Receptor kinases convey diverse environmental and developmental inputs by sensing extracellular ligands. In plants, one group of receptor‐like kinases (RLKs) is characterized by extracellular leucine‐rich repeat (LRR) domains, which interact with various ligands that include the plant hormone brassinosteroid and peptides of the CLAVATA3/EMBRYO SURROUNDING REGION (CLE) type. For instance, the CLE45 peptide requires the LRR‐RLK BARELY ANY MERISTEM 3 (BAM3) to prevent protophloem formation in Arabidopsis root meristems. Here, we show that other proposed CLE45 receptors, the two redundantly acting LRR‐RLKs STERILITY‐REGULATING KINASE MEMBER 1 (SKM1) and SKM2 (which perceive CLE45 in the context of pollen tube elongation), cannot substitute for BAM3 in the root. Moreover, we identify MEMBRANE‐ASSOCIATED KINASE REGULATOR 5 (MAKR5) as a post‐transcriptionally regulated amplifier of the CLE45 signal that acts downstream of BAM3. MAKR5 belongs to a small protein family whose prototypical member, BRI1 KINASE INHIBITOR 1, is an essentially negative regulator of brassinosteroid signaling. By contrast, MAKR5 is a positive effector of CLE45 signaling, revealing an unexpected diversity in the conceptual roles of MAKR genes in different signaling pathways.
Unlike BKI1, a negative regulator of brassinosteroid signaling, the BKI1‐related MAKR5 protein is a positive effector of CLE45 perception, revealing an unexpected diversity in the conceptual roles of MAKR genes in different signaling pathways.
SKM1 or SKM2 cannot replace BAM3 in CLE45 perception in the root.
The BKI1 and MAKR5 N‐terminal domains are interchangeable, but their C‐terminal domains are not because they are responsible for signaling pathway specificity.
MAKR5 is a positive effector of BAM3‐dependent CLE45 perception in protophloem development.
- Received March 29, 2016.
- Revision received May 28, 2016.
- Accepted June 2, 2016.
- © 2016 The Authors