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 o f the flesh, depreciates greatly the commercial quality of early-season Cha rentais cantaloupe melons (Cucumis melo L. cv. Talma), Although it is accep ted that the genotype and a number of physiological and environmental facto rs play a role in the development of the syndrome, the intimate mechanisms responsible for water-soaking remain unknown. We report here on an integrat ed study of the development of water-soaking in fruit. Using nuclear magnet ic 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 wh ich ethylene action was blocked by 1-methylcyclopropene remained sensitive to eater-soaking. Moreover, the expression of two genes encoding key enzyme s in ethylene biosynthesis remained unchanged in response to water-soaking, The major changes observed concerned a protein implicated in calcium signa lling processes, While the amount of total calmodulin, the ubiquitous calci um binding protein, was not modified, a particular calmodulin-binding prote in (CaM-BP) was absent in water-soaked but not in sound mature tissues. Thi s CaM-BP may be a marker or a determinant of this physiological disorder.