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Pectin de-esterification and fruit softening: revisiting a classical hypothesis Luis F Goulao Centro de Ecofisiologia, Bioquímica e Biotecnologia Vegetal, Instituto de Investigação Científica Tropical (IICT), Av. da República - Quinta do Marquês, Oeiras, Portugal Abstract Purpose of review: Pectin methylesterase (PME)-mediated de-esterification of pectins is considered a crucial event in fruit softening during ripening. However, PMEs act biochemically to produce different, and sometimes opposite, effects on the polysaccharide’s structure, according to different regulation mechanisms. Therefore, the exact role of PMEs in fruit softening needs to be further elucidated. This article reviews recent and classical knowledge concerning PME’s mode of action and pectin microstructure and provocatively discusses possible complementary pathways resulting from PME action or pectin de-esterification associated with fruit ripening. Findings: There is extensive literature associating enzymatic activity, mRNA transcription and protein accumulation of several cell wall-modifying enzyme families with fruit softening in a large number of fruit species. However, functional analysis and clear-cut demonstration of hypotheses are still needed to understand the exact role of each family in the process. Recent models concerning pectin microstructure and network superstructure, the use of mutants and genotypes with distinct softening behaviours belonging to the same species, the association of new players like PME inhibitors or pectin acetylesterases, and the occurrence of other events unrelated to enzymatic or cell wall modification have been reported in isolated research lines. A compilation of recent and classical findings regarding PME regulation and mode of action should be collectively considered to unravel the specific role of pectin de-esterification in the fruit softening event. Directions for future research: Specific aspects of PME action on the pectic network have been extensively discussed. The integration of the results obtained from different approaches and the testing of different hypotheses should be done and collectively re-evaluated to provide knowledge aimed at planning novel directions and improved strategies for the postharvest management of fleshy fruits with the objective of increasing the shelf-life of produce and meeting consumer satisfaction. Keywords: cell wall; de-esterification; fruit softening; pectin Stewart Postharvest Review 2010, 1:7 Published online 01 March 2010 doi: 10.2212/spr.2010.1.7 |