A kiwifruit bHLH149 transcription factor modulates carotenoid biosynthesis by directly activating <i>LCYB</i> during postharvest ripening

The carotenoid content in kiwifruit undergoes dynamic changes during the ripening process, yet the underlying mechanism remains unclear. Herein, the variation in carotenoid content was investigated during the postharvest ripening process of kiwifruit (Actinidia valvata). The results demonstrated a gradual increase in total carotenoid content during postharvest ripening and softening, accompanied by a deepening of the yellow flesh. The levels of all five carotenoid components also exhibited an upward trend with fruit ripening, with lutein content being the highest among them, thus indicating its pivotal role as the most significant carotenoid component in kiwifruit. The expression analysis of genes associated with carotenoid metabolism during postharvest storage revealed an upward trend in the expression levels of biosynthesis-related genes, while a downward trend was observed for degradation-related genes. Notably, the lycopene beta-cyclase (LCYB) gene exhibited the highest expression levels, which were significantly and positively correlated with carotenoid content. A basic helix-loop-helix transcription factor protein, bHLH149, capable of binding to the LCYB promoter was identified through yeast one-hybrid library screening. The expression of bHLH149 increased during fruit ripening and showed a positive correlation with carotenoid content. As a nuclear localization protein, bHLH149 acts as a transcriptional activator and significantly enhances LCYB promoter activity, thereby promoting carotenoid biosynthesis. The collective findings suggest that bHLH149 may play a role in the carotenoid biosynthesis pathway during kiwifruit ripening by positively regulating the transcription of LCYB, thereby enhancing our comprehension of the regulatory network governing carotenoid biosynthesis in kiwifruit.