For shade‐intolerant plants, changes in light quality through competition from neighbors trigger shade avoidance syndrome (SAS): a series of morphological and physiological adaptations that are ultimately detrimental to plant health and crop yield. Phytochrome‐interacting factor 7 (PIF7) is a major transcriptional regulator of SAS in Arabidopsis; however, how it regulates gene expression is not fully understood. Here, we show that PIF7 directly interacts with the histone chaperone anti‐silencing factor 1 (ASF1). The ASF1‐deprived asf1ab mutant showed defective shade‐induced hypocotyl elongation. Histone regulator homolog A (HIRA), which mediates deposition of the H3.3 variant into chromatin, is also involved in SAS. RNA/ChIP‐sequencing analyses identified the role of ASF1 in the direct regulation of a subset of PIF7 target genes. Furthermore, shade‐elicited gene activation is accompanied by H3.3 enrichment, which is mediated by the PIF7‐ASF1‐HIRA regulatory module. Collectively, our data reveal that PIF7 recruits ASF1‐HIRA to increase H3.3 incorporation into chromatin to promote gene transcription, thus enabling plants to effectively respond to environmental shade.

Light and temperature are two key environmental signals that share several molecular components that, in turn, regulate plant growth. Darkness and high ambient temperatures promote skoto-and thermomorphogenesis, including stem elongation. Heat shock proteins 90 (HSP90s) facilitate the adaptation of organisms to various adverse environmental stimuli.Here, we showed that HSP90s are required for hypocotyl elongation during both skotoand thermomorphogenesis.When HSP90s activities are impaired by the knockdown of HSP90s expression or the application of HSP90 inhibitors, the expression levels and protein abundance of PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) markedly decreased. EARLY FLOWERING 3 (ELF3) deficiency was resistant to the inhibition of HSP90s activities. Furthermore, HSP90s interacted with and destabilized ELF3. In the CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) mutant, the changes in endogenous PIF4 and ELF3 protein levels caused by the inhibition of HSP90s activities were abolished. HSP90s enhanced the interaction between COP1 and ELF3, reduced ELF3 functional effects on PIF4 and modulated hypocotyl elongation during skotoand thermomorphogenesis.Our results indicated that HSP90s participate in light and temperature signalling via the COP1-ELF3-PIF4 module to regulate hypocotyl growth in changing environments.

Shade avoidance syndrome (SAS) commonly occurs in plants experiencing vegetative shade, triggering a series of morphological and physiological changes for the plants to reach more light. A number of positive regulators, such as PHYTOCHROME-INTERACTING 7 (PIF7), and negative regulators, such as PHYTO-CHROMES, are known to ensure appropriate SAS. Here, we identify 211 shade-regulated long non-coding RNAs (lncRNAs) in Arabidopsis. We further characterize PUAR (PHYA UTR Antisense RNA), a lncRNA produced from the intron of the 5 0 UTR of the PHYTO-CHROME A (PHYA) locus. PUAR is induced by shade and promotes shade-induced hypocotyl elongation. PUAR physically associates with PIF7 and represses the shade-mediated induction of PHYA by blocking the binding of PIF7 to the 5 0 UTR of PHYA. Our findings highlight a role for lncRNAs in SAS and provide insight into the mechanism of PUAR in regulating PHYA gene expression and SAS.