Cell separation in kiwifruit without development of a specialised detachment zone

Abstract

BackgroundUnlike in abscission or dehiscence, fruit of kiwifruit Actinidia eriantha develop the ability for peel detachment when they are ripe and soft in the absence of a morphologically identifiable abscission zone. Two closely-related genotypes with contrasting detachment behaviour have been identified. The ‘good-peeling’ genotype has detachment with clean debonding of cells, and a peel tissue that does not tear. The ‘poor-peeling’ genotype has poor detachability, with cells that rupture upon debonding, and peel tissue that fragments easily.ResultsStructural studies indicated that peel detachability in both genotypes occurred in the outer pericarp beneath the hypodermis. Immunolabelling showed differences in methylesterification of pectin, where the interface of labelling coincided with the location of detachment in the good-peeling genotype, whereas in the poor-peeling genotype, no such interface existed. This zone of difference in methylesterification was enhanced by differential cell wall changes between the peel and outer pericarp tissue. Although both genotypes expressed two polygalacturonase genes, no enzyme activity was detected in the good-peeling genotype, suggesting limited pectin breakdown, keeping cell walls strong without tearing or fragmentation of the peel and flesh upon detachment. Differences in location and amounts of wall-stiffening galactan in the peel of the good-peeling genotype possibly contributed to this phenotype. Hemicellulose-acting transglycosylases were more active in the good-peeling genotype, suggesting an influence on peel flexibility by remodelling their substrates during development of detachability. High xyloglucanase activity in the peel of the good-peeling genotype may contribute by having a strengthening effect on the cellulose-xyloglucan network.ConclusionsIn fruit of A. eriantha, peel detachability is due to the establishment of a zone of discontinuity created by differential cell wall changes in peel and outer pericarp tissues that lead to changes in mechanical properties of the peel. During ripening, the peel becomes flexible and the cells continue to adhere strongly to each other, preventing breakage, whereas the underlying outer pericarp loses cell wall strength as softening proceeds. Together these results reveal a novel and interesting mechanism for enabling cell separation.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-017-1034-2) contains supplementary material, which is available to authorized users.

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

“…The role which pectate lyase plays in kiwifruit softening remains largely unknown. A study by Prakash et al (2017) which used an antibody directed against kiwifruit pectate lyase did not reveal the presence of protein in the kiwifruit genotypes analyzed, despite gene expression being present. In this current study, while the expression of PL was significantly higher in 'AC-F' at FC4, it is unlikely this would influence pectin solubilization or the softening rate, as much of these processes had already occurred by this point.…”

“…Neutral sugar content was determined by gas chromatography (GC) after hydrolysis in trifluoroacetic acid (TFA), followed by the conversion of monosaccharides to alditol acetates ( Albersheim et al., 1967 ) as described in Prakash et al. (2017) .…”

“…Neutral sugar content was determined by gas chromatography (GC) after hydrolysis in trifluoroacetic acid (TFA), followed by the conversion of monosaccharides to alditol acetates (Albersheim et al, 1967) as described in Prakash et al (2017). Each neutral sugar analysis was carried out twice per firmness category with two technical replicates per biological replicate.…”

“…Xyloglucan endotransglycosylase (XET), xyloglucanase, BGal, and PME activity were extracted from frozen tissue as described in Prakash et al (2017), using a low salt (LS)-high salt (HS) buffer extraction approach. Enzyme activity assays were carried out using at least three extracts derived from bulk tissue (~10 pieces of fruit per softening stage) collected over 'season 2', with three to five technical replicates for each assay including appropriate controls.…”

“…Xyloglucanase activity was assayed by gel diffusion as described in Prakash et al (2017). The radius of the clearance zones after staining of the agar with Congo Red was digitally measured after photographing the plates (Nikon D80 digital SLR camera, with a 60 mm macro Nikkor lens, Nikon Corporation, Japan) using a program written in the programming language MATLAB ® which analyzes the image of a plate in RGB (red, green, blue) format.…”