Overexpression of Pectin methylesterase inhibitors in Arabidopsis affects defence against pathogens

Pectins are a family of complex galacturonic acid rich polysaccharides present in the primary cell wall and intercellular spaces of higher plants. Pectins are synthesized in the Golgi and released in muro as highly methylesterified forms and soon thereafter de-esterified by pectin methylesterase (PME). PMEs produced by plants take part in important physiological processes and also have a role in resistance to fungi and bacteria as well as in the systemic spread of tobacco mosaic virus. In addition to the transcriptional control, PME activity is post-translationally regulated by specific proteinaceous inhibitors (PMEIs). PMEIs are encoded in Arabidopsis by a small gene family of two members named AtPMEI-1 and AtPMEI-2. PMEIs typically inhibit PMEs of plant origin and do not affect the activity of microbial enzymes. To understand the role of AtPMEI-1 and AtPMEI-2 in resistance to pathogens we have stably over-expressed AtPMEI-1 and AtPMEI-2 in Arabidopsis plants. The overexpression of the inhibitors resulted in a decreased PME activity and in a significant increase in the degree of pectin methylesterification. Transformed plants were more resistant to the necrotrophic fungus Botrytis cinerea. The analysis of the resistance of the transformed plants to bacterial and viral pathogens is in progress.