A phase-separated nuclear GBPL circuit controls immunity in plants

Liquid-liquid phase separation (LLPS) has emerged as a central paradigm
for understanding how membrane-less organelles compartmentalize diverse
cellular activities in eukaryotes. Here, we identified a new superfamily
of plant Guanylate-Binding Protein-Like GTPases (GBPLs) that assemble
LLPS-driven condensates within the nucleus to protect against infection
and autoimmunity. In Arabidopsis thaliana, two family members - GBPL1 and
GBPL3 - undergo phase transition behavior to control transcriptional
responses as part of an allosteric switch triggered by exposure to biotic
stress. GBPL1, a pseudoGTPase, sequesters catalytically-active GBPL3 under
basal conditions but is displaced by GBPL3 LLPS when it enters the nucleus
following immune cues to drive formation of unique membrane-less
organelles termed GDACs (GBPL Defense-Activated Condensates) that we
visualized by in situ cryo-electron tomography. Within these mesoscale
GDAC structures, native GBPL3 directly bound defense gene promoters and
recruited specific transcriptional coactivators of the Mediator complex
plus RNA Pol II machinery to massively reprogram host gene expression for
disease resistance. Together, our study identifies a new GBPL circuit that
reinforces the biological importance of phase-separated condensates, in
this case, as indispensable players in plant defense.

液-液相分离（Liquid-liquid phase separation, LLPS）已成为阐释真核生物中无膜细胞器如何区室化多样细胞生命活动的核心研究范式。本研究鉴定得到一类全新的植物鸟苷酸结合蛋白样GTP酶（Guanylate-Binding Protein-Like GTPases, GBPLs）超家族，该家族蛋白可在细胞核内组装由LLPS驱动的凝聚体，以抵御病原体感染与自身免疫应答。在拟南芥中，该家族的两个成员——GBPL1与GBPL3——会发生相变行为，作为生物胁迫触发的变构开关的一部分调控转录响应。假GTP酶GBPL1在基础生理条件下会结合并隔离具有催化活性的GBPL3；当免疫信号触发后，GBPL3发生LLPS并进入细胞核，从而将GBPL1置换，进而驱动一类被称为GDAC（GBPL Defense-Activated Condensates，GBPL防御激活凝聚体）的独特无膜细胞器的形成，我们通过原位冷冻电子断层扫描对该结构进行了可视化观测。在这些介尺度的GDAC结构中，天然状态的GBPL3可直接结合防御基因启动子，并招募中介体复合物（Mediator complex）的特异性转录共激活因子与RNA聚合酶II（RNA Pol II）系统，以大规模重编程宿主基因表达以获得抗病性。综上，本研究鉴定得到一条全新的GBPL信号通路，该通路进一步证实了相分离凝聚体的生物学重要性——在本研究中，它们是植物防御过程中不可或缺的关键参与者。