Fruit From Two Kiwifruit Genotypes With Contrasting Softening Rates Show Differences in the Xyloglucan and Pectin Domains of the Cell Wall

Abstract

Fruit softening is controlled by hormonal and developmental cues, causing an upregulation of cell wall-associated enzymes that break down the complex sugar matrices in the cell wall. The regulation of this process is complex, with different genotypes demonstrating quite different softening patterns, even when they are closely related. Currently, little is known about the relationship between cell wall structure and the rate of fruit softening. To address this question, the softening of two Actinidia chinensis var. chinensis (kiwifruit) genotypes (a fast 'AC-F' and a slow 'AC-S' softening genotype) was examined using a range of compositional, biochemical, structural, and molecular techniques. Throughout softening, the cell wall structure of the two genotypes was fundamentally different at identical firmness stages. In the hemicellulose domain, xyloglucanase enzyme activity was higher in 'AC-F' at the firm unripe stage, a finding supported by differential expression of xyloglucan transglycosylase/hydrolase genes during softening. In the pectin domain, differences in pectin solubilization and location of methyl-esterified homogalacturonan in the cell wall between 'AC-S' and 'AC-F' were shown. Side chain analyses and molecular weight elution profiles of polyuronides and xyloglucans of cell wall extracts revealed fundamental differences between the genotypes, pointing towards a weakening of the structural integrity of cell walls in the fast softening 'AC-F' genotype even at the firm, unripe stage. As a consequence, the polysaccharides in the cell walls of 'AC-F' may be easier to access and hence more susceptible to enzymatic degradation than in 'AC-S', resulting in faster softening. Together these results suggest that the different rates of softening between 'AC-F' and 'AC-S' are not due to changes in enzyme activities alone, but that fundamental differences in the cell wall structure are likely to influence the rates of softening through differential modification and accessibility of specific cell wall polysaccharides during ripening.

Cell Wall Calcium and Hemicellulose Have a Role in the Fruit Firmness during Storage of Blueberry (Vaccinium spp.)

“…In this regard, the researchers studied the metabolism of the hemicelluloses of pears in refrigerated storage, determining that in the level of changes correlated with the extension of time in storage [ 61 ]. Similarly, kiwifruit with different softening rates were compared, and it was found that the slow softening genotype showed higher concentrations of glucose and xylose, hemicellulose associated components, in the later stages of softening [ 62 ]. This aspect is relevant, given that in cold storage the cell wall continues to undergo modifications that affect fruit firmness.…”

Preharvest Application of Chitosan Improves the Postharvest Life of ‘Garmrok’ Kiwifruit through the Modulation of Genes Related to Ethylene Biosynthesis, Cell Wall Modification and Lignin Metabolism

“…In addition, fruit firmness is closely associated with changes in the biochemical properties of the cell wall. Polygalacturonase (PG) is a key enzyme involved in cell wall modifications during fruit ripening, with its important functions in degradation of soluble pectin and depolymerization of solubilized pectin [50][51][52]. In kiwifruit, there are three polygalacturonase genes, namely, PGA, PGB, and PGC.…”

“…In kiwifruit, there are three polygalacturonase genes, namely, PGA, PGB, and PGC. Of these, PGC is a predominant gene expressed throughout the softening of kiwifruit [50,51,53]. The suppression of the AdPGC gene in preharvest chitosan-treated 'Garmrok' kiwifruit during the early phase of storage (i.e., 30 and 60 days) may be associated with the delayed kiwifruit softening and retained firmness.…”

“…The suppression of the AdPGC gene in preharvest chitosan-treated 'Garmrok' kiwifruit during the early phase of storage (i.e., 30 and 60 days) may be associated with the delayed kiwifruit softening and retained firmness. Expansins (EXPs) are non-enzymatic proteins that have been considered to play a role in loosening and extension of the cell wall (the cellulose-xyloglucan hydrogen bonds) [50,52,54]. Yang et al [54] reported that two expansin genes AdEXP1 and AdEXP2 are actively involved in kiwifruit (A. deliciosa 'Bruno') ripening and softening.…”

Consensus co-expression network analysis identifies AdZAT5 regulating pectin degradation in ripening kiwifruit

“…At physiological level, the volume of intercellular air spaces was proven to be enlarged while cell wall material shrinked together with reduction of xyloglucan molecular weight [28] , [29] . Furthermore, the activities of critical enzyme, such as XTH, PG, PME, β-Gal were investigated by several reports [19] , [30] , [31] , [32] , [33] , although there seem to be some conflict among them. At molecular level, it was reported that AdXTH5 can be actively regulated by AdEIL2 (EIN3-Like) and AdEIL3 to promote softening [34] .…”