Phytochrome B Nuclear Bodies Respond to the Low Red to Far-Red Ratio and to the Reduced Irradiance of Canopy Shade in Arabidopsis

Abstract

The current consensus is that plant responses to canopy shade involve the perception of low red to far-red ratios (R:FRs) by phytochrome B (phyB), which leads to the direct activation of auxin synthesis genes by PHYTOCHROME INTERACTING FACTORs (PIFs). In addition to its effect on R:FRs, shade also reduces irradiance, but whether shade-induced drops in irradiance affect phyB activity has not been demonstrated. To address this issue, we investigated whether irradiance and R:FRs have similar effects on the nuclear distribution of phyB in petiole cells of light-grown plants. Under high-irradiance white light, phyB formed large nuclear bodies. Lowering irradiance without changing R:FRs or lowering R:FRs by adding far-red light led to the appearance of small nuclear bodies containing phyB. Large nuclear bodies remained but with some concomitant reduction in diameter. The appearance of small nuclear bodies was rapid, stable, and reversible upon the return to high irradiance and high R:FRs. High levels of red light but not of blue light were enough to restrain the formation of small phyB nuclear bodies. Irradiance was effective within the range found in natural canopies and even under relatively low R:FRs. The promotion of leaf hyponasty by lowering irradiance was impaired in phyB and pif mutants, as previously reported for the response to R:FRs. The expression of auxin-related genes showed a similar hierarchy of response to low R:FRs and low irradiance. We propose that phyB is able to perceive not only the low R:FRs, but also the low irradiance of shade.

Integration of Light and Photoperiodic Signaling in Transcriptional Nuclear Foci

“…HMR is involved in the degradation of phyA and PIF TFs (Qiu et al, 2015). On the contrary, the localization of TZP in NBs does not affect its protein stability (Figure 1B) suggesting that TZP-phyB NBs may have a distinct function compared to the nuclear sites of PIF degradation, whereas the extremely small TZP-PIF4 NBs could have a storage role similar to the one described for small phyB NBs during shade avoidance response at later developmental stages (Trupkin et al, 2014). …”

Bottom-up Assembly of the Phytochrome Network

“…These results suggest that phyC forces phyB to localize to the nucleus, after periods of darkness and in a light-quality independent manner. However, in Arabidopsis nuclei, phyB remains nuclear under prolonged periods of darkness, low quality or low irradiance, but changes its pattern of localization in nuclear bodies, from large to small nuclear bodies [36, 37]. To address the behavior of phyB in the presence of phyC in Arabidopsis, we generated phyB-GFP lines either in the quintuple phytochrome mutant background or in a background having only phyC, by crossing phyB-GFP phyA phyB phyC phyD phyE lines with either the quintuple phyA phyB phyC phyD phyE mutant or the phyA phyB phyD phyE quadruple mutant bearing only phyC.…”

PhytochromeBdynamics departs from photoequilibrium in the field

“…Plants sense features of the light environment to obtain information about the temporal and spatial conditions. Phytochrome B (phyB) is one of the most important photo-sensory receptors, able to sense light quantity (irradiance) and quality (spectral composition; Galvão & Fankhauser, 2015;Trupkin, Legris, Buchovsky, Rivero, & Casal, 2014). Phytochromes have two forms, P r and P fr .…”