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MAPK Cascade in Stomatal Specification

Stomata are special structures in plant leaves that allow gas and water vapor exchange between plants and the environment. Stomata are separated by pavement cells. Wang et al. used genetic approaches to show that in Arabidopsis thaliana stomatal specification involves a mitogen-activated protein kinase (MAPK) cascade. Because double knockout of MPK3 and MPK6 is embryonic-lethal and single loss-of-function mutants do not exhibit any phenotype, Wang et al. created single-knockout mpk6−/− plants in which MPK3 was reduced by RNA interference. These plants arrested as seedlings and showed clustered stomata instead of the normal arrangement in which stomata are always separated by one pavement cell. A similar phenotype was observed in mpk3, mpk6 double-mutant plants that were rescued with a steroid-inducible MPK6 gene. MKK4 and MKK5 are the upstream kinases to MPK3 and MPK6 in stress response pathways, and RNA interference of these two MAPKKs simultaneously caused the seedling leaves to have all stomata with no pavement cells. Gain-of-function experiments were performed using a steroid-inducible, constitutively activated mutant of the tobacco homolog of MKK4 and MKK5 (GVG-Nt-MEKDD). These plants had no stomata, but when crossed into either mpk6−/− or mpk3−/− mutant background, then normal stomata and stomatal arrangement were restored. The dose of MAPK signaling appears important for proper specification of stomatal cell fate. Plants deficient in the MAPKKK Yoda (YDA) also have a clustered stomata phenotype, and GVG-Nt-MEKDD suppressed this clustered phenotype in a dose-dependent manner, with higher concentrations of steroid suppressing the clustering more effectively than lower concentrations. In-gel kinase assays showed that the activity of MPK3 and MPK6 was increased in plants in which YDA was constitutively activated. Examination of cotyledons in the various mutant plants at various times during development showed that the stomatal phenotypes were associated with aberrations in asymmetric cell divisions. Fewer asymmetric cell divisions occurred in plants in which the pathway was hyperactivated (GVG-Nt-MEKDD) and ectopic cell divisions occurred, all producing small stomatal cells in plants in which the pathway was inactivated (rescued mpk3, mpk6 double mutants). H. Wang, N. Ngwenyama, Y. Liu, J. C. Walker, S. Zhang, Stomatal development and patterning are regulated by environmentally responsive mitogen-activated protein kinases in Arabidopsis. Plant Cell 19, 63-73 (2007). [Abstract] [Full Text]