The tiny pores, or stomata, that open and close on a leaf's surface allow for the exchange of gases according to the needs of the plant's physiology. The number of stomata formed during development is a result of competing signals that activate or repress pore formation. Lampard et al. now show how these competing signals converge so that their inputs result in one question: Will there or won't there be a stoma placed here? The transcription factor SPEECHLESS, which activates stomatal development programs, can be phosphorylated by certain mitogen-activated protein kinases (MAPKs), a group of kinases that, among myriad other functions, repress formation of stomata. All the phosphorylation sites are contained within the 93–amino acid MAPK target domain of SPEECHLESS, which thus integrates positive and negative signals.
G. R. Lampard, C. A. MacAlister, D. C. Bergmann, Arabidopsis stomatal initiation is controlled by MAPK-mediated regulation of the bHLH SPEECHLESS. Science 322, 1113–1116 (2008). [Abstract] [Full Text]