PECTIN METHYLESTERASE34 Contributes to Heat Tolerance through Its Role in Promoting Stomatal Movement.

Pectin, a major component of the primary cell wall, is synthesized in the Golgi apparatus and exported to the cell wall in a highly methylesterified form, then is partially demethylesterified by pectin methylesterases (PMEs; EC 3.1.1.11). PME activity on the status of pectin methylesterification profoundly affects the properties of pectin and, thereby, is critical for plant development and the plant defense response, although the roles of PMEs under heat stress (HS) are poorly understood. Functional genome annotation predicts that at least 66 potential genes are contained in Arabidopsis (). Thermotolerance assays of gene T-DNA insertion lines revealed two null mutant alleles of (At3g49220) that both consistently showed reduced thermotolerance. Nevertheless, their impairment was independently associated with the expression of HS-responsive genes. It was also observed that transcription was induced by abscisic acid and highly expressed in guard cells. We showed that the mutation has a defect in the control of stomatal movement and greatly altered PME and polygalacturonase (EC 3.2.1.15) activity, resulting in a heat-sensitive phenotype. has a role in the regulation of transpiration through the control of the stomatal aperture due to its cell wall-modifying enzyme activity during the HS response. Hence, PME34 is required for regulating guard cell wall flexibility to mediate the heat response in Arabidopsis.