Leucine-Rich-Repeat (LRR) Receptor-Like Kinases (RLKs). In addition, other RLKs are required to the full sensing of the peptides. CLAVATA2 (CLV2) and CORYNE(CRN) are Receptor-Like Protein (RLP) and Receptor-Like Kinase (RLK), respectively, that form dimers playing essential role in the perception of CLE peptides in a yet unknown way. We have recently showed, that CLE peptides suppress root growth by preventing protophloem differentiation, which requires local activity of CLV2 and CRN. In this context, the protophloem serves as a systemic organizer to sense the CLE peptides.

In the Arabidopsis root, two plasma membrane-associated proteins, BREVIS RADIX (BRX) and OCTOPUS (OPS) serve as master regulators of the phloem sieve element differentiation. In addition, the second-site suppressor screen with the short-root brx mutant has revealed the components of the CLE45-BAM3 (BARELY ANY MERISTEM3) RLK pathway that suppress the protophloem differentiation. Interestingly, one of the suppressors found in the same screen, was an OPS gain-of-function mutant, that shows resistance to CLE45 peptide. We recently found that this OPS loss-of-function mutant shows resistance to all root-active CLE peptides. Using in vivo FRET/FLIM system, we have found, that OPS interacts with CRN and BAM3 in the plasma membrane. Our results imply, that OPS activity affects the abundance of CRN protein and therefore impacts on the overall protophloem-specific CLE peptides perception. In this context, OPS dampens the CLE signaling, promoting the differentiation.