植物学报 Ch/ne B i///e f/>7 of Bofany 2021, 56 (2): 142-146, w w
d o i: 10.11983/CBB21042
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两类免疫受体强强联手筑牢植物免疫防线
王伟，唐定中，
福建农林大学植物免疫中心，福州350002珠海旅游攻略
摘要植物先天免疫系统在抵御病原菌入侵过程中发挥至关重要的作用，主要包括两个层次，即病原菌相关分子模式和效 应因子分别触发的PTI和ETI免疫反应。PTI和ETI分别由植物细胞膜表面模式识别受体(PRRs>和胞内免疫受体(NLRs)激活, 具有特异的激活机制，但是两者激活的下游免疫事件相互重叠。PTI和ETI是否为泾渭分明的两道防线，以及ETI与PTI下游 事件为何如此相似，一直是植物免疫领域最受关注的问题之一。最近，中国科学院分子植物科学卓越创新中心辛秀芳团队 与合作者利用拟南芥(Arab/'ctops/s fha//ana)与丁香假单胞杆菌(Puctomonas syringae)互作系统对PTI和ETI在机制上的联 系进行了研宄。他们发现PRRs和共受体参与E T I，而活性氧的产生是联系PRRs和NLRs所介导的免疫早期信号事件。他们 还发现NLRs信号能够迅速增强PTI关键因子的转录和蛋白水平，PTI的增强在ETI免
疫反应中不可或缺。该研宄从机制上解 析了植物免疫领域中长期悬而未决的PTI与ETI相似性之谜，是该领域的一项突破性进展，为未来作物分子设计育种提供了 新的启示。
关键词植物免疫，病原菌相关分子模式触发的免疫反应(P T I)，效应因子触发的免疫反应(E T I)，活性氧，N L R s，模式识别 受体(PRRs)
王伟，唐定中（2021).两类免疫受体强强联手筑牢植物免疫防线.植物学报56, 142-146.
当受到病原菌侵染时，植物会利用位于细胞膜表 面的模式识别受体（pattern-recognition receptors, PRRs)迅速识别病原菌保守的相关分子模式(pathogen-associated molecular patterns,PAMPs)，通过与共受体(co-receptor)结合并磷酸化激活胞质型 类受体激酶（receptor-like cytoplasmic kinas, RLCKs)，如 BIK1(BOTRYTIS-INDUCED KINASE 1)和BSK1(BR-SIGNALING KINASE1) (Lu et al., 2010; Zhang et al” 2010; Shi et al.，2013)，进而激 活丝裂原激活蛋白激酶(mitogen-activated protein kinas,MAPKs)级联信号通路、钙依赖蛋白激酶 (calcium dependent protein kinas,CDPKs)通路和 引起活性氧(reactive oxygen species,ROS)爆发等，这些反应统称为PTI免疫反应(pattern-triggered immunity)(Couto and Zipfel, 2016; Tang et al., 2017; Wang et al., 2020; Zhou and Zhang, 2020)。
为了抑制植物PTI免疫反应，一些病原菌向宿主 细胞分泌效应因子(effectors)以千扰免疫进程，使植物感病(Jones and Dangl,2006)。面对病原菌的入侵，植物进化出细胞内受体蛋白NLRs (nucleotide-bin
ding site and leucine-rich repeat domain receptors)，通过识别效应因子激活更强烈的ETI免疫反应(effector-triggered immunity)，通常在病原菌入侵位点诱导超 敏反应(hypernsitive respon,HR)抑制病原菌的 生长（Cui et al.,2015; Monteiro and Nishimura, 2018)。ETI免疫反应同样可以引起MAPKs信号通路 激活和ROS爆发(Qi e ta l.,2011)。这些过程的调控与 PTI相关组分是否有关尚不清楚，PTI和ETI之间的联 系也有待研究。最近，中国科学院分子植物科学卓越 创新中心辛秀芳团队与合作者利用拟南芥(AraWc/ops/s Catena)与丁香假单胞杆菌(Pudomonas syr/'ngae)互作系统开展研宄，发现拟南芥ETI免疫反应的发生 也依赖PTI通路中的多种元件，进而揭示了 PRRs与 NLRs协同激活植物免疫的新机制，增进了人们对植 物先天免疫系统的认识，是植物免疫研究领域的一项 突破性进展(Yuan et al., 2021)(图1)。
收稿日期：2021-03-02；接受日期：2021-03-05基金项目：国家自然科学基金(No.31830077) * 通讯作者。E-mail:dztang@genetics_
ac
天妃E伟等：两类免疫受体强强联手筑牢植物免疫防线143
图1植物先天免疫系统示意图少儿英语教学
植物通过细胞膜表面PRRs受体和共受体识别病原菌PAMPs，并磷酸化胞质型类受体激酶BIK1。激活的BIK1通过磷酸化RbohD 正调控活性氧(ROS)爆发，抑制病原菌生长。成功入侵的病原菌通过向植物细胞内分泌效应因子抑制免疫反应。宿主细胞内的NLRs 受体识别特定的效应因子后，不仅能够激活下游免疫反应，而且能提升BIK1和RbohD等免疫关键蛋白的丰度，增强活性氧爆发和 免疫通路重要基因的表达，导致更加强烈的免疫反应。
Figure 1Schematic reprentation of the plant immune system
Plants recognize the PAMPs of pathogens via the cell surface-localized PRRs, leads to phosphorylation of the receptor-like cytoplasmic kina BIK1. The activated BIK1 positively regulates reactive oxygen species (ROS) burst by phosphorylation of RbohD, resulting in the inhibition of pathogen growth. Successful invaded pathogens deliver effectors into plant cell to inhibit immune respons. Upon perception of specific effectors, the host intracellular NLRs activate downstream im
mune respons, and boost the abundance of key immune-related proteins such as BIK1 and RbohD to enhance ROS burst and the transcripts of important genes in immune signaling, leading to robust immune respons.
野生型拟南芥通过NLRs蛋白识别特定效应因子, 引起强烈的ETI免疫反应，抑制丁香假单胞杆菌Pfo DC3000相应菌株的生长（Cesari,2018; Monteiro and Nishimura,2018)。但是，辛秀芳团队的研究发 现，在缺失多种重要PRRs或共受体的拟南芥突变体 fls2/efrlcerk1 (fee)和 bak1/bkk1lcerk1 (bbc)中，由效应因子avrRpt2、avrPphB和avrRps4诱导的ETI免疫 反应明显受到影响，说明ETI免疫反应也受PRRs受 体的调控，暗示ETI依赖PTI(Yuan et al., 2021)。为 了排除细菌内源效应因子等对免疫反应的影响，他们 利用敲除了所有36个内源效应因子和冠菌素(coronatine)合成基因的Pto DC3000突变菌株D36E (Wei et al.,2015)进行研究。通过将awRpf2转入 D36E，并侵染拟南芥不同材料，发现D36E(a^Rpf2) 在野生型植物中的生长数量明显少于D36E菌株；而在缺失PRRs或共受体的突变体中，2种菌株的生长数 量无显著差异(Yuan et al.,2021)，进一步说明avrRpt2诱导的ETI免疫反应依赖PRRs及其共受体。
细菌效应因子avrRpt2可以切割细胞内的RIN4 (RPM1-INTERACTING PROTEIN4)，激活N L R g 白RPS2 (RESISTANT TO P.SYRINGAE 2) (Day et al.,2005; Kim et al.,2005)。而辛秀芳团队研宄发现, 与野生型相比，PRRs或共受体多重突变体中D36E (awRpf2)诱导的RIN4蛋白减少和RPS2转录本增加
均未受影响，且MPK3/6的磷酸化程度也一致。为了 深入探宄PRRs或共受体多重突变体如何影响ETI免 疫反应，他们重点研究了在PTI和ETI反应中都发挥 重要作用的ROS产生(Qi et al.,2017)。非常有意思的 是，通过对地塞米松(dexamethasone,DEX)诱导型 转基因植株进行研宄，他们发现flg22和
DEX
144植物学报56(2> 2021
共处理不仅可以快速诱导ROS的爆发(PTI-ROS)，而 且在处理2_3小时后还出现了第2次持续性ROS爆发, 被称为ETI-ROS。ETI-ROS爆发同样依赖于flg22处 理，且在共受体多重突变体中ETI-ROS爆发几乎丧 失，说明PRRs信号通路的持续激活是ETI-ROS爆发 的关键。同时，flg22处理可以加速拟南芥中avrRpt2 诱导的HR反应，说明ETI-ROS爆发可能促进ETI免 疫反应(Yuan et al., 2021)。
PTI-ROS和ETI-ROS的产生是否依赖相同的亚 细胞组分依然未知。早期有研宄报道，植物NADPH氧 化酶和过氧化物酶(peroxidas)在病原菌诱导的PTI-ROS爆发过程中发挥关键作用(Torres et al., 2002; Daudi et al., 2012)。随后，辛秀芳团队通过化 学抑制剂筛选处理，发现ETI-ROS的产生需要NADPH 氧化酶RbohD(respiratory burst oxida homolog D)。同时，在D36E (a^Rpf2)诱导的ETI免疫反应中, Rfoo/iD转录本水平出现上调(Yuan et al., 2021)。通 过后续的遗传学和生物化学实验，他们进一步明确 ETI-R O S的产生依赖BIK1，而BIK1通过磷酸化
R bohD第343和347位丝氨酸（S343/347)调控ET卜ROS的爆发(Yuan et al.,2021 )。前期已有研宄表 明，BIK1的S343/347磷酸化在细菌侵染时PTI反应中 的ROS产生以及ETI介导的抗病反应中发挥重要作用 (Kadota et al.,2019)。辛秀芳团队的研宄结果进一步 揭示了 PRRs信号和NLRs信号通过协调Rb
公园的一角ohD活性 和丰度来调控ETI-ROS爆发的重要性。
为了进一步探明是否存在其它PTI免疫通路成员 参与调控ETI反应，辛秀芳团队利用RNA-Seq技术分 析了拟南芥野生型和共受体多重突变体分别接种
D36E和D36E (ai/rRpf2)菌株3小时后的转录组差异。他们发现D36E(a\/rRpf2)处理后，ETI的激活可以在 很大程度上恢复共受体多重突变体中PTI相关基因表 达的缺陷。但是，与野生型拟南芥相比，D36E (awRpf2)处理后的共受体多重突变体中仍有272个 基因表达存在差异，其中包括许多PTI免疫通路标志 基因，如H/RKY22/29和F R K f，且这些基因的表达同 样受到BIK1的调控，说明BIK1是ETI免疫反应中ROS爆发和免疫基因表达调控的一个重要节点。此 外，无论是在野生型还是在共受体多重突变体中，D36E(a\/rR以2)处理均引起大量PTI免疫通路关键元 件编码基因的表达上调(Yuan e ta l.,2021)。最近，英国塞恩斯伯里实验室Jonathan DG Jones课题组也发现PRRs和NLRs两类免疫受体互相协作，单独激活 PRRs或NLRs介导的抗性都不能有效抵御Pto DC3000入侵(Ngou et al.,2021)。相关研究成果以 “背靠背”形式发表在A/aftyre杂志上。在以往的研究 中，PTI和ETI被认为是两条平行的免疫通路。但以上 研宄结果表明，PTI和ETI并非泾渭分明的两道防线，ETI的激活需要PRRs和共受体参与，ETI免疫信号可 以调动PTI免疫通路，进而放大PTI反应，表明植物免 疫反应的全面激活需要PRRs信号和NLRs信号的强 强联手。这项研宄工作从机制上阐明了植物免疫领域 中长期悬而未决的PTI与ETI相似性之谜，增进了人 们对植物先天免疫系统的理解，为未来作物的抗病育 种提供了新
的启示。
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排球的规则
146植物学报56(2) 2021
Synergistic Cooperation Between Cell Surface and Intracellular
Immune Receptors Potentiates to Activate Robust Plant Defen
Wei Wang,Dingzhong Tang
Plant Immunity Center, Fujian Agriculture and Forestry University, Fuzhou 350002, China
Abstract Innate immune system plays a crucial role to defend against pathogens attack and is classif父亲礼物
ied into two layers, which include pathogen-associated molecular pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). The PTI and ETI are activated by cell-surface localized pattern-recognition receptors (PRRs) and mostly intracellu- larly-localized nucleotide-binding, leucine-rich repeat receptors (NLRs), respectively, with specific activation mechanisms, but largely overlapped downstream immune events and components. One of the top unanswered questions in the field of plant immunity is whether ETI and PTI are really distinct, considering the high similarity of the downstream of the recognition process and components. Recently, a team led by Prof. Xiufang Xin, CAS Center for Excellence in Molecular Plant Sciences, Chine Academy of Sciences, ud the Arabidopsis thaliana and Pudomonas syringae pathosystem to study the functional link between PTI and ETI, and demonstrated that PRRs and the co-receptor of PRRs contribute to ETI, and the production of reactive oxygen species (ROS) is the early signal event that connects PTI and ETI. They also showed that ETI enhances the transcript and protein levels of key components of PTI, and the incread PTI is crucial for full activation of ETI. This study provides mechanistic explanation to a long-lasting enigma in the field of plant immunity regarding the mechanistic connections of PTI and ETI, and the high similarity of the two layers of immunity. This work reprents an important breakthrough in the field of plant immunity, and will have implications for the future molecular breeding in crops.
Key words plant immunity, pattern-triggered immunity (PTI), effector-triggered immunity (ETI), reactive oxygen species, nucleotide-binding site and leucine-rich repeat domain receptors (NLRs), pattern-recognition receptors (PRRs)
Wang W, Tang DZ (2021). Synergistic cooperation between cell surface and intracellular immune receptors potentiates to activate robust plant defen. Chin Bull Bot56, 142-146.
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