Overexpression of pectin methylesterase inhibitors in Arabidopsis restricts fungal infection by Botrytis cinerea.

Pectin, one of the main components of plant cell wall, is secreted in a highly methylesterified form and demethylesterified in muro by pectin methylesterase (PME). The action of PME is important in plant development and defence and makes pectin susceptible to hydrolysis by enzymes such as endopolygalacturonases (endoPGs). Regulation of PME activity by specific protein inhibitors (PMEIs) can, therefore, play a role in plant development as well as in defence by influencing the susceptibility of the wall to microbial endoPGs. To test this hypothesis we have constitutively expressed the genes AtPMEI-1 and AtPMEI-2 in Arabidopsis and targeted the proteins into the apoplast. The overexpression of the inhibitors resulted in a decrease of PME activity in transgenic plants and two PMEs isoforms were identified that interacted with both inhibitors. While the content of uronic acids in transformed plants was not significantly different from that of WT, the degree of pectin methylesterification was increased by about 16%. Moreover differences in the fine structure of pectins of transformed plants were observed by enzymatic fingerprinting. Transformed plants showed a slight but significant increase in root length and were more resistant to the necrotrophic fungus Botrytis cinerea. The reduced symptoms caused by the fungus on transgenic plants were related to its impaired ability to grow on methylesterified pectins.