Stomata are excellent model systems for examining the mechanisms that regulate cell fate determination and pattern formation. It has recently been demonstrated that brassinosteroids control stomatal development by regulating both the MAPK kinase kinase YODA and the basic helix-loop-helix transcriptional factor SPEECHLESS. Here, we show that these plant regulators positively regulate stomatal formation in the hypocotyl and also accelerate their development. Hormone tests, reporter gene studies and mutant analyses revealed that brassinosteroids act upstream of the transcriptional factors CAPRICE and GLABRA2. These plant regulators control an earlier stage of stomatal production than those regulated by the membrane receptor TOO MANY MOUTHS. This work highlights differences in the genetic control of stomatal development between cotyledons or leaves and hypocotyls.

Stomata are pores on the plant surface that enable gas exchange with the atmosphere. In Arabidopsis thaliana, brassinosteroids, which function upstream of the TTG/bHLHs/MYBs/GL2 transcriptional network, positively regulate stomatal formation in the hypocotyl. Gibberellins also promote stomatal development in the embryonic stem. Here, we investigated the hypothetical interactions between the gibberellin and brassinosteroid signaling pathways during stomatal formation. By combining genetics and gene expression studies, we found that gibberellins promote stomatal development, functioning upstream of both brassinosteroids and the TTG/bHLHs/MYBs/GL2 network. Gibberellins also regulate an earlier step in stomatal development than that regulated by the receptor-like membrane protein TOO MANY MOUTHS.