Kiwifruit softening: A cell wall study

Abstract

Cell walls are important for providing structural integrity to fruit tissues during softening. While there are reports on changes relating to fruit softening in kiwifruit cell walls, this study was the first to examine two closely related genotypes of Actinidia chinensis with different softening rates, MPM (a fast softener) and ACS (a slow softener), using an integrated approach of cell wall composition analysis, enzyme activity assays, microscopy and molecular techniques. The aim was to identify whether the different softening rates between genotypes were influenced by fundamental variations in the chemistry of the cell walls, or by comparative increases or decreases of gene expression or activity of the enzymes that cause the same cell wall degradative processes to occur at different speeds. While the pectin domain of the cell wall showed several dissimilarities between MPM and ACS, the main differences observed were in the xyloglucan domain. Compared to MPM, ACS appeared to have a higher xyloglucan content in the tightly bound 4 M KOH fraction and the cell wall residue, a result supported by a higher distribution of monoclonal antibody LM15 epitopes (related to xyloglucan) in all cell types in ACS. These results suggest an increased cell wall structural integrity in ACS which may resist the changes occurring to the xyloglucan-cellulose structure in MPM. Xyloglucan-modifying enzymes also varied between MPM and ACS. MPM had a higher xyloglucan endohydrolase (XEH) activity from the unripe stage, suggesting a weakened network which might be more easily accessible to other cell wall hydrolases from early in softening. Differences in the expression patterns of xyloglucan transglycosylase/hydrolase (XTH) genes showed that several were predominantly expressed in MPM and not ACS, suggesting that different isoforms of XTH were present in these genotypes. Results showed that softening of these genotypes was indeed influenced by differences in cell wall composition, gene expression and enzyme activity. Knowledge of the cell wall components, structural properties and genes expressed during softening can be applied to identify chemical and genetic markers for breeding kiwifruit to enable early stage selection of genotypes with a preferred softening rate, giving the New Zealand kiwifruit industry a competitive edge in the global kiwifruit market.