Expression patterns of cell wall-modifying enzymes during grape berry development
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During ripening of grape (Vitis vinifera L.) berries, softening occurs concomitantly with the second growth phase of the fruit and involves significant changes in the properties of cell wall polysaccharides. Here, the activities of enzymes that might participate in cell wall modification have been monitored throughout berry development. α-Galactosidase (EC 220.127.116.11), β-galactosidase (EC 18.104.22.168) and pectin methylesterase (EC 22.214.171.124) activities were present, but no polygalacturonase (EC 126.96.36.199), cellulase (EC 188.8.131.52), xyloglucanase (xyloglucan-specific cellulase EC 184.108.40.206) or galactanase (EC 220.127.116.11) could be detected. The accumulation of mRNAs encoding wall-modifying enzymes was examined by northern hybridization analysis. Transcripts for β-galactosidase, pectin methylesterase, polygalacturonase, pectate lyase (EC 18.104.22.168) and xyloglucan endotransglycosylase (EC 22.214.171.124) were present during ripening, although polygalacturonase activity had not been detected in berry extracts. Cellulases could not be detected in ripening berries, either at the enzyme or mRNA levels. The increase in β-galactosidase activity and mRNA is consistent with the observed decrease in type-I arabinogalactan content of the walls during ripening, and the detection of polygalacturonase and pectate lyase mRNAs might explain the increased solubility of galacturonan in walls of ripening grapes. Thus, the modification of cell wall polysaccharides during softening of grape berries is a complex process involving subtle changes to different components of the wall, and in many cases only small amounts of enzyme activity are required to effect these changes.
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