Manipulating the Light Systemic Signal HY5 Greatly Improve Fruit Quality in Tomato.

Fruit ripening in tomato fruits comprises dramatic metabolic changes that are regulated by environmental factors. Light not only drives photosynthesis but also acts as a critical signal regulating plant growth, development, and the quality of produce. However, it is unclear how plants sense light signals in the environment to regulate fruit quality. It is demonstrated that the accumulation of Long Hypocotyl 5 (HY5) protein peaks at the breaker stage of fruit maturity, independent of fruit bagging. Genetic manipulation of HY5 reveals that its knockout delays carotenoid synthesis and sucrose conversion, while its overexpression promotes fruit ripening. Molecular and biochemical analyses show that HY5 directly activates the transcript of the key carotenoid synthesis genes, such as Phytoene Synthase 1 (PSY1) and Phytoene Desaturase (PDS), as well as the sucrose metabolism genes, including Lycopersicum Invertase (LIN5, LIN6), Vacuolar Invertase (VI) and Sucrose Synthase (SS1, SS7). Importantly, grafting experiments reveal that HY5 acts as a systemic signal, translocating from leaves to fruits to promote ripening. Furthermore, nightly lighting with red or blue LED greatly improves fruit quality. In summary, the results establish that HY5 as a mobile protein that mediates the systemic light regulation of fruit ripening, offering practical applications for improving fruit quality.