Abstract

Identification of a Mitogen Activated Protein Kinase Phosphatase Controlling Stomatal Development in Arabidopsis thaliana
Hassan Damen, M.Sc.
Stomata are tiny openings on all above ground plant epidermis except for petals and stamen filaments. They regulate gas exchange and control water loss, which enables plants to adapt to environmental changes and facilitates photosynthesis. In Arabidopsis, stomata are formed through a series of differentiation events according to positional cues mediated by cell-to-cell signalling that contains mitogen-activated protein kinase (MAPK) cascade components, an YDA (MAPKKK), MKK4/MKK5 (MAPKKs), MPK3/MPK6 (MAPKs). Although much is known about the upstream kinases and biological role of MAPKs (MPK3 and MPK6) during stomatal development, the process by which they are inactivated remains unclear. In mammals, some members of the dual-specificity phosphatases (DSPs), which dephosphorylate both phosphoserine/threonine and phosphotyrosine residues of their substrates, have been recognized as key players in deactivating MAPKs. Five DSP-type phosphatases with described or predicted MAPK phosphatase (MKP) function are encoded in the genome of Arabidopsis, but their role in stomatal patterning and differentiation has not yet been investigated.
Here, I report the identification of MKP1, one of the five Arabidopsis MKPs, as a key regulator promoting stomatal cell fate differentiation. A loss-of-function mkp1 mutation occasionally developed clusters of small cells that failed to differentiate into stomata. I determined the phosphatase activity of MKP1 is essential and required for totally normal stomatal development. A genetic analysis showed that the redundantly functioning kinases, MPK3 and MPK6, are epistatic to MKP1, consistent with the predicted function of MKP1 as a MAPK phosphatase. Furthermore, our cell type-specific rescue experiments using the promoters of the genes encoding the stomatal basic-helix-loop-helix (bHLH) transcription factors revealed that early stomatal cell-specific expression of MKP1 is sufficient for proper stomatal development. This data implies that MKP1 controls a MAPK cascade in the first out of three division and/or differentiation “steps” of stomatal development. Taken together, these findings help determine the role of phosphatase in controlling MAPK signaling specificity in stomatal development, and thus providing further insights into the cell fate regulation by this intracellular signalling cascade.