Photobody Localization of Phytochrome B Is Tightly Correlated with Prolonged and Light-Dependent Inhibition of Hypocotyl Elongation in the Dark    

Abstract

Photobody localization of Arabidopsis (Arabidopsis thaliana) phytochrome B (phyB) fused to green fluorescent protein (PBG) correlates closely with the photoinhibition of hypocotyl elongation. However, the amino-terminal half of phyB fused to green fluorescent protein (NGB) is hypersensitive to light despite its inability to localize to photobodies. Therefore, the significance of photobodies in regulating hypocotyl growth remains debatable. Accumulating evidence indicates that under diurnal conditions, photoactivated phyB persists into darkness to inhibit hypocotyl elongation. Here, we examine whether photobodies are involved in inhibiting hypocotyl growth in darkness by comparing the PBG and NGB lines after the red light-to-dark transition. Surprisingly, after the transition from 10 mmol m 22 s 21 red light to darkness, PBG inhibits hypocotyl elongation three times longer than NGB. The disassembly of photobodies in PBG hypocotyl nuclei correlates tightly with the accumulation of the growth-promoting transcription factor PHYTOCHROME-INTERACTING FACTOR3 (PIF3). Destabilizing photobodies by either decreasing the light intensity or adding monochromatic far-red light treatment before the light-to-dark transition leads to faster PIF3 accumulation and a dramatic reduction in the capacity for hypocotyl growth inhibition in PBG. In contrast, NGB is defective in PIF3 degradation, and its hypocotyl growth in the dark is nearly unresponsive to changes in light conditions. Together, our results support the model that photobodies are required for the prolonged, light-dependent inhibition of hypocotyl elongation in the dark by repressing PIF3 accumulation and by stabilizing the far-red light-absorbing form of phyB. Our study suggests that photobody localization patterns of phyB could serve as instructive cues that control light-dependent photomorphogenetic responses in the dark.

Bottom-up Assembly of the Phytochrome Network

“…However, after 15 h in the dark, phyB-GFP remained in large nuclear bodies in the presence of phyC, whereas in the absence of phyC, phyB-GFP showed a diffused pattern within the nuclei (Fig 6F and 6G). It was recently shown that phytochrome nuclear bodies are required to inhibit hypocotyl elongation during a prolonged dark period [37]. In the light of these findings, our results strongly suggest that phyC is important to maintain phyB in these active nuclear bodies.…”

“…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.…”

“…To further test if phyC could increase the activity of phyB at the end of the night phase, we measured the expression of genes that respond to either light quality or dawn cues during the dark to light transition (S11C–S11F Fig), a condition where phyC promotes de accumulation of phyB in large nuclear bodies (Fig 6F and 6G). The mRNA levels of PIF3-LIKE 1 ( PIL1 ) and ARABIDOPSIS THALIANA HOMEOBOX PROTEIN 2 ( ATHB2 ) were repressed by the photoreceptors phyB and phyC to a lower level than phyB alone, even during the last hour of the dark period (S11C and S11D Fig) [37]. The morning-expressed clock genes NIGHT LIGHT-INDUCIBLE AND CLOCK-REGULATED1 ( LNK1 ) and CIRCADIAN CLOCK ASSOCIATED 1 ( CCA1 ) [38, 39] showed the strongest response to light when both phyB and phyC were present (S11E and S11F Fig).…”

Seedling Establishment: A Dimmer Switch-Regulated Process between Dark and Light Signaling

“…During the first night hours, phyB Pfr persists and inhibits PIF accumulation while slowly dark reverting to inactive Pr (Sweere et al, 2001;Rausenberger et al, 2010;Medzihradszky et al, 2013). The photoactivated phyB Pfr forms dynamic nuclear photobodies together with Hemera (HMR) to induce rapid phosphorylation of PIFs, leading to their degradation (Kircher et al, 2002;Chen et al, 2010;Van Buskirk et al, 2014). phyB also is found in tandem zinc knuckle/ plus3-dependent photobodies that also contain members of the EC (Kaiserli et al, 2015;Huang et al, 2016aHuang et al, , 2016b but apparently not in HMR, which could indicate the existence of specialized phyB-containing photobodies that might regulate PIF accumulation or transcription separately.…”

HEMERA Couples the Proteolysis and Transcriptional Activity of PHYTOCHROME INTERACTING FACTORs in Arabidopsis Photomorphogenesis

“…The steady state pattern of phyB-GFP photobodies is directly regulated by light quality and quantity (Chen et al, 2003). The phyB-containing photobodies persist into darkness and are tightly correlated with phyB-mediated repression of PIF3 accumulation and hypocotyl growth in the dark (Rausenberger et al, 2010;Van Buskirk et al, 2014). In a genetic screen aimed at identifying factors required for the localization of phyB-GFP to photobodies, we recently uncovered a phy signaling component named HEMERA (HMR) (Chen et al, 2010b).…”