Phytochrome B photobody components

The phytochrome B signaling regulates salt-mediated seedling growth in the dark

Light is an essential environmental factor that facilitates the robust upward growth of post-germinative seedlings emerging from buried seeds that is partly mediated by the photoreceptors. Salinity stress hampers plant growth and development and reduces yield. However, the involvement and regulatory role of photoreceptors and light signaling factors to salt stress are largely unknown. Here, we report that mutants of the phytochrome B (phyB) photoreceptor showed reduced sensitivity to salt-inhibited hypocotyl elongation in darkness, and that PHYTOCHROME-INTERACTING FACTOR 3 (PIF3) acts downstream of phyB in regulating this process in Arabidopsis thaliana. We also show that SALT OVERLY SENSITIVE 2 (SOS2) regulates phyB protein accumulation under salt stress in darkness. Surprisingly, salt treatment induces phyB nuclear body formation in darkness. Moreover, we found that the phosphorylation at residue Ser-86 of phyB is essential for its function, and the scaffold protein 14-3-3κ is involved in the regulation of phyB under salt stress in darkness. Taken together, our study reveals a regulatory role of the phyB-PIF3 module in mediating post-germination growth in darkness in response to salt stress.

Phytochrome B regulates cortical microtubule arrangement to control cotyledon polar expansion by repressing LONGIFOLIAs

Light promotes the expansion and controls the directionality of expansion in cotyledons, transforming small oval cotyledons into larger orbicular shapes. However, the cellular basis underlying this polar expansion remains unclear. We report that cotyledon polar expansion in Arabidopsis (Arabidopsis thaliana) is primarily associated with the polar expansion of pavement cells, rather than with polar cell proliferation. Phytochrome B (phyB) promotes this polar expansion by inhibiting PHYTOCHROME INTERACTING FACTORs (PIFs), which normally suppress expansion and inversely regulate its directionality. PIFs exert their control over directionality partly through the activation of their target genes, LONGIFOLIAs (LNGs). At the cellular level, phyB decreases the number of transversely arranged cortical microtubules, while increasing the number of longitudinally arranged microtubules. This phyB-induced change in microtubule arrangement would strengthen transverse expansion while weakening longitudinal expansion. In contrast, PIFs regulate microtubule arrangements in the opposite manner. Downstream of the phyB-PIF pathway, LNGs preferentially increase transversely arranged cortical microtubules. Overall, our data support that the regulation of cortical microtubule orientation by the phyB-PIF-LNG pathway underlies how phyB weakens longitudinal expansion relative to transverse expansion while promoting pavement cell expansion to make orbicular cotyledons in the light.

The LUX-SWI3C module regulates photoperiod sensitivity in Arabidopsis thaliana

In plants, the photoperiod sensitivity directly influences flowering time, which in turn affects latitudinal adaptation and yield. However, research into the mechanisms underlying photoperiod sensitivity, particularly those mediated by epigenetic regulation, is still in its nascent stages. In this study, we analyzed the regulation of photoperiod sensitivity in Arabidopsis thaliana. We demonstrate that the evening complex LUX ARRYTHMO (LUX) and the chromatin remodeling factor SWITCH/SUCROSE NONFERMENTING 3C (SWI3C) regulate GI locus chromatin compaction and H3K4me3 modification levels at the GIGANTEA locus under different photoperiod conditions. This mechanism is one of the key factors that allow plants to distinguish between long-day and short-day photoperiods. Our study provides insight into how the LUX-SWI3C module regulates photoperiod sensitivity at the epigenetic level.

Light and high temperatures control epigenomic and epitranscriptomic events in Arabidopsis

Light and temperature are two key environmental factors that control plant growth and adaptation by influencing biomolecular events. This review highlights the latest milestones on the role of light and high temperatures in modulating the epigenetic and epitranscriptomic landscape of Arabidopsis to trigger developmental and adaptive responses to a changing environment. Recent discoveries on how light and high temperature signals are integrated in the nucleus to modulate gene expression are discussed, as well as highlighting research gaps and future perspectives in further understanding how to promote plant resilience in times of climate change.

The BBX7/8-CCA1/LHY transcription factor cascade promotes shade avoidance by activating PIF4

Sun-loving plants undergo shade avoidance syndrome (SAS) to compete with their neighbors for sunlight in shade conditions. Phytochrome B (phyB) plays a dominant role in sensing the shading signals (low red to far-red ratios) and triggering SAS. Shade drives phyB conversion to inactive form, consequently leading to the accumulation of PHYTOCHROMEINTERACTING FACTOR 4 (PIF4) that promotes plant growth. Here, we show B-box PROTEIN 7 (BBX7)/BBX8 and CIRCADIAN CLOCK ASSOCIATED 1 (CCA1)/LATE ELONGATED HYPOCOTYL (LHY) positively regulate the low R : FR-induced PIF4 expression and promote the low R : FR-triggered hypocotyl growth in Arabidopsis. Shade interferes the interactions of phyB with BBX7 or BBX8 and triggers the accumulation of BBX7 and BBX8 independent of phyB. BBX7 and BBX8 associate with CCA1 and LHY to activate their transcription, the gene produces of which subsequently upregulate the expression of PIF4 in shade. Genetically, BBX7 and BBX8 act upstream of CCA1, LHY, and PIF4 with respect to hypocotyl growth in shade conditions. Our study reveals the BBX7/8-CCA1/LHY transcription factor cascade upregulates PIF4 expression and increases its abundance to promote plant growth and development in response to shade.

TANDEM ZINC-FINGER/PLUS3: a multifaceted integrator of light signaling

TANDEM ZINC-FINGER/PLUS3 (TZP) is a nuclear-localized protein with multifaceted roles in modulating plant growth and development under diverse light conditions. The unique combination of two intrinsically disordered regions (IDRs), two zinc-fingers (ZFs), and a PLUS3 domain provide a platform for interactions with the photoreceptors phytochrome A (phyA) and phyB, light signaling components, and nucleic acids. TZP controls flowering and hypocotyl elongation by regulating gene expression and protein abundance in a blue, red, or far-red light-specific context. Recently, TZP was shown to undergo liquid-liquid phase separation through its IDRs, thus promoting phyA phosphorylation. Collectively, TZP is an emerging regulator of diverse light signaling pathways; therefore, understanding its biochemical function in integrating environmental signaling networks is key for optimizing plant adaptation.

BBX9 forms feedback loops with PIFs and BBX21 to promote photomorphogenic development

Light is one of the most essential environmental factors that tightly and precisely control various physiological and developmental processes in plants. B-box CONTAINING PROTEINs (BBXs) play central roles in the regulation of light-dependent development. In this study, we report that BBX9 is a positive regulator of light signaling. BBX9 interacts with the red light photoreceptor PHYTOCHROME B (phyB) and transcription factors PHYTOCHROME-INTERACTING FACTORs (PIFs). phyB promotes the stabilization of BBX9 in light, while BBX9 inhibits the transcriptional activation activity of PIFs. In turn, PIFs directly bind to the promoter of BBX9 to repress its transcription. On the other hand, BBX9 associates with the positive regulator of light signaling, BBX21, and enhances its biochemical activity. BBX21 associates with the promoter regions of BBX9 and transcriptionally up-regulates its expression. Collectively, this study unveiled that BBX9 forms a negative feedback loop with PIFs and a positive one with BBX21 to ensure that plants adapt to fluctuating light conditions.