Involvement of AOX and UCP pathways in the post-harvest ripening of papaya fruits

Abstract

Enhanced respiration during ripening in climacteric fruits is sometimes associated with an uncoupling between the ATP synthesis and the mitochondrial electron transport chain. While the participation of two energy-dissipating systems, one of which is mediated by the alternative oxidase (AOX) and the other mediated by the uncoupling protein (UCP), has been linked to fruit ripening, the relation between the activation of both mitochondrial uncoupling systems with the transient increase of ethylene synthesis (ethylene peak) remains unclear. To elucidate this question, ethylene emission and the two uncoupling (AOX and UCP) pathways were monitored in harvested papaya fruit during the ripening, from green to fully yellow skin. The results confirmed the typical climacteric behavior for papaya fruit: an initial increase in endogenous ethylene emission which reaches a maximum (peak) in the intermediate ripening stage, before finally declining to a basal level in ripe fruit. Respiration of intact fruit also increased and achieved higher levels at the end of ripening. On the other hand, in purified mitochondria extracted from fruit pulp the total respiration and respiratory control decrease while an increase in the participation of AOX and UCP pathways was markedly evident during papaya ripening. There was an increase in the AOX capacity during the transition from green fruit to the intermediate stage that accompanied the transient ethylene peak, while the O2 consumption triggered by UCP activation increased by 80% from the beginning to end stage of fruit ripening. Expression analyses of AOX (AOX1 and 2) and UCP (UCP1-5) genes revealed that the increases in the AOX and UCP capacities were linked to a higher expression of AOX1 and UCP (mainly UCP1) genes, respectively. In silico promoter analyses of both genes showed the presence of ethylene-responsive cis-elements in UCP1 and UCP2 genes. Overall, the data suggest a differential activation of AOX and UCP pathways in regulation related to the ethylene peak and induction of specific genes such as AOX1 and UCP1.

Alternative oxidase (AOX) constitutes a small family of proteins in Citrus clementina and Citrus sinensis L. Osb

“…The relatively high expression of CsAOXa and CSAOXd in fruits (>15 RPKM; Fig 6) may be related to the expression of AOX genes from other species producing fruits such as tomato, papaya or mango [55–58]. Some AOX genes were related to fruit maturation, ripening and post-harvest ripening in association with ethylene peak emission (climacteric fruits) [56, 57], while other AOX genes were associated to gametophyte development [58]. Some AOX genes related to climacteric fruit ripening presented elements responsive to ethylene in their promoter sequences [57].…”

“…Some AOX genes were related to fruit maturation, ripening and post-harvest ripening in association with ethylene peak emission (climacteric fruits) [56, 57], while other AOX genes were associated to gametophyte development [58]. Some AOX genes related to climacteric fruit ripening presented elements responsive to ethylene in their promoter sequences [57]. Here, the CsAOXa and CSAOXd genes did not present any elements responsive to ethylene in their promoter sequences; this could be related to the fact that citrus are non-climacteric fruits, or may suggest an involvement of these CsAOX genes in fruit formation more than in fruit ripening (Figs 2 and 6).…”

Glyoxylic acid overcomes 1-MCP-induced blockage of fruit ripening in Pyrus communis L. var. ‘D’Anjou’

“…This expression trend is reminiscent of the pre-climacteric maxima of AOX transcription in non-1-MCP treated pear fruit that had undergone full conditioning [15,23]. AOX pathway activity has been shown to contribute to the respiratory climacteric during ripening in tomato, papaya, and mango [16, [24][25][26]. While GLA activated AOX, the results of this transcriptomic analysis suggest that AOX activity in 1-MCP treated pear fruit is potentially regulated in a feedforward manner as a result of carbon metabolism upstream [24,25,27].…”

Genetic Research Progress of Fruit Ripening and Quality Control

“…The two energy consumption systems involved, one of which was mediated by alternating oxidase (AOX) and the other by uncoupling protein (UCP), were both related to fruit maturation (Seymour et al, 2013). To illustrate this problem, Oliveira et al (2015) monitored ethylene emissions and two uncoupling (AOX and UCP) pathways during maturation of papaya (from green to yellow). The results showed that papaya was a typical respiration climacteric fruit: endogenous ethylene emissions initially increased, reached a peak at the intermediate stage of maturity, and finally returned to basic levels before maturing.…”