CONICET | Buscador de Institutos y Recursos Humanos

Phytocgrome A mediates two photobiologically discrete phases of response: The very-low-fluence response (VLFR), which saturates with brief exposures to light and the high-irradiance response (HIR), which requires continuous excitation with higher fluence rates. These discrete outcomes are important in different biological contexts but the mechanisms that initiate the HIR are not fully understood. We have identified mutations at the PHYA photoreceptor gene itself that selectively impair the HIR. Also, fusions containing the N-terminal and not the C-terminal domain of phyA are biologically active but they are unable to initiate a HIR. These results indicate that the divergence between VLFR and HIR has its origin in the photoreceptor itself. However, by analysing the promoter of a light-responsive target gene we identified a motif necessary for HIR but not for VLFR, indicating that downstream events are also critical for HIR. This motif binds the Bell-like homeodomain 1 (BLH1) transcription factor in in vitro and in yeast 1-hybrid assays. blh1 mutants showed reduced HIR and retained normal responses to transient light signals. BLH1 enhanced BLH1 expression and its promotion by far-red light. We propose a model where under continuous far-red light a BLH1 feed-back loop increases BLH1 expression above a threshold required to initiate HIR. Consistently with this model, overexpression of BLH1 enhanced the responses to transient FR, a condition where the blh1 mutants showed no phenotype.