PECTIN METHYLESTERASE51 Affects Stomatal Dimensions, Rosette Area, and Root Length in Arabidopsis thaliana.

Pectins are abundant in the cell walls of eudicot plants and have been implicated in determining the development and biomechanics of stomatal guard cells, which expand and contract dynamically to open and close stomatal pores on the plant surface, modulating photosynthesis and water transport. Pectic homogalacturonan is delivered to the cell wall in a methylesterified form but can be demethylesterified in the wall by pectin methylesterases, increasing both its ability to form crosslinks via calcium and its susceptibility to degradation by endogenous pectinases. Although a few pectin methylesterases have been implicated in stomatal development and function, this large family of proteins has not been fully characterized with respect to how they modulate stomatal guard cells. Here, we characterized the function of PECTIN METHYLESTERASE51 (PME51), a pectin methylesterase-encoding gene that is expressed in developing guard cells, in stomatal morphogenesis in seedlings and adult plants of Arabidopsis thaliana. Overexpressing PME51 led to smaller adult plants with smaller stomatal complexes and subtle changes in initial responses to opening and closure stimuli, whereas knocking out PME51 resulted in smaller stomatal complexes and longer roots in seedlings. We observed changes in pectin labeling in knockout and overexpression plants that imply a specific function for PME51 in modulating the degree of methylesterification for homogalacturonan. Together, these findings expand our understanding of how pectin modification by pectin methylesterases affects the development and function of stomatal guard cells, which must maintain a balance of strength and flexibility to optimize plant growth.