Involvement of pectin methyl-esterase during the ripening of grape berries: partial cDNA isolation, transcript expression and changes in the degree of methyl-esterification of cell wall pectins

Grape berries (Vitis vinifera L., ev Ugni blanc) were harvested at 12 diffe rent weeks of development in 1996 and 1997. Ripening was induced at veraiso n, the crucial stage of berry softening, and was followed by a rapid accumu lation of glucose and fructose and an increase of pH. Total RNAs, crude pro teins and cell wall material were isolated from each developmental stage. A partial length cDNA (pme 1, accession number AF 159122, GenBank (TM)) enco ding a pectin methyl-esterase (PME, EC 3. 1. 1.11) was cloned by RT-PCR wit h degenerate primers. Northern blots revealed that mRNAs coding for PME acc umulate from one week before the onset of ripening until complete maturity, indicating that this transcript represents an early marker of veraison and could be involved in berry softening. However, PME activity was detected d uring all developmental stages. Total activity per berry increased, whereas "specific" activity, on a fresh weight basis, decreased during development . The amount of cell wall material (per berry and per g of berry) followed the same pattern as that of PME activity (total and "specific" respectively ), indicating they were tightly correlated and that PME levels varied very little in the cell walls. Nevertheless, the degree of methyl-esterification of insoluble pectins decreased throughout the development from 68% in gree n stages to less than 20% for the ripe berries, and this observation is con sistent with the induction of PME mRNAs during ripening. Relations between transcript expression, PME activity, the DE of insoluble pectic polysacchar ides and their involvement in grape berry ripening are discussed. (C) 2001 Elsevier Science Ltd. All rights reserved.