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.

光和温度从幼苗生长的最早阶段开始起作用，建立了 skotomorphogenesis、photomorphogenesis 和 thermomorphogenesis 的发育策略。在深色生长的植物中，skotomorphogenesis 是主要的生长策略，其特征是下胚轴伸长、抑制叶片扩张、叶片低钠和抑制幼苗寻找光线时叶绿体发育。相比之下，当暴露在光线下时，幼苗将其发育程序转变为光形态发生，其特征是下胚轴缩短、叶片膨胀和叶绿体生物发生。在温暖的条件下，特别是温暖的阴凉条件下，植物在发育阶段表现出热形态生长，其中包括一些 skotomorphogeno 发生的标志，例如下胚轴的伸长和叶柄的伸长、叶形的变化、低生和早开花。包括 phyB 在内的光感受器在调节这些发育途径中起关键作用。PhyB 对于感应红光与远红光的比率至关重要，从而能够从 skoto- 过渡到光形态发生。此外，最近的工作表明，phyB 还充当环境温度的直接温度传感器，在热形态发生中起关键作用（Jung等 人，2016 年;Legris et al.， 2016）。PhyB 包含四吡咯发色团植物粒体蛋白，它能够吸收红光 （660 nm） 和远红光 （730 nm） 光。 phyB 的非活性胞质形式 Pr 被红光吸收激活，并发生构象变化为活性状态 Pfr，这可能暴露了核定位信号，导致 phyB 的 Pfr 形式的核转位和在称为光体的细胞核中形成离散的 phyB 点。吸收远红光后，Pfr 活性形式切换回非活性 Pr 状态，phyB 光体解离。此外，在称为热回归的过程中，活性和非活性状态之间的转变直接受到温度的影响，其中较高的温度会增加从活性形式到非活性形式的转变速率。PhyB 活性和定位也受到其磷酸化状态的强烈影响，以前的研究将 FERONIA 确定为一种受体样激酶，可磷酸化 phyB，触发从活性形式转变为非活性形式和光体解离（Liu等人 ，2023 年）。

在 Yang 等 人的文章中，作者表明 phyB 是 BIN2 激酶的直接靶标，BIN2 激酶是油菜素类固醇信号转导的关键调节因子，可磷酸化油菜素唑-RESISTANT1 （BZR1） 转录因子，导致其降解和植物生长抑制。最近的研究还确定 BIN2 磷酸化植物色素相互作用因子 4，这是一种参与光和温度反应途径的转录因子，也是光形态发生基因的负调节因子，靶向其降解（Bernardo-García等 人，2014 年）。因此，BIN2 作为生长的负调节因子的作用非常复杂，不仅涉及激素信号通路，还涉及 PIF 的积累和 phyB 光体的稳定和/或组装。然而，在较高的环境温度下，PIF 会积累并减轻生长抑制。BIN2 在较高温度下的酶活性降低可能在 PIF 积累中起关键作用。

白天，PIF 结合 phyB 等植物色素并浓缩到光体中，随后被磷酸化并靶向降解。BIN2 对 phyB 的磷酸化稳定了光体，并使 phyB 能够更强烈地与另一种直接温度传感器 ELF3 相互作用。ELF3 募集泛素结合蛋白 HEMERA 并进一步促进光体的形成和/或稳定。phyB-ELF3-HEMERA 复合物负向调节热形态发生，可能通过 PIF 的降解（Qiu等 人，2015 年）。在夜间，当光体解离时，由 ELF3、ELF4 和 LUX ARRYTHMO 组成的抑制性夜间复合物 （EC） 抑制 PIF 的转录，进一步限制 PIF 蛋白的积累（Ezer等 人，2017 年）。光体和 EC 都会因温度升高而不稳定，尽管机制不同。而 ELF3 在较温暖的条件下形成核体（Jung等 人，2020 年;Hutin等 人，2023 年），这降低了 EC 的活性，较高的温度会解离 phyB 光体，部分原因是 BIN2 的活性较低和 phyB 磷酸化水平降低。Yang 等 人。证明 BIN2 作为较高温度的函数形成无活性的寡聚体，导致 phyB 磷酸化水平降低，并为由于磷酸化状态变化而导致的光体不稳定提供了机制模型。这项研究强调 BIN2 介导的 phyB 磷酸化对植物热形态发生至关重要，并通过蛋白质活性和结构的变化为温度感应机制提供了见解（图 1）。

进一步的研究将揭示温度如何诱导 BIN2 的结构变化以促进寡聚化，以及这是否取决于细胞的氧化还原状态（Lu等 人，2022 年）。需要解决温度如何影响光电体的其他组件，例如温度传感器 ELF3，以及这是否也有助于在较高温度下的光电体解离。光体的组成和活动随温度变化的动力学和变化是该领域的一个主要挑战。确定光体组装、稳定性和解离的新调节因子以及光体和信号转导途径之间潜在的串扰是了解植物如何根据不断变化的环境条件改变其生长的关键步骤。Yang 等 人的工作。进一步了解了植物用来感知环境并优化不同温度条件下的发育程序的复杂信号网络和重叠参与者。