Phosphorylation of phyB by GSK3s, a key mechanism that brings temperature sensors together

Abstract

Light and temperature act from the earliest stages of seedling growth, establishing the developmental strategies of skotomorphogenesis, photomorphogenesis and thermomorphogenesis. In dark-grown plants, skotomorphogenesis is the dominant growth strategy and is characterized by the elongation of the hypocotyl, repression of leaf expansion, leaf hyponasty and inhibition of chloroplast development as the seedling searches for light. By contrast, when exposed to light, seedlings switch their development program to photomorphogenesis, characterized by shortened hypocotyls, leaf expansion and chloroplast biogenesis. Under warm conditions, particularly warm shade conditions, plants across developmental stages exhibit thermomorphogenic growth which includes some of the hallmarks of skotomorphogenesis such as elongation of hypocotyls as well as petiole elongation, changes in leaf shape, hyponasty and early flowering. Photoreceptors including phyB play key roles in regulating these developmental pathways. PhyB is essential for sensing the ratio of red to far-red light, enabling the transition from skoto- to photomorphogenesis. In addition, recent work has shown that phyB also acts as a direct thermosensor of ambient temperature, playing a key role in thermomorphogenesis (Jung et al., 2016; Legris et al., 2016). PhyB contains a tetrapyrrole chromophore, phytochromobilin, that is able to absorb red (660 nm) and far-red (730 nm) light. The inactive cytosolic form of phyB, Pr, is activated by the absorption of red light and undergoes a conformational change to an active state, Pfr, which likely exposes a nuclear localization signal, resulting in nuclear translocation of the Pfr form of phyB and the formation of discrete phyB puncta in the nucleus called photobodies. Upon absorption of far-red light, the Pfr active form switches back to the inactive Pr state and phyB photobodies dissociate. In addition, the transition between active and inactive states is directly affected by temperature in a process called thermal reversion, in which higher temperatures increase the transition rate from the active to the inactive form. PhyB activity and localization are also strongly affected by its phosphorylation state, with previous studies identifying FERONIA as a receptor-like kinase that phosphorylates phyB, triggering reversion from the active to inactive form and photobody dissociation (Liu et al., 2023).

In the article by Yang et al., the authors show that phyB is a direct target of the BIN2 kinase, a key regulator of brassinosteroid signaling that phosphorylates the BRASSINAZOLE-RESISTANT1 (BZR1) transcription factors, resulting in their degradation and plant growth repression. Recent studies have also determined that BIN2 phosphorylates PHYTOCHROME INTERACTING FACTOR 4, a transcription factor involved in light and temperature response pathways and a negative regulator of photomorphogenic genes, target it for degradation (Bernardo-García et al., 2014). Thus, the role of BIN2 as a negative regulator of growth is highly complex, involving not only hormone signaling pathways, but also the accumulation of PIFs and the stabilizing and/or assembly of phyB photobodies. However, under higher ambient temperature conditions, PIFs accumulate and growth repression is alleviated. The reduced enzymatic activity of BIN2 under warmer temperatures likely plays a key role in PIF accumulation.

During the day, PIFs bind phytochromes such as phyB and are concentrated into photobodies where they are subsequently phosphorylated and targeted for degradation. Phosphorylation of phyB by BIN2 stabilizes photobodies and allows phyB to more strongly interact with ELF3, another direct temperature sensor. ELF3 recruits HEMERA, a ubiquitin-binding protein, and further facilitates the formation and/or stabilization of photobodies. The phyB-ELF3-HEMERA complex negatively regulates thermomorphogenesis, likely via the degradation of the PIFs (Qiu et al., 2015). During the night, when photobodies have dissociated, the repressive Evening Complex (EC), consisting of ELF3, ELF4 and LUX ARRYTHMO, represses transcription of PIFs, further limiting PIF protein accumulation (Ezer et al., 2017). Both photobodies and the EC are destabilized by warmer temperatures, although due to different mechanisms. Whereas ELF3 forms nuclear bodies under warmer conditions (Jung et al., 2020; Hutin et al., 2023), and this decreases the activity of the EC, warmer temperatures dissociate phyB photobodies due, in part, to the lower activity of BIN2 and decreased levels of phyB phosphorylation. Yang et al. demonstrate that BIN2 forms inactive oligomers as a function of higher temperature, resulting in lower levels of phyB phosphorylation and providing a mechanistic model for photobody destabilization due to changes in phosphorylation state. This research highlights that the BIN2-mediated phosphorylation of phyB is vital for plant thermomorphogenesis and provides insights into the mechanisms of temperature sensing via changes in protein activity and structure (Fig. 1).

Further studies will reveal how temperature may induce structural changes in BIN2 to promote oligomerization and whether this is dependent on the redox state of the cell (Lu et al., 2022). How temperature affects other components of photobodies, such as the thermosensor ELF3, and whether this also contributes to photobody dissociation under warmer temperatures will need to be addressed. The dynamics and changes in composition and activity of photobodies as a function of temperature is a major challenge in the field. The identification of new regulators of photobody assembly, stability and dissociation as well as potential cross-talk between photobodies and signal transduction pathways is a critical step in understanding how plants alter their growth as a function of changing environmental conditions. The work by Yang et al. furthers our knowledge of the complex signaling networks and overlapping players plants use to sense their environment and optimize their developmental programs under different temperature regimes.