Fruit development and maturation involve many physiological and biochemical changes. Some apple cultivars show undesirable ripening process leading to mealiness, which is characterized by texture deterioration resulting in soft, grainy and dry fruit.
To understand the molecular bases of mealiness development, we used the 120k AryANE microarray (1). Based on sensorial analysis over 6 years, six genotypes with contrasted fruit texture (mealy or not) were selected among a progeny. A global transcriptome analysis was performed over four time points during fruit development and cold storage. Interestingly, this analysis revealed one transcript coding a pectin methyl esterase (PME) which displayed an important modification in its expression level in accordance with the development of mealiness in fruits. Protein fusion experiment showed that this specific MdPME is secreted to the apoplast. Microscopic analysis revealed a progressive loss of cell to cell adhesion in mealy fruits in accordance with specific cell wall disassembly. Biochemical analysis pointed out specific alteration of pectin during fruit ripening. However, no global significant changes in pectin structural properties could be ascribed to mealiness. Consistent with the observations of Ng et al (2), we hypothesize that mealiness may be partially due to qualitative and spatial variations of pectin microarchitecture rather than quantitative pectin differences, and that these changes may occur early in fruit development. Overall, these data support the role of PME in cell wall remodeling during fruit maturation and suggest that PME act locally. This MdPME could be an early marker of texture disorder in apple.
(1) Celton et al., New Phytol, 2014. In Press.
(2) Ng et al., BMC Plant Biol. 2013.