One more role for the brassinosteroid regulators: BZR1 and BES1 inhibit stomatal development in Arabidopsis cotyledons

Stomatal density affects growth by regulating gaseous exchange and affects defense by controlling pathogen entry.The development of guard cells is initiated from the differentiation of meristemoid mother cells to become guard mother cells (GMCs), which further divide to form a pair of guard cells surrounding the stoma.The cell differentiation and division processes are regulated by various hormones, including brassinosteroid (BR).BR promotes guard cell formation in the hypocotyl but represses it in cotyledons (Wei et al. 2021).The tissue-specific effects suggest the delicate signaling regulation, which has remained unclear.In this issue of Plant Physiology, Li et al. report the role of BR in stomatal development in cotyledons in the dark (Li et al. 2024).The homologous transcription factors (TFs) BZR1 and BES1 are the master BR signaling components (Wang et al. 2002;Yin et al. 2002;Li et al. 2018).The TFs also play a role in stomatal development, and their gain-of-function mutations (bzr1-1D and bes1-D respectively) inhibited stomatal development in Arabidopsis cotyledons (Li et al. 2024).To search for the target genes of BZR1 and BES1, the authors selected candidate stomatal development genes with BR regulatory elements in their promoters.bzr1-1D and bes1-D had promoted expression of MKK9 but repressed expression of FAMA.MKK9 encodes a motigen-activated protein kinase, while FAMA encodes a bHLH TF (Ohashi-Ito and Bergmann 2006;Lampard et al. 2009).BR inhibits stomatal development by promoting the expression of MKK9 while repressing that of FAMA (Ohashi-Ito and Bergmann 2006;