Differential expression of cell-wall-modifying genes and novel cDNAs in apple fruit during storage

During postharvest storage, apple [Malus pumila P. Mill.] fruit softens and its texture changes noticeably, with adverse effects on fruit quality. These changes are a result of degradation of the cell wall and middle lamella. Enzymes that cause changes in the cell walls have been characterized, but temporal distribution of their activities and their molecular regulation during storage is not well understood. 'Honeycrisp' fruit does not soften significantly during storage in contrast to fruit from 'Macoun', which softens significantly during storage. Contrasting phenotypes of 'Honeycrisp' and 'Macoun' were analyzed for changes in transcript levels of four cell-wall-modifying genes in fresh and 3-month-stored fruit from both cultivars. A suppression-subtractive hybridization experiment identified 15 cDNAs differentially expressed in fresh or 3-month-stored 'Macoun' fruit. Transcript levels of these 15 cDNAs were further quantified by quantitative real-time polymerase chain reaction (qRT-PCR) in fresh and 3-month-stored fruit from both 'Macoun' and 'Honeycrisp'. The combination of a late increase in MdEXPA2 and decreased levels of MdPG and MdAFase1 transcript levels in 'Honeycrisp' fruit during storage may lead to its nonsoftening phenotype. Three cDNAs, potentially important for postharvest changes in apple fruit were also identified based on their different expression patterns in fresh and 3-month-stored 'Macoun' and 'Honeycrisp' fruit.