The Role of PME2 and PME3 in Arabidopsis Stomatal Development and Morphology

Pectin methylesterases (PMEs) are enzymes, encoded by multigene families, that catalyze the demethylesterification of cell wall homogalacturonans. The removal of methyl groups, if performed block-wise in large series of methylesters, leads to the production of homogalacturonans that can be cross-linked with calcium bridges. This fine modulation of the methylesterification status of the pectin network alters the mechanical properties of the cell wall and has been proven crucial for stomatal complex ontogenesis and function. Considering this significant role of PMEs, as well as their involvement in numerous plant development processes, we investigated the phenotypic implications of two Arabidopsis thaliana PME compromised mutants (pme2 and pme3) and the corresponding double mutant (pme2 pme3) in stomatal development and morphology. The cotyledons of the double mutant were larger and wider, while the ratio of length/width was smaller compared to that of WT plants. The stomatal patterning was also affected since the pme2 pme3 mutant displayed a higher number of mature stomata as well as a higher percentage of stomatal clustering. Furthermore, the guard cells of the double mutant displayed a lower ratio of cell length to width, indicating alterations in the morphology of mature stomata. As far as the cell wall matrix composition is concerned, callose and pectins’ epitope distribution displayed significant differences in pme single and double mutants compared to WT plants. Taken together, our results underline the indispensable role of PME2 and PME3 in stomatal development, since their functional disruption affects not only stomatal patterning but also the morphology and function of the guard cells.