Light Signaling-Dependent Regulation of Photoinhibition and Photoprotection in Tomato.

Photoreceptor-mediated light signaling plays a critical role in plant growth, development, and stress responses but its contribution to the spatial regulation of photoinhibition and photoprotection within the canopy remains unclear. Here, we show that low-red/far-red (-/) ratio light conditions significantly alleviate PSII and PSI photoinhibition in the shade leaves of tomato () plants. This protection is accompanied by a phytochrome A-dependent induction of LONG HYPOCOTYL5 (HY5). HY5 binds to the promoter of 5 (), triggering ()-dependent HO production in the apoplast. Decreased levels of , , and transcripts increased cold-induced photoinhibition and abolished -/-induced alleviation of photoinhibition. -/ illumination induced nonphotochemical quenching (NPQ) of chlorophyll fluorescence and increased the activities of Foyer-Halliwell-Asada cycle enzymes and cyclic electron flux (CEF) around PSI. In contrast, decreased , , and transcript levels abolished the positive effect of -/ on photoprotection. Loss of -dependent CEF led to increased photoinhibition and attenuated -/-dependent NPQ. These data demonstrate that HY5 is an important hub in the cross talk between light and cold response pathways, integrating ABA and reactive oxygen species signaling, leading to the attenuation of photoinhibition by enhanced induction of photoprotection in shade leaves.