Examination of expansin genes as related to apple fruit crispness

The possible relationship of seven expansin genes to maintenance of apple (Malus × domestica Borkh.) crispness during storage was examined. Expression of expansin genes MdEXPA1, MdEXPA2, MdEXPA3, MdEXPA4, MdEXPA5, and MdEXPA7 was quantified among apple cultivars that remained crisp or lost crispness between harvest and after 8 weeks of cold storage. MdEXPA1 was expressed in low quantities in fresh fruit while expression of MdEXPA4, MdEXPA5, and MdEXPA7 was not detected among fresh or cold-stored apple cultivars. Among the expansin isoforms studied, MdEXPA2 and MdEXPA3 were the most abundantly expressed. A new expansin isoform, with homology to PcEXPA1 and low expression levels in fresh and cold-stored apples, was isolated in this study and designated MdEXPA8. Two alleles of MdEXPA2 were identified with different 3′UTR sequences, and 41 apple genotypes were allelotyped for MdEXPA2 gene to determine to what extent a relationship existed between allelotype and loss of fruit crispness. Paired t tests suggested that there was no relationship between allelotype and crispness maintenance. However, genotype sample size was small and may have been inadequate to detect any effect. The gene for 1-aminocyclopropane 1-carboxylate synthase, MdACS1, was also allelotyped in the 41 apple genotypes to determine if there might be an additive effect with MdEXPA2 allelotype and was found to be unsuitable as a marker for maintenance of apple fruit crispness for the crosses used in this study. Furthermore, fragment analyses showed that MdEXPA2 allelotype had no effect on transcript stability, and protein blots suggested no effect on translation.