Pectin methylesterase and polygalacturonase in the developing grape skin

Ripening of grape (Vitis vinifera L.) berry immediately precedes harvesting and the evolution of the skin tissue is important as it contains the key compounds for wine quality. Grape softening is thought to result from extensive cell wall modifications that occur during ripening. These modifications result from the activity of different cell wall-modifying enzymes. Two of the most significant pectin-degrading enzymes are pectin methylesterase (EC 3.1.1.11) and polygalacturonase (EC 3.2.1.15). In this work, the activities of both enzymes were monitored in skin tissue throughout berry development. Pectin methylesterase activity was present before the onset of veraison and increased during skin maturation. No polygalacturonase activity could be detected. The accumulation of mRNA encoding a pectin methylesterase and two polygalacturonase isoforms was examined using RT-PCR. Transcripts for pectin methylesterase were present in all stages analyzed with a maximal accumulation at the end of color change. Accumulation of VvPG1 transcript was closely correlated with berry softening, and expression of this gene was markedly increased during the color change. VvPG2 mRNA accumulation began before veraison and was low during skin ripening. A phylogenic analysis showed that this gene is classified in a different group than VvPG1. These findings suggest that both genes are associated with different mechanisms during skin development. VvPG1, in particular, is most likely to play a role in skin softening and VvPG2 in triggering the ripening process.