Cell wall changes during strawberry fruit ripening

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The primary cell wall of fleshy fruits shows structural and compositional changes during ripening. These changes lead to loss of firmness and facilitate the attack of pathogens, decreasing postharvest life. Cell wall of dicots is a complex structure constituted by three groups of polysaccharides: cellulose, hemicelluloses and pectins. Cellulose polymers are assembled in microfibrils that form a network. Each microfibril is coated and cross-linked together by hemicelluloses. The spaces in the cellulose/hemicellulose matrix are filled by the pectins, which form an additional network. During ripening, cell wall polymers show an increased solubility, depolymerization and the loss of neutral sugars, which have been associated to the action of a number of enzymes and proteins. Most of the biochemical and molecular studies about cell wall degradation during ripening have been done in tomato, the model of climacteric fruit. However, the relative extent and timing of cell wall changes may vary among species. This review covers most of the studies about cell wall degradation during ripening of strawberry, which has been chosen as a model of nonclimacteric fruit. Strawberry shows a fast growth, intense softening and, as a consequence, a short postharvest life. As in other soft fruit, total cell wall content decreases during ripening, but analysis using solid-state 13C NMR showed that the cellulose content remains almost constant during ripening, indicating that the other cell wall components must be degraded. Studies about metabolism of hemicelluloses and pectins have shown contradictory results. Primary researches done in the 80ies detected important depolymerization in hemicelluloses. In this sense, an enhanced endoglucanase activity and endoglucanase gene expression were detected. However, more recent works done in different cultivars have detected scarce hemicellulose depolymerization. Moreover, antisense lines with a strong suppression of an endoglucanase gene did not show fruit firmness changes. On the contrary, primary reports indicated that the solubility of pectins increased during ripening but their depolymerization was low. Congruently, polygalacturonase activity was not detected in some firm cultivars. However, cultivars with intense softening showed a detectable polygalacturonase activity and an important depolymerization of pectins. More recently, a fruit-specific polygalacturonase gene was described and a lower pectin solubilization in strawberries with antisense expression of a pectate lyase gene was described. Additionally, genes related with metabolism of hemicelluloses (expansins; β-xylosidase) or pectins (β-galactosidase) were also detected and characterized as fruit specific and ripening related. Información suministrada por el agente en SIGEVA