Chloroplasts modulate elongation responses to canopy shade by retrograde pathways involving HY5 and ABA

Plants use light as energy for photosynthesis but also as a signal of competing vegetation. By using different concentrations of norflurazon and lincomycin, we found that the response to canopy shade in Arabidopsis thaliana was repressed even when inhibitors only caused a modest reduction in the level of photosynthetic pigments. High inhibitor concentrations resulted in albino seedlings that were unable to elongate when exposed to shade, in part due to attenuated light perception and signaling via phytochrome B and phytochrome-interacting factors. The response to shade was further repressed by a GUN1-independent retrograde network with two separate nodes represented by the transcription factor HY5 and the carotenoid-derived hormone ABA. The unveiled connection between chloroplast status, light (shade) signaling, and developmental responses should contribute to achieve optimal photosynthetic performance under light-changing conditions.