Characterization of changes in pectin methylesterase expression and pectin esterification during tomato fruit ripening

Abstract

The production, accumulation, and in situ location of pectin methylesterase (EC 3.1.11) was examined in ripening fruit of the processing tomato cv. UC82B. Pectin methylesterase detected with a monoclonal antibody (PME-1) first appeared adjacent to seeds in immature green fruit and was later detected only in tissue adjacent to the cuticle (i.e., exopericarp) during ripening. Enzyme-linked immunosorbant assay and Western blot analysis using PME-1 demonstrated that the fresh-market cultivars Celebrity and Better Boy accumulated lower levels of this immunologically detectable pectin methylesterase during maturation than did processing cv. UC82B, and that the immunologically detected pectin methylesterase and the total detectable pectin methylesterase activities of 'Celebrity' and 'Better Boy' increased throughout ripening. In contrast, processing cv. UC82B displayed a total detectable pectin methylesterase activity profile that peaked during the breaker stage, a finding supported immunologically by tissue-printing. To correlate pectin methylesterase expression during ripening to the degree of methylesterification of pectins in exopericarp cell walls, we subjected exopericarp tissue from 'UC82B' fruit to an immunocytochemical and ultrastructure study. Esterified pectin decreased in some regions of the exopericarp cell walls during fruit development but persisted in some regions as well. Less-esterified pectin was localized in the middle lamella of exopericarp cell walls during preripe stages, while in ripe fruit, this labeling was largely absent.Key words: pectin methylesterase (PME), immunocytochemistry, tissue-print, pectin esterification, Lycopersicon esculentum.