Evidence for the Involvement of Vernalization-related Genes in the Regulation of Cold-induced Ripening in ‘D’Anjou’ and ‘Bartlett’ Pear Fruit

Abstract

Physiological studies have characterized the conditioning requirements for a range of European pear cultivars under defined conditioning temperatures, exogenous ethylene application regimes, and other pre-harvest treatments 8-11. While exogenous ethylene treatment reduces cold conditioning needs, in most European pear cultivars it does not entirely supplant the need for cold conditioning. This indicates that cold-dependent mechanisms are partly responsible for regulating the development of ripening competency, which in turn impacts the quality and marketability of pear fruit. Interestingly, in contrast to P. communis and P. bretschneideri, many Japanese pear (Pyrus pyrifolia L.) varieties have no conditioning requirements and are also regarded as non-climacteric fruits because they do not display the characteristic S1 to S2 transition during ripening 12. Additionally, 'Bosc' , unique from other European pear cultivars, acquires competency for ripening with exogenous ethylene only, needing no chilling to ripen 8. Requirement of cold exposure in pear to induce ripening is reminiscent of other natural cold temperature-dependent developmental phenomena, such as the vernalization and stratification that are needed for flowering and seed germination, respectively. The genes that regulate vernalization and stratification have been well-studied in model organisms, including Arabidopsis, wheat, and barley 13-15 ; however, similar gene homologues have not yet been reported in cold-induced fruit ripening. With respect to flowering, the process of developmental initiation following an environmentally governed dormant state is known as endodormancy release 16. Thus far, endodormancy release has not been used to characterize ripening after chilling, although the two processes share many similarities with regards to the timing and environmental nature of cold required, suggesting that similar genetic and regulatory mechanisms may govern these processes. Various forms of the chilling requirement for ripening have been described in avocado and mango, although to a lesser degree than in pear, as the former are more prone to chilling injury 17,18. Few recent studies have utilized a transcriptomics approach to characterize the molecular underpinnings of cold-induced S1 to S2 transition in pear. A complex interaction of genes is involved in regulating phytohormones, secondary messengers, signaling pathways, respiration and chromatin modification that underlies cold-induced progression of ripening 19-23. Genes associated with phytohormones such as abscisic acid (ABA), auxin, and jasmonic acid, along with several transcription factors were implicated in low-temperature-mediated enhancement of ripening in 'Bartlett' 21. In 'Passe Crassane, ' the impact of low temperature-induced ethylene (LT) and the effects of exogenous ethylene treatments were evaluated using a transcriptomics approach 24. It was observed that the expression of a subset of the LT-induced differentially expressed genes was disrupted by 1-MCP treatment, indicating that ...

Beyond Ethylene: New Insights Regarding the Role of Alternative Oxidase in the Respiratory Climacteric

“…Recent independent studies, as well as reviews, have indicated that such modifications to the classical climacteric ripening implicate several pathways that operate concomitantly with ethylene biosynthesis and perception, including AOX respiration, signaling by reactive oxygen species (ROS), and pathways that may be triggered by changes in epigenetic signatures (Perotti et al, 2014;Kumar et al, 2016;Farinati et al, 2017;Bucher et al, 2018;Hendrickson et al, 2019;Hewitt et al, 2020b). New advances in genome editing have provided further insight regarding potential sites for variation in ripening, as manipulation of upstream transcriptional regulators (Ito et al, 2017) leads to alterations in fruit texture, photoperiodic response, and posttranscriptional regulation of ripening-related genetic elements (Martín-Pizarro and Posé, 2018).…”

“…In zucchini and sweet pepper, cold-induced activation of AOX results in the mitigation of chilling injury (Aghdam, 2013;Hu et al, 2014;Carvajal et al, 2015). Recently, it was demonstrated that completion of cold conditioning, facilitating the S1-S2 transition, in European pear coincides with preclimacteric maxima in AOX transcript accumulation (Hendrickson et al, 2019;Dhingra et al, 2020;Hewitt et al, 2020b).…”

“…Such ROS-induced retrograde signaling leads to modulation of AOX expression, as has been shown in potato and pea. It has been hypothesized that in this way, ROS may facilitate crosstalk between respiration and ethylene via AOX activation in fruit ripening (Marti et al, 2009;Hewitt et al, 2020b;Hua et al, 2020).…”

“…When AOX was inhibited via RNA interference, the expression of these transcriptional regulators decreased (Xu et al, 2012). Because CNR acts upstream of ethylene biosynthesis and the NR receptor acts downstream, this observation suggests that AOX plays a yet uncharacterized role in ripening mediated by transcriptional regulators that affect ethylene biosynthesis, signal transduction, and response (Giovannoni, 2004;Seymour et al, 2013b;Hewitt et al, 2020b). Interestingly, it has been hypothesized that the activation of both NOR and RIN may be linked, either directly or indirectly, to jasmonic acid signaling (Czapski and Saniewski, 1992).…”

“…Other fruits can be induced to ripen more quickly through appropriate cold storage (e.g., avocado, 14 days, 5-10°C; dashed blue arrow; Eaks, 1983). Expression and activity of alternative oxidase (AOX) have been implicated in preclimacteric and/or climacteric stages of some fruits (e.g., European pear, papaya, banana, and tomato; Woodward et al, 2009;Oliveira et al, 2015;Hendrickson et al, 2019;Hewitt et al, 2020b). Peak AOX expression, as indicated in the cited studies, is indicated by vertical dashed red lines.…”