HFR1/PIF module balances chlorophyll biosynthesis to promote greening during de-etiolation in Arabidopsis.

During de-etiolation, dark-grown seedlings are exposed to light, which triggers chlorophyll biosynthesis and greening of the cotyledons. LONG HYPOCOTYL IN FAR-RED 1 (HFR1) is known to interact with PHYTOCHROME INTERACTING FACTORs (PIFs) to regulate many light-mediated developmental processes in Arabidopsis. Here, we found that seedlings overexpressing HFR1 [HFR1(ΔN)-OE] showed photo-oxidative bleaching and reduced greening during de-etiolation, which is similar to pif1-1. To elucidate the role of HFR1 in regulating de-etiolation and greening, transcriptome analyses were performed on seedlings of hfr1-5, HFR1(ΔN)-OE, and pif mutants under 0, 1, and 6 h of de-etiolation. We found that PIFs and HFR1 exert opposing regulation of genes related to chlorophyll biosynthesis, photosynthesis, and oxidative stress during de-etiolation. Importantly, HFR1 promoted the expression of genes related to antioxidant activity and inhibition of programmed cell death, along with reduced protochlorophyllide (Pchlide) accumulation, potentially explaining the attenuated photobleaching observed in HFR1(ΔN)-OE, as compared to pifq. Further analysis of the tetrapyrrole biosynthetic pathway revealed that gene regulation by HFR1 and PIFs at 6 h de-etiolation coincides with their photo-oxidative phenotypes. While HFR1 suppresses the tetrapyrrole biosynthesis genes, PIFs promote their expression, which influences the accumulation of protochlorophyllide and burst of singlet oxygen during de-etiolation, thereby causing photobleaching.