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 whichPHYA has an established role in responses to FR. To study whether and how PHYs may influence each other’stranscription, PHY-Luciferase reporter plants (pPHYA:LUC, pPHYB:LUC, pPHYC:LUC, pPHYD:LUC and pPHYE:LUC) wereconstructed. Subsequently, reporter lines representative for each PHY were crossed into each of the five single phymutant backgrounds. Reporter activities in WT and phy mutant was studied under diurnal mixed (R, B, FR), R, FR or BLED light in seedling or rosette plants. Both pPHYA:LUC and pPHYB:LUC show strong induction under FR. Full FRupregulation 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 ofendogenous PHYA and PHYB, albeit with different dynamics compared to the LUC reporters. Profiling of pPHYA:LUCand pPHYB:LUC reporters suggest gating of FR responses. Manipulation of PHY expression levels by FR may provide anovel basis for manipulating plant growth in controlled environments.