Figure 9.

Model for spch-5 phenotype and BR effects. A, Current simplified view of SPCH-mediated stomata promotion. SPCH is negatively regulated by the YODA cascade and by BIN2. In the initiation of the stomatal cell lineage, the YODA cascade is triggered by EPF2 through TMM and ERECTA family receptor complexes. SPCH activates EPF2 transcription, creating a feedback repression loop. BIN2 is a negative regulator of the repressive YODA cascade, and BIN2 activity is negatively regulated by BRs through the receptor BRI1. B, BR-dependent regulation of the SPCH-5 phosphorylation status. In Col-0, the negative feedback loop through EPF2 compensates for the negative and positive effects of BIN2 on SPCH activity; thus, the stomatal phenotype is independent of BL. In spch-5, because EPF2 and the negative loop are absent, the repressive YODA pathway operates very limitedly and most SPCH-5 phosphorylation is under BIN2 control. BL treatment inactivates BIN2, releasing the main phosphorylation-mediated repression on SPCH-5 and promoting protein accumulation and stomata development.