Transgenic phytochrome D cooperates with phytochrome B to promote photomorphogenesis at low temperatures.

The function of the phytochrome B photoreceptor is enhanced by its homologous counterpart, phytochrome D, at low temperatures, thereby increasing phenotypic plasticity in response to fluctuating environmental conditions. Light is a dynamically changing environmental factor that influences nearly every aspect of plant growth and development. Photoreceptors detect light, with phytochromes (PHY) specifically monitoring red and far-red wavelengths. Arabidopsis thaliana has five phytochromes, named phyA through phyE. phyA and phyB are extensively studied key mediators of most phytochrome responses; the precise functions of the other phytochromes remain poorly understood. To investigate phyD signaling, we overexpressed phyD-YFP alone and together with phyB-CFP in a genetic background lacking other phytochromes. Photomorphogenic responses, protein accumulation, and transcriptome changes were compared at low and high temperatures. We found that phyD-YFP alone cannot facilitate photomorphogenesis, but it significantly promotes red light signaling of both phyB-CFP and endogenous phyB in a temperature-dependent manner, being effective at low but not at high temperatures. In parallel, co-expressing phyD-YFP and phyB-CFP triggered substantially more robust light-induced transcriptome changes at low temperatures than expressing either transgene alone. Because phyD-YFP is more light-stable than phyB, and both PHYB-CFP and endogenous PHYB accumulate to higher levels alongside PHYD-YFP in light, phyD likely stimulates phyB signaling via PHYB protein stabilization. To explain how PHYD-YFP enhances PHYB levels at both temperatures yet stimulates signaling exclusively at low temperatures, we propose that phyD and phyB form light-stable heterodimers at both temperatures and that these heterodimers are functional only at low temperatures. Our findings demonstrate that phyB and phyD functionally cooperate to mediate temperature-dependent light signaling responses, providing plants with developmental flexibility under changing environmental conditions.