Spatial-resolved analysis of histological and biochemical alterations induced by water-soaking in melon fruit

Water-soaking, a physiological disorder characterised by a glassy texture of the flesh, depreciates greatly the commercial quality of early-season Charentais cantaloupe melons (Cucumis melo L. cv. Talma), Although it is accepted that the genotype and a number of physiological and environmental factors play a role in the development of the syndrome, the intimate mechanisms responsible for water-soaking remain unknown. We report here on an integrated study of the development of water-soaking in fruit. Using nuclear magnetic resonance (NMR) imaging, we have shown that water mobility increased in the diseased tissues. Alteration of the cell wall and the presence of large intercellular spaces were correlated with a severe depletion of cell wall calcium. Water-soaking developed during the late stages of fruit ripening, but no correlation was found with ethylene biosynthesis, Thus, fruits in which ethylene action was blocked by 1-methylcyclopropene remained sensitive to eater-soaking. Moreover, the expression of two genes encoding key enzymes in ethylene biosynthesis remained unchanged in response to water-soaking, The major changes observed concerned a protein implicated in calcium signalling processes, While the amount of total calmodulin, the ubiquitous calcium binding protein, was not modified, a particular calmodulin-binding protein (CaM-BP) was absent in water-soaked but not in sound mature tissues. This CaM-BP may be a marker or a determinant of this physiological disorder.