The impact of growing season on the ethylene biosynthesis and signaling pathways of a heat tolerant tomato during off-vine postharvest ripening

Ethylene initiates and mediates fruit ripening throughout a cascade of signal transductions from ethylene re-ceptors (ETRs) to ethylene response factors (ERFs). High temperatures impair fruit ripening by suppressing ethylene biosynthesis and parts of physiological and biochemical processes. This study made use of the ethylene inhibitor (1-MCP) and exogenous ethylene to investigate the ethylene biosynthesis and signaling pathways of a heat tolerant tomato cultivar, cv. 'Savior', during off-vine postharvest ripening during two distinct growing seasons. Mature green 'Savior' tomatoes grown in winter (mild temperature) and summer (high temperature) were harvested at and treated with 1-MCP or exogenous ethylene. During off-vine ripening, summer fruit showed a different ethylene biosynthesis profile compared to winter fruit. The application of exogenous ethylene did not accelerate fruit ripening of the heat tolerant cultivar. 1-MCP suppressed ethylene production for a long time, and delayed fruit ripening of winter tomato, while for summer fruit, a limited effect of 1-MCP on inhibiting ethylene biosynthesis and delaying ripening was observed despite the strong suppression of ACC oxidase (ACO) activity. Our work also showed that a 1-MCP treatment reduced gene expression of several ethylene receptors (ETR) but did not have a strong effect on constitutive triple response (CTR), except CTR1, and ethylene-insensitive protein 2 (EIN2). Overall, our results indicate that i) ethylene biosynthesis is differentially regulated in winter as compared to summer fruit by ethylene signaling, ii) a high cultivation temperature impacts ethylene biosynthesis and ethylene sensitivity during fruit ripening despite the fact that this is a heat tolerant cultivar, iii) the expression of most ETRs and CTRs, and especially EIN2 did not respond to the 1-MCP or ethylene treatment.