Phytochrome gene expression studies: PHYE is required for FR-induced expression of PHYA and PHYD suppresses expression of PHYA

Arabidopsis has five phytochrome (PHY) genes for sensing the Red:Far Red (R:FR) ratio in ambient light, of which PHYA has an established role in responses to FR. To study whether and how PHYs may influence each others transcription, PHY-Luciferase reporter plants (pPHYA:LUC, pPHYB:LUC, pPHYC:LUC, pPHYD:LUC and pPHYE:LUC) were constructed. Subsequently, reporter lines representative for each PHY were crossed into each of the five single phy-mutant backgrounds. Reporter activities in WT and phy mutant was studied under diurnal mixed (R, B, FR), R, FR or B LED light in seedling or rosette plants. Both pPHYA:LUC and pPHYB:LUC show strong induction under FR. Full FR upregulation of both PHYA and PHYB is dependent on PHYE, identifying PHYE as a novel sensor for FR light responses. Results also show that PHYA expression is strongly suppressed by PHYD. Results were confirmed for expression of endogenous PHYA and PHYB, albeit with different dynamics compared to the LUC reporters. Profiling of pPHYA:LUC and pPHYB:LUC reporters suggest gating of FR responses. Manipulation of PHY expression levels by FR may provide a novel basis for manipulating plant growth in controlled environments.