ABA-mediated reprogramming of carotenoid metabolism under heat stress impairs tomato fruit ripening

Heat stress is a major limiting factor for fruit crop productivity and quality, yet the injury mechanisms in developing fruits remain poorly understood. To investigate this, tomato fruits harvested at the mature green stage were subjected to postharvest ripening under control (21 °C) and heat stress conditions (35 °C and 42 °C). At 21 °C, fruits ripened normally, developing full red coloration. In contrast, fruits exposed to 35 °C developed an orange hue due to reduced lycopene accumulation, which was attributed to suppressed precursor synthesis and increased lycopene-converting enzyme activity. At 42 °C, fruits failed to ripen altogether. Heat stress significantly disrupted hormonal dynamics: abscisic acid (ABA), ethylene, and auxin levels peaked prematurely and declined sharply, especially ethylene during late ripening stages, leading to impaired hormonal coordination. Additionally, heat stress triggered extensive metabolic reprogramming, with increased accumulation of flavonoids, phenolic acids, and specific amino acids, indicative of enhanced defense responses. Transcriptomic analysis revealed the activation of stress-related genes and suppression of ripening-associated pathways. Overall, our findings demonstrate that (1) heat stress accelerates ABA biosynthesis and signaling, which suppress lycopene accumulation through transcriptional and enzymatic regulation, and (2) tomato fruits prioritize stress adaptation over ripening under elevated temperatures, ultimately compromising fruit quality and marketability.