Pectins, Hemicelluloses and Celluloses show Specific Dynamics in the internal and external surfaces of Grape Berry Skin During Ripening

Grapevine berry skin is a complex structure that contributes to the final size and shape of the fruit and affects its quality traits. The organization of cell wall polysaccharides in situ and their modification during ripening are largely uncharacterized. Polymer structure of Corvina berry skin, its evolution during ripening and related modifying genes were determined by combing mid-infrared micro-spectroscopy and multivariate statistical analysis with transcript profiling and immunohistochemistry. Spectra were acquired in situ using a surface-sensitive technique on internal and external sides of the skin without previous sample pre-treatment allowing comparison of the related cell wall polymer dynamics. The external surface featured cuticle-related bands, the internal one showed more adsorbed water. Application of surface-specific normalization revealed the major molecular changes related to hemicelluloses and pectins in the internal surface and to cellulose and pectins in the external one and that they occur between mid-ripening and full-ripening in both skin sides. Transcript profiling of cell wall modifying genes pointed out a general suppression in cell wall metabolism during ripening. Genes related to pectin metabolism - aβ-galactosidase, a pectin(methyl)esterase and a pectate lyase - and a xyloglucan endotransglucosylase/hydrolase, involved in hemicellulose modification, showed enhanced gene expression. In agreement with Fourier transform infrared spectroscopy, patterns due to pectin methyl-esterification provided new insights about the relation between pectin modifications and the associated transcript profile during skin ripening. This study proposes an original description of polymer dynamics in grape berries during ripening, highlighting differences between internal and external skin sides.