PHYTOCHROME-INTERACTING FACTOR 7 and RELATIVE OF EARLY FLOWERING 6 act in shade avoidance memory in Arabidopsis

Shade avoidance helps plants maximize their access to light for growth under crowding. It is unknown, however, whether a priming shade avoidance mechanism exists that allows plants to respond more effectively to successive shade conditions. Here, we show that the shade-intolerant plant Arabidopsis can remember a first experienced shade event and respond more efficiently to the next event on hypocotyl elongation. The transcriptional regulator PHYTOCHROME-INTERACTING FACTOR 7 (PIF7) and the histone H3K27-demethylase RELATIVE OF EARLY FLOWERING 6 (REF6) were identified as being required for this shade avoidance memory. RNA-sequencing analysis revealed that shade induction of shade memory-related genes was impaired in the pif7 and ref6 mutants. Based on the enrichments of H3K27me3, REF6 and PIF7, we found that priming shade treatment induced PIF7 accumulation, which further recruited REF6 to demethylate H3K27me3 on the chromatin of certain memory-related genes, leading to a state poised for their transcription. Upon the second shade treatment, enhanced shade-mediated induction of these genes resulted in stronger hypocotyl growth responses. We conclude that the transcriptional memory mediated by epigenetic modification plays a key role in the ability of primed plants to remember previously experienced shade and acquire enhanced responses to recurring shade conditions. For REF6-HA binding ChIP, proREF6::REF6-HA/ref6-1 (N_WL and N_SH 8 h) and proREF6::REF6-HA/pif7-1 (N_SH 8 h) seedlings were prepared for each genotype of plants grown under white light and shade conditions (N, 4-day-old seedlings + 8 h white light + 40 h white light + 8 h triggering shade), using an anti-HA antibody (Lumiprobe sc-7392 X).