Adjustment of the PIF7‐HFR1 Transcriptional Module Activity Controls Plant Shade Adaptation

Shade caused by the proximity of neighboring vegetation triggers a set of acclimation responses to either avoid or tolerate shade. Comparative analyses between the shade‐avoider Arabidopsis thaliana and the shade‐tolerant Cardamine hirsuta revealed a role for the atypical basic‐helix‐loop‐helix LONG HYPOCOTYL IN FR 1 (HFR1) in maintaining the shade tolerance in C. hirsuta, inhibiting hypocotyl elongation in shade and constraining expression profile of shade‐induced genes. We showed that C. hirsuta HFR1 protein is more stable than its A. thaliana counterpart, likely due to its lower binding affinity to CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), contributing to enhance its biological activity. The enhanced HFR1 total activity is accompanied by an attenuated PHYTOCHROME INTERACTING FACTOR (PIF) activity in C. hirsuta. As a result, the PIF‐HFR1 module is differently balanced, causing a reduced PIF activity and attenuating other PIF‐mediated responses such as warm temperature‐induced hypocotyl elongation (thermomorphogenesis) and dark‐induced senescence. By this mechanism and that of the already‐known of phytochrome A photoreceptor, plants might ensure to properly adapt and thrive in habitats with disparate light amounts.