HRB2-BBX21 interaction regulates stomatal aperture

Stomata must maintain a balance between opening for carbon uptake and closing to prevent dehydration and pathogen invasion during times of stress. Kang et al. investigate this relationship, utilizing light and ABA signaling to modulate stomatal dynamics. HBR2 was identified as a chromatin-remodeling factor, corresponding to the gene PKL. The mutant hrb2 has defects in hypocotyl growth under light conditions similar to that of pkl-1 (tested in this work) and other pkl alleles (referenced). In the absence of ABA, hbr2 had reduced stomatal apertures, whereas bbx21-1 mutants were WT-like. BBX21 is a B-box protein involved in various light responses. Both mutants showed reduced stomatal apertures in the presence of ABA. Furthermore, HRB2 upregulates the expression of BBX21, which is light inducible. To further elucidate the role of HRB2 in the ABA pathway, expression of the ABA signaling gene ABI5 was investigated and found to be upregulated in hbr2, pkl-1, and bbx21-1 leaves. Overexpression of ABI5 reduces stomatal apertures and may account for the phenotype of hrb2. BBX21 was found to associate with the ABI5 promoter. This leads to a model in which BBX21 binds to the ABI5 promoter and recruits HRB2 to modulate chromatin at this locus (Summary by Alecia Biel) Plant Cell Environ. 10.1111/pce.13336.