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.

Conserved and differentiated functions of CIK receptor kinases in modulating stem cell signaling in Arabidopsis.

The shoot apical meristem (SAM) and root apical meristem (RAM) act as pools of stem cells that give rise to aboveground and underground tissues and organs in higher plants, respectively. The CLAVATA3 (CLV3)-WUSCHEL (WUS) negative-feedback loop acts as a core pathway controlling SAM homeostasis, while CLV3/EMBRYO SURROUNDING REGION (ESR) 40 (CLE40) and WUSCHEL-RELATED HOMEOBOX5 (WOX5), homologs of CLV3 and WUS, direct columella stem cell fate. Moreover, CLV3 INSENSITIVE KINASES (CIKs) have been shown to be essential for maintaining SAM homeostasis, whereas whether they regulate the distal root meristem remains to be elucidated. Here, we report that CIKs are indispensable for transducing the CLE40 signal to maintain homeostasis of the distal root meristem. We found that the cik mutant roots displayed disrupted quiescent center and delayed columella stem cell (CSC) differentiation. Biochemical assays demonstrated that CIKs interact with ARABIDOPSIS CRINKLY4 (ACR4) in a ligand-independent manner and can be phosphorylated by ACR4 in vitro. In addition, the phosphorylation of CIKs can be rapidly induced by CLE40, which partially depends on ACR4. Although CIKs act as conserved and redundant regulators in the SAM and RAM, our results demonstrated that they exhibit differentiated functions in these meristems.