Dissecting the molecular differences that lead to apple and pear texture

Abstract

During ripening in the closely related pome fruit apple (Malus x domestica) and pear (Pyrus spp.), different cultivars soften to textures ranging from firm and crisp through to soft and melting. The primary determinant of this process is the modification of cell wall polysaccharides by enzymes, which will be controlled in part at the level of gene transcription and will involve production of the plant hormone ethylene. The aim of this thesis was to better understand the genetic elements underlying these textural differences. The first section measured the changes in fruit firmness and ethylene production in 3 melting and 3 crisp pear cultivars during fruit ripening. In the second section specific genes were identified which were likely to be involved in the development of different textures, from a class of cell wall-modifying genes known as the glycosyl hydrolases, using a whole-genome approach. Another cell wall-related family, the expansins, was also included. Thirdly the regulation of genes identified as differentially expressed between the melting and crisp fruit was assessed by a trans-activation assay of the gene promoters by the top 80 most highly expressed transcription factors in ripening apple. The results uncovered a pear cultivar which did not develop a melting texture despite producing ethylene. In this cultivar expression profiles of cell wall modifying genes were more similar to that of non-ethylene producing varieties, suggesting a decoupling of ethylene regulation from cell wall-modifying genes. Characterisation of whole glycosyl hydrolase families revealed differences in the expression of many different glycosyl hydrolase genes during ripening between crisp- and melting-textured apples and pears (putative β-glucosidase, β- glucosylceramidase, xyloglucan endotransglycosylase, polygalacturonase, xylan 1,4-β- xylosidase, β-galactosidase, and cellulase encoding genes). Analysis of the transcriptional regulation of two of these genes (EXPA2, XTH1) uncovered common transcription factors capable of activating expression of both of these genes as well as a transcription factor with a PHD finger domain which was capable of activating expression of the promoter of EXPA2 in the melting pear cultivar ‘Comice’, but not its homologue from the crisp apple cultivar ‘Royal Gala’. Analysis of the expression of the activating transcription factors uncovered differences in expression in a MADS-box and an AP2 transcription factor between the fruit differing in texture. Together, these results suggest that differences in the regulation of cell wall-modifying genes at the level of transcription may play an important role in the development of different textures in closely-related apple and pear cultivars.