Convergence of CONSTITUTIVE PHOTOMORPHOGENESIS 1 and PHYTOCHROME INTERACTING FACTOR signalling during shade avoidance

Summary Shade‐avoidance responses require CONSTITUTIVE PHOTOMORPHOGENESIS 1 ( COP 1) but the mechanisms of action of COP 1 under shade have not been elucidated. Using simulated shade and control conditions, we analysed: the transcriptome and the auxin levels of cop1 and phytochrome interacting factor 1 ( pif1 ) pif3 pif4 pif5 ( pifq ) mutants; the dynamics of ELONGATED HYPOCOTYL 5 ( HY 5) and LONG HYPOCOTYL IN FAR ‐ RED ( HFR 1) proteins; and the epistatic relationships between cop1 and pif3 , pif4 , pif5 , hy5 and hfr1 mutations in Arabidopsis thaliana . Despite severely impaired shade‐avoidance responses, only a few genes that responded to shade in the wild‐type failed to do so in cop1 . Shade enhanced the convergence between cop1 and pifq transcriptomes, mainly on shade‐avoidance marker genes. Shade failed to increase auxin levels in cop1 . Residual shade avoidance in cop1 was not further reduced by the pif3 , pif4 or pif5 mutations, suggesting convergent pathways. HFR 1 stability decreased under shade in a COP 1‐dependent manner but shade increased HY 5 stability. The cop1 mutant retains responses to shade and is more specifically impaired in shade avoidance. COP 1 promotes the degradation of HFR 1 under shade, thus increasing the ability of PIF s to control gene expression, increase auxin levels and promote stem growth.