Phytochrome C And Low Temperature Promote The Expression And Red Light Signaling Of Phytochrome D

Light is an ever-changing environmental parameter affecting almost every aspects of plant growth and development. It is perceived by photoreceptors, among them phytochromes (PHY) are responsible for monitoring the red and far-red part of the spectrum. Arabidopsis thaliana possesses five phytochromes genes, named through phyA-phyE. Whereas the function of phyA and phyB – that mediate most of the phytochrome responses – is extensively studied, our knowledge on other phytochromes are still rudimentary. To analyze phyD function we expressed it at high levels in different phytochrome deficient genetic backgrounds. We found that overexpressed phyD governs effective light signaling at low temperatures but only in cooperation with functional phyC. Under these conditions, opposite to phyB, phyD accumulates to high levels and this pool is stable under light illumination. Furthermore, the detectable photoconvertible phyD amount is proportional with the available protein amount indicating that the phyD pool contains fully functional photoreceptors. The thermal reversion of phyD is very fast suggesting that the thermosensing of phyD is based on its protein amount and not on its Pfr conformer stability, which was described for phyB. We also found that phyD and phyB associate to identical genomic locations and mediate similar gene expression changes, however the efficiency of phyD is lower. Taken together our data suggest that under certain conditions synergistic interaction of phyD and phyC substitutes phyB function thus increases the ability of plants to respond more flexible to environmental changes. 4-day-old seedlings were grown on half-strength MS plates at 17 or 27°C in the dark and illuminated for 1 or 24 h. 0.1 g plant material was snap frozen in liquid nitrogen from etiolated or 1 h or 24 h red light (50 µmol m-2 s-1) illuminated seedlings. Total RNA was extracted using RNeasy Miniprep Kits (Qiagen) according to the manufacturer's instructions. RNA sequencing was done by Novogene, Cambridge, United Kingdom.