Laurence Geny

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.

Grape berry skin features related to ontogenic resistance to Botrytis cinerea.

This work investigated the structural and biochemical changes during grape berry development which account potentially for the onset and increase in susceptibility to Botrytis cinerea. Using the cv. Sauvignon blanc, we quantified at seven developmental growth stages from herbaceous to over-mature berries: (1) fruit ontogenic resistance using three strains (II-transposa), (2) the morphological and maturity fruit characteristics and (3) preformed biochemical compounds located in the berry skin. From the mid-colour change stage onwards, susceptibility of unwounded fruit increased sigmoidally in both rot and sporulation severities at the berry surface. A principal component analysis identified a very close connection between fruit susceptibility and the level of fruit maturity. Berry susceptibility was significantly and positively correlated with the phenolic compounds in the skin cell walls and negatively correlated with the total tannin content in the skin and with water activity (Aw) at the fruit surface. On the berry, Aw decreased from 0.94 at bunch closure to 0.89 at berry maturity, with a relatively low value (0.90) at the stage of mid-colour change. Using artificial media, different Aw levels led to significant differences in mycelial growth (Aw ≤0.95 resulted in the lowest growth rate ≤0.34xa0mm day−1). Thus, besides the level of fruit maturity, both water activity on the fruit and the total tannin content in the skin may affect fungal growth and berry colonisation. The potential of these variables for use as indicators of grape berry susceptibility as well as associated mechanisms for the development of disease are discussed.