Comparison of cell wall changes of two different types of apple cultivars during fruit development and ripening

Fruit development and ripening is a complex procedure (Malusxdomestica Borkh.) and can be caused by various factors such as cell structure, cell wall components, and cell wall hydrolytic enzymes. In our study, we focused on the variations in fruit firmness, cell wall morphology and components, the activity of cell wall hydrolytic enzymes and the expression patterns of associated genes during fruit development in two different types of apple cultivars, the hard-crisp cultivar and the loose-crisp cultivar. In this paper, the aim was to find out the causes of the texture variations between the different type cultivars. Cell wall materials (CWMs), hemicellulose and cellulose content were strongly associated with variations in fruit firmness during the fruit development. The content of water soluble pectin (WSP) and chelator soluble pectin (CSP) gradually increased, while the content of ionic soluble pectin (ISP) showed inconsistent trends in the four cultivars. The activities of polygalacturonase (PG), beta-galactosidase (beta-gal), cellulase (CEL), and pectate lyase (PL) gradually increased in four cultivars. And the activities of PG, beta-gal, and CEL were higher in 'Fuji' and 'Honeycrisp' fruit with the fruit development, while the activity of PL of 'Fuji' and 'Honeycrisp' was lower than that of 'ENVY' and 'Modi'. Both four cultivars of fruit cells progressively became bigger as the fruit expanded, with looser cell arrangements and larger cell gaps. According to the qRT-PCR, the relative expression levels of MdACO and Md beta-gal were notably enhanced. Our study showed that there were large differences in the content of ISP and hemicellulose, the activity of PL and the relative expression of Md beta-gal between two different types of apple cultivars, and these differences might be responsible for the variations in the texture of the four cultivars.