Genetics of Fruit Softening

Abstract

Ripe kiwifruit have a soft melting texture that, combined with distinctive flavours, aromas and colours, has made the fruit an international success. A key feature of successful New Zealand cultivars, e.g. Actinidia chinensis var. deliciosa 'Hayward' and Actinidia chinensis var. chinensis 'Hort16A', is their ability to be stored for long periods both at ambient temperature and in cool storage. In these cultivars, the majority of fruit softening occurs in the apparent absence of ethylene production. In contrast, late ripening is associated with autocatalytic ethylene production, where fruit enter the eating window and then proceed to senescence. The decline in fruit firmness during kiwifruit ripening is largely attributed to the disassembly of the fruit cell wall, which provides cellular rigidity and is responsible for intercellular adhesion. In this review, we consider the key genetic changes that occur during fruit softening in kiwifruit and relate these to changes in the cell wall during the same time period. Understanding these relationships is essential to the development of new kiwifruit cultivars with good postharvest characteristics, as long-storing fruit are relatively rare in A. chinensis var. chinensis and A. chinensis var. deliciosa germplasm material, and most other Actinidia species are characterised by rapid softening and limited shelf life.

Genome-Wide Identification and Expression Analysis of Polygalacturonase Gene Family in Kiwifruit (Actinidia chinensis) during Fruit Softening

“…It is well documented that cell wall disassembly is caused by the synergistic actions of a multitude of cell wall enzymes. In kiwifruit 'Hayward', the activities of cell wall associated enzymes and the change of cell wall component have been extensively studied [8,9,33]. It is mainly concluded that the activity of PG enzyme increases with ripening process and becomes abundant when fruit ripen [11,12], although a slight increase of PG activity during kiwifruit softening was previously reported [10].…”

“…Accompanied with the kiwifruit softening progress, the change of cell wall composition and the activities of cell wall enzymes occurred [8,9]. The main biochemical changes include pectin solubilisation, galactose loss, soluble pectin degradation, and a reduction in the molecular weight of xyloglucan [9]. The change of PG activity during 'Hayward' softening have been described previously [10][11][12], but their results seemed somewhat contradictory.…”

“…The previous studies indicated that the pattern of kiwifruit softening is sigmoidal with three distinct phases: Phase 1 slow, Phase 2 fast, and Phase 3 slow [6,8]. Accompanied with the kiwifruit softening progress, the change of cell wall composition and the activities of cell wall enzymes occurred [8,9]. The main biochemical changes include pectin solubilisation, galactose loss, soluble pectin degradation, and a reduction in the molecular weight of xyloglucan [9].…”

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

“…Kiwifruit is an unusual climacteric fruit as many ripening processes, including the majority of fruit softening, occur independently of climacteric ethylene ( Richardson et al., 2011 ). Climacteric (endogenous) ethylene production starts at the last stage of kiwifruit ripening ( Atkinson and Schroder, 2016 ), when fruit is becoming eating-soft, but can also be induced by exogenous ethylene or chilling exposure ( Minas et al., 2016 ). When ethylene is suppressed, the last stages of ripening including the production of aroma volatiles do not occur ( Atkinson et al., 2011 ).…”

Postharvest metabolomic changes in Pyrus ussuriensis Maxim. wild accession ‘Zaoshu Shanli’

“…For producers, however, providing fruit during the optimum window of consumption can be challenging. Fruit softening that occurs too quickly affects shelf life, is susceptible to physical injury and pathogen attack as well as increases transport and storage costs . Hence, there are strong economic drivers for understanding the fruit softening process.…”