Signalling peptides have important roles in cell-to-cell communication. One of the largest family of signalling peptides in plants is formed by the CLE peptides (32 members in Arabidopsis). However, receptors that perceive and transduce the peptide signals are much more diverse, which suggests that the same peptides may bind to different receptors and induce tissue-specific signals. Therefore, identifying peptide–receptor–coreceptor complexes is of major importance in deciphering tissue-specific signalling pathways. CLE peptides bind to leucine rich repeat receptor-like kinases, of which there are over 200 in Arabidopsis. CLE19 was known to have tissue-specific roles in the root, vasculature, embryo and pollen; however, the receptor–coreceptor pair remained unknown.
The researchers found that CLE19 interacts with the receptor PXY-LIKE 1 (PXL1) and, to a lesser extent, with PXL2, but not with the closely related PHLOEM INTERCALATES WITH XYLEM (PXY). CLE19 binding induces PXL1 phosphorylation, but only when CLE19 is functionally active. Expression of a dominant-negative PXL1 leads to higher expression of genes that are involved in exine formation and to pollen defects that are reminiscent of plants that express dominant-negative CLE19. Pollen defects caused by CLE19 overexpression can be suppressed by dominant-negative PXL1, which suggests that PXL1 is required for CLE19 function. The coreceptor SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE 1 (SERK1) can interact with PXL1, depending on the presence of CLE19, but genetic analysis suggests that there is functional redundancy with SERK2 and SERK3. Together, the protein interaction and genetic analyses suggest that CLE19 forms a peptide–receptor–coreceptor complex with PXLs and SERKs to transduce a signal that limits, and thus balances, the formation of the pollen exine.
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