Matrix glycan depolymerisation and xyloglucan endohydrolase activities in ethylene‐treated watermelon (Citrullus lanatus) fruit

Abstract

AbstractBACKGROUND: Ethylene‐induced placental tissue water soaking in harvested watermelon fruit is accompanied by cell separation and collapse, depolymerisation of water‐ and chelator‐soluble pectins, a reducton in total uronic acids and increased polygalacturonase activity. The aim of this study was to investigate whether ethylene‐induced changes in placental tissue cell walls also affected matrix glycans (hemicelluloses).RESULTS: Polymers from 0 and 6 day air‐treated fruits were similar in molecular mass distribution, with the majority of polymers eluting within the void volume of Sepharose 6B. Polymers from ethylene‐treated fruit showed significant molecular mass downshifts involving xyloglucan. Xylose and glucose together comprised nearly 70% of 2 mol L−1 alkali‐soluble neutral sugars. Treatment of watermelon fruit with ethylene was not accompanied by changes in matrix glycan composition, indicating that in situ depolymerisation did not result in altered solubility and/or loss of xyloglucan. Cell‐free protein extracts from watermelon placental tissue from both air‐ and ethylene‐treated fruits degraded tamarind seed xyloglucan, resulting in significant molecular mass downshifts without production of oligomers and monomers. The capacity of cell‐free extracts to degrade tamarind xyloglucan and watermelon matrix glycans was similar for 6 day ethylene‐ and air‐treated fruits.CONCLUSION: Collectively the data suggest that enhanced depolymerisation of xyloglucan in ethylene‐treated watermelon fruit arises from factors other than altered enzyme levels. Accordingly, the depolymerisation of pectin and matrix glycans would not appear to be solely responsible for ethylene‐induced water soaking in watermelon. Copyright © 2007 Society of Chemical Industry