A CLE-BAM-CIK signalling module controls root protophloem differentiation in Arabidopsis

Exogenous application of CLAVATA3 (CLV3)/EMBRYO SURROUNDING REGION (CLE) peptides suppresses protophloem differentiation and leads to the consumption of the proximal root meristem. However, the exact CLE peptides and the corresponding receptor complex regulating protophloem differentiation have not yet been clarified. Through expression pattern and phylogenetic analyses, CLE25/26/45 were identified as candidate peptides. Further genetic analyses, physiological assays and specific protophloem marker observations indicated that CLE25/26/45, BARELY ANY MERISTEM1/3 (BAM1/3) and CLV3 INSENSITIVE KINASEs (CIKs) are involved in regulating protophloem differentiation. The cle25 26 45 and cik2 3 4 5 6 mutation can greatly rescue the root defects of brevis radix (brx) and octopus (ops) mutants. The protophloem differentiation and proximal root meristem consumption of clv1 bam1 3 and cik2 3 4 5 6 were insensitive to CLE25/26/45 treatments. cle25 26 45, clv1 bam1 3 and cik2 3 4 5 6 displayed similar premature protophloem. In addition, CLE25/26/45 induced the interactions between BAMs and CIKs in vivo. Furthermore, CLE25/26/45 enhanced the phosphorylation levels of CIKs, which were greatly impaired in clv1 bam1 3 mutant. Our work clarifies that the CLE25/26/45-BAM1/3-CIK2/3/4/5/6 signalling module genetically acts downstream of BRX and OPS to suppress protophloem differentiation.