Unraveling the MAPK Signaling Network in Stomatal Development

role in biotic and abiotic responses and embryo and floral organ development. The answer, in large part, may lie with cell-specific differences in the expression and activation of various components of the MAPK network under different sets of circumstances. Lampard et al. devised a cell-specific assay for assessing the activity and regulation of MAPK modules in stomatal development based a panel of dominant-negative and constitutively active MAPKK variants expressed in discrete stomatal lineage cell types through the use of cell-type specific promoters. The approach was designed to allow investigation into cell-specific aspects of MAPK signaling without inducing pleiotropic phenotypes, which likely result from ubiquitous activation of MAPK signaling. The authors identified expanded roles for MKK4 and MKK5 in negative regulation of stomatal development and, unexpectedly, uncovered both positive and negative regulatory roles for MKK7 and MKK9 at different stages of stomatal development. MKK7 and MKK9 were found to function in inhibition of the first two stages of stomatal development and in promotion of guard cell proliferation at the terminal stages of stomatal development (see figure). The authors present a model of MAPK control of stomatal development that integrates developmental and environmental cues to offer a reasonable explanation of experimental results and highlight unknown components. This work provides significant insight into regulation of stomatal development and is also of broader significance to our understanding of MAPK networks. It shows that MAPK networks can be dissected by modulating the expression of individual components in specific cell types, in this case stomatal lineage cell types. This cell-specific approach should prove useful for the study of other complex signal transduction pathways as well.