作为一种主要粮食作物，水稻面临着多种病害的危害，如病原真菌引起的稻瘟病和纹枯病、细菌引起的白叶枯病、病毒引起的水稻黑条矮缩病和水稻条纹叶枯病、线虫引起的稻根结线虫病等。这其中，稻瘟病菌Magnaporthe oryzae(syn.Pyricularia oryzae)引起的稻瘟病为世界性水稻主要病害。除此之外，稻瘟病菌还可侵害其他多种禾本科作物，每年都造成大量损失[1-2]。稻瘟病菌作为一种重要的植物病原物，除了因经济重要性受重视外，还因易于开展遗传分析，近年来逐渐成为研究丝状真菌生长发育的重要模式生物之一[3-6]。同时，稻瘟病菌与水稻的互作也成为研究植物病原真菌-寄主互作较为理想的模式系统之一[4-7]。本文综述了近年来水稻与稻瘟病菌相互作用的分子模式，以期为水稻抗稻瘟病育种研究提供借鉴。
1 PMAPs介导的水稻PTI免疫反应植物在自然界中可为其他病原微生物提供营养来源，并受到一定的胁迫。在与病原微生物互作的进化过程中，植物不断产生一些复杂的免疫应答反应来抵御病原微生物的侵害。植物体内存在两种层次防御反应，分别为病原菌相关模式(Pathogen associated molecular pattern，PAMP)诱发的免疫反应(PAMP-triggered immunity，PTI)和效应因子诱发的免疫反应(Effector-triggered immunity，ETI)，二者在抵御病害过程中起重要作用。PAMP是一类具保守特征的小分子物质(如细菌的鞭毛蛋白短肽flg22、内毒素脂多糖、真菌鞘脂及几丁质等)，可为植物模式受体(Pattern recognition receptor，PRR)(如FLS2和EFR蛋白)识别，引发PTI反应[8-9]，如MAPK信号途径的激活以及活性氧爆发、胼胝质积累等防御现象[10]。除了PAMPs之外，病菌编码的一些分泌蛋白也能诱发PTI反应，即激发子(Elicitor)。传统定义上来讲，“激发子”是指能诱导植物产生植保素的一些分子。随着科技的发展，这个名词所适用的范围更广，如今多指能刺激植物防御的所有物质，包括来源于病原菌(外源性激发子)以及在侵染过程中植物自身产生的物质(内源性激发子)，最终提高植物抗病能力[11-13]。
稻瘟病菌激发子类型多样，有糖蛋白[14-15]和脂蛋白等[16]。现已证实菌丝细胞壁、细胞膜、分生孢子及菌丝发酵液中均含有激发子[17-20]。细胞膜成分鞘脂类物质可诱导水稻合成植保素、细胞死亡[21-23]。菌丝发酵液中的MoHrip1[24]和MoHrip2蛋白[25]引起烟草细胞程序性死亡，并且增强水稻免疫力。稻瘟病菌坏死-乙烯诱导蛋白1(Necrosis and ethylene-inducing peptide 1，Nep1)[26]和类Nep1(Nep1-like proteins，NLPs)蛋白MoNLP1、MoNLP2、MoNLP4[27]能引起烟草细胞发生细胞坏死。在对稻瘟病菌侵染阶段的转录分析中，Chen等[28]筛选到4个可诱导水稻和烟草组织产生细胞坏死的稻瘟分泌蛋白(MoCDIP1、MoCDIP2、MoCDIP3和MoCDIP4)。此外，Wang等[29]利用质外体流技术，在发病水稻质外体中分离出多个稻瘟病菌激发蛋白。其中，MSP1蛋白引发植物细胞坏死反应，并提高水稻抵抗稻瘟病能力。稻瘟病菌激发子基因在植物中的持续表达可增强广谱抗病性，如MgSM1转基因拟南芥、水稻对细菌和真菌病菌均有较好的抵抗能力[30-31]。
细胞壁成分几丁质所介导的PTI信号途径研究得较为全面。作为经典的PAMP，几丁质可激发植物防御反应，如植保素的合成[32, 33]、pH变化[34]、细胞膜稳定性[33-35]、防御基因诱导表达[36-37]等方面。然而，并不是所有类型的几丁质都能激发宿主PTI反应。据报道，聚合度6-8的几丁质对水稻细胞才有活性，而聚合度小于5的几丁质短链不足以引起水稻防御反应，并且诱导效应随着聚合度的提高而增强[35, 37]。植物细胞膜上分布着不同的受体蛋白，各司其职，以识别不同的信号。蛋白结合实验证明了水稻几丁质受体OsCEBiP蛋白可特异结合几丁质寡糖(GlcNAc)8[38-42]。OsCEBiP为水稻抗病反应所需，持续表达OsCEBiP基因可提高水稻抗稻瘟病和白叶枯病能力[43]。相反，OsCEBiP基因沉默后，水稻细胞无法识别几丁质(GlcNAC)8，最终导致水稻PTI免疫反应受抑制，丧失了抗病力[44]。OsCEBiP蛋白编码两个LysM结构域和一个跨膜结构域[44]，但单靠这两种结构不足以将几丁质信号由胞外往胞内转化。受体激酶OsCERK1则可协助OsCEBiP完成信号的转换[45]。在拟南芥中，AtCERK1识别并结合几丁质，在抵抗真菌病原菌过程中起关键作用[46-48]。作为AtCERK1的同源蛋白，虽然OsCERK1编码LysM和磷酸激酶结构域，但并不直接结合几丁质[49-50]。然而激酶结构域的存在，使胞外几丁质信号得以向胞内转换。OsCERK1为几丁质信号通路所必需，水稻Oscerk1突变体中几丁质信号传导受阻，抗病能力下降[51-53]。
OsRac编码鸟苷酸三磷酶(GTPase)，属于Rho-GTPase家族，在水稻抵御病原菌过程中起重要作用。OsRac GTPase在信号转导途径中充当分子开关，调控多种细胞生命活动。水稻基因组编码7个OsRac蛋白，其中OsRac1为关键调控因子。OsRac1响应PAMPs并参与PTI反应。当几丁质或真菌鞘脂后处理水稻原生质体之后，OsRac1快速聚集到细胞膜上[54-55]。持续表达型激活态(Constitutive active，CA)-OsRac1基因，可诱发水稻细胞内ROS的爆发、细胞凋亡、植保素合成以及相关防御基因表达，最终提高了水稻对稻瘟病的抵抗能力。反之，在水稻中持续表达该基因的失活态(Dominant negative，DN)，使OsRac1丧失活性，则抑制了上述的防御反应，抵消了水稻抗稻瘟病能力[54]。进一步研究发现，OsRac1通过两种途径来调控胞内活性氧(Reactive oxygen species，ROS)的水平。一方面，CA-OsRac1正调控OsRbohB，与之发生相互作用后，细胞内Ca2+水平迅速提高。累积的Ca2+激活了NADPH氧化酶，使后者不断产生活性氧ROS[56]。另一方面，OsRac1负调控ROS清除相关基因(比如OsMT2b)的表达以保证ROS的积累[56-57]，可见OsRac1在调节水稻细胞ROS爆发以及细胞死亡过程中起重要作用。此外，水稻与稻瘟病菌非亲和互作中，OsRac1为水稻NB-LRR抗病蛋白Pit直接激活[58]，这说明了OsRac1在水稻PTI和ETI反应中均起到重要作用。
与Rho家族成员一样，OsRac1的失活型(GDP结合型)和激活态(GTP结合型)构象之间的转换由鸟苷酸交换因子(Guaninenucleotide Exchanging Factors，GEFs)催化。据报道，两类鸟苷酸交换因子(OsSWAP70A和OsRacGEF1)参与OsRac1蛋白的激活[59-60]。水稻OsSWAP70A和OsRacGEF1分别编码Db1(diffuse B-cell lymphoma)-homology(DH)和PRONE(Plant-specific Rac/Rop)类型GEF。这两个基因的过表达均加剧了OsRac1介导的活性氧爆发，增强了几丁质介导的PTI反应和水稻抗稻瘟病菌能力[59-60]。
水稻进化出多个蛋白复合体来识别并转化PAMP信号[59-65]，以实现几丁质信号自外向内的转导。总的来讲，这些复合体主要由以下蛋白组成:OsCEBiP-OsCERK1、OsRac1、OsRacGEF1、热激蛋白Hsp90和Hsp70、分子伴侣Hop/Sti1、支架蛋白OsRACK1、级联反应相关OsMAPK3/OsMAPK6、转录因子RAI1/Rap2.6等[64-67]。OsRac1参与多个复合体的构成，在热激蛋白Hsp90、分子伴侣辅助因子Hop/Sti1和支架蛋白OsRACK1的协助下，OsRac1与OsRAR1、Hsp90、Hsp70组成复合体[64-67]。后续研究发现，在几丁质介导的PTI反应中，OsCERK1、Hop/Sti1a、Hsp90、Hsp70和OsRac1以复合体的形式在内质网和细胞膜上执行功能。分子伴侣和支架蛋白的存在则有助于承接OsCERK1与OsRac1之间的信号传导。
OsRac1和OsCERK1复合体成员在水稻免疫反应中起重要作用。在功能上，OsRAR1与OsRac1相互影响。植物RAR1(for required for Mla12 resistance)为多个R基因调控的抗病反应所需，如Mla、RPM1、RPS2、RPS5[68-75]。OsRAR1参与水稻抗稻瘟病菌过程，在水稻基础抗性中也起了重要作用。进一步研究发现，OsRAR1和Hsp90共同协助OsRac1调控稻瘟病菌鞘脂介导的PTI反应。持续表达CA-OsRac1基因可转录上调OsRAR1和OsSGT1(for suppressor of the G2 allele of skp1)，引起水稻细胞ROS爆发，激活抗病反应；OsRac1基因的沉默则抑制了OsRAR1基因的表达，这说明了OsRac1正向调控OsRAR1。OsRAR1也可影响OsRac1的功能发挥，OsRAR1基因的沉默削弱了CA-OsRac1转基因水稻的抗病力。可见，这两个基因之间存在某种程度的相互调控作用[64]。另外，分子伴侣辅助因子Hop/Sti1a也参与了几丁质介导的水稻抗病反应。过表达Hop/Sti1a基因明显提高了水稻对稻瘟病菌的抵抗能力；该基因的沉默则降低了水稻的抗病能力[63]。作为复合体中的支架蛋白，OsRACK1同样参与调控ROS爆发和PTI反应。过表达OsRACK1基因明显提高了水稻对稻瘟病菌的抗性[67, 76]。
丝裂原活化蛋白激酶(Mitogen-activated protein kinase，MAPK)级联反应参与水稻生长发育与基础抗病过程。作为MAPK的激酶，活性态OsMKK4(即OsMKK4-dd)可激活OsMAPK3/OsMAPK6，并且这3个蛋白均响应几丁质处理[77-78]。据报道，OsRac1正向调控OsMAPK3、OsMAPK6与转录因子OsRAI1。OsRAI1(bHLH transcription factor Rac Immunity 1)参与水稻抵抗稻瘟病[62]。OsRAI1和OsRac1均可与OsMAPK3、OsMAPK6发生直接互作，但尚未文章报道OsRAI1和OsRac1是否发生互作。在水稻原生质体细胞持续表达OsMKK4-dd与OsMAPK3/6后，防御相关基因OsPAL1、OsWRKY19转录水平明显提高[62]。因此OsRac1可能通过OsMAPK3、OsMAPK6来激活OsRAI1，而磷酸化的OsRAI1结合靶标基因的启动子区域，以启动相关防御基因的表达[26, 62]。
总的来讲，当水稻细胞尚未感知几丁质时(即非激活状态下)，OsCEKR1由内质网经囊泡运输至细胞膜，与伴侣蛋白、OsRacGEF1、Hop/Sti1和失活态的OsRac1组合成一个蛋白复合体。当膜受体OsCEBiP识别几丁质之后，OsCERK1立即与之形成二聚体。随后，OsRacGEF1-OsCERK1-分子伴侣形成的复合体从内质网转运到细胞膜。OsCERK1结合OsRacGEF1并对其进行磷酸化，后者则进一步识别并激活OsRac1。通过MAPK级联放大反应，OsRac1将信号逐步传到细胞核中，激活防御基因表达，诱导免疫反应(图 1)[79]。
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| 图 1 几丁质介导的水稻PTI信号传导 |
为了维持胞内稳态，植物进化出一些负调控因子，来抑制细胞的过激反应。水稻U-box E3连接酶OsSPL11(Spotted leaf11)负调控细胞程序性死亡和免疫反应。spl11突变体水稻广谱抗菌、体内防御基因转录水平和ROS含量偏高[80-82]。后续实验发现OsSPL11与GTP酶激活蛋白(GTPase-activating proteins，GAPs)RhoGAP SPIN6发生相互作用并将后者进行泛素化降解。SPIN6催化小GTP酶OsRac1由GTP结合态向GDP结合态转变，使其失活。SPIN6基因的沉默导致了活性态OsRac1的积累，激活了OsRac1复合体中其他基因(如OsSGT1和OsRAR1)转录表达，使得胞内活性氧水平剧增，引起细胞程序性死亡，对PAMPs(flg22和几丁质)更加敏感，最终提高了水稻对稻瘟病菌和白叶枯病菌的抵抗力[83]。OsRac1 GEF1催化OsRac1由失活态向激活态转化，正向调控OsRac1介导的水稻免疫反应[60]。相比之下，SPIN6则控制活性OsRac1的积累，防止过度免疫事件的发生，维持细胞内环境的稳定(图 1)。SPIN6对水稻PTI的影响则助于完善OsRac1复合体的功能，如SPIN6是否与OsRac1、OsRac1 GEF1相互作用，SPIN6是否与OsCERK1存在功能上的关联等。
2 稻瘟病菌效应蛋白的分泌在由几丁质介导的PTI反应中，几丁质短链并未进入水稻细胞中，而是通过细胞膜外受体识别，进而将几丁质信号由胞外往胞内转换。在与宿主相互作用过程中，稻瘟病菌往往通过分泌一系列效应蛋白来促进在水稻体内的增殖。稻瘟病菌的效应因子编码序列呈现多样化，但是根据其分泌途径的差异，效应因子可分为两类[84]:可进入植物细胞的胞质型效应蛋白(Cytoplasmic effector)[85-87]、不进入植物细胞的质外体效应蛋白(Apoplastic effectors)[86]。胞质型效应蛋白主要通过Biotropic Interfacial Complex(BIC)[88]进入水稻细胞中。BIC是一种源自植物细胞膜的多层膜结构，与初级侵染菌丝毗邻。随着侵染菌丝的扩展，BIC结构又转移到接近侵染菌丝顶端的位置。胞质型效应效应蛋白在BIC积累到一定程度后，转运到Extrainvasive Hyphal Membrane(EIHM)[88]后再进入植物细胞。这个过程则需要植物细胞囊泡运输系统(如Sso1 t-SNARE和exocyst复合体中的Exo70、Sec5[84, 89])协助完成。胞质型效应蛋白一般在侵染菌丝破坏植物细胞膜之前就分泌到植物细胞中，为后续侵染做准备，如抑制宿主免疫反应。效应因子PWL2[88]和AvrPiz-t[90]为典型的胞质型分泌蛋白，均可经过BIC结构分泌到水稻细胞中。
与胞质型效应蛋白相比，质外体效应蛋白不进入宿主细胞，而是停留或是分散在EIHM膜中，并包围整个侵染菌丝。EIHM也是一种源于植物细胞膜的膜结构[86, 88-89]。在侵染早期，这种膜结构可将整个肿胀侵染菌丝包围住。这期间质外体效应蛋白经过内质网-高尔基体这一传统分泌途径进入胞外间隔层中[91, 88-89]。效应因子BAS4[86, 92]和Slp1[92]为质外体型分泌蛋白，并未进入水稻细胞中。
3 稻瘟病菌致病因子介导的水稻抗性反应 3.1 效应因子介导的水稻ETI、ETS反应几乎所有的病原菌都带有PAMPs，然而植物仍然遭受侵染，这说明某些病原菌可以克服植物的PTI。病原菌通过分泌一些效应蛋白，绕过宿主的抵御防线，抑制PTI的产生，这个过程称为效应因子引发的感病反应(Effector Triggered Susceptible reaction，ETS)[93-95]。与此同时，植物也进化出基于R蛋白的第二道防线，直接和间接识别并结合病原菌的无毒蛋白(Avr)，即效应因子激发的免疫反应(Effector-triggered immunity，ETI)，主要表现出植物组织强烈的过敏性反应[93-95]。ETI反应模式符合基因-基因假说[95]，当与含相应R蛋白的宿主发生反应，由无毒基因编码或是加工的效应蛋白才显示出无毒的表型，即非亲和反应。近20年，水稻抗稻瘟病基因和稻瘟病菌无毒基因的克隆工作并驾齐驱。已克隆的水稻抗病基因普遍含有NBS-LRR结构域[96]，如Pib、Pita、Pi-kh(Pi54)、Pid2、Pi9、Piz-t、Pi2、Pi36、Pi37、Pi-km、Pi5、Pi21、Pit、Pid3、Pish、Pik、Pik-p、Pia、Pi25、Pil[97]以及Pi-CO39[98]、Pi41[99]、Pi55(t)[100]、Pi50(t)[101]等。目前超过10个稻瘟病菌无毒基因得到克隆与鉴定，如PWL2、AvrPita、Avr-CO39、AvrPiz-t、AVR-Pii、Avr-Pia、AVR-Pik/km/kp[97]和ACE1[102]、AVR-Pik-m[103]、AvrPi9[104]、AvrPib[105]。在水稻与稻瘟病菌的互作过程中，抗病基因与无毒基因之间可产生直接、间接物理相互作用，以启动高级防御反应。据报道，Pita/AvrPita、Pik/AvrPik、Pi-CO39/Avr1-CO-39、Pia/AvrPia[93, 106-107]等基因组合可发生直接互作。以Pita/AvrPita为例，稻瘟病菌无毒基因Avr-Pita编码的依赖于锌的金属蛋白酶，该蛋白的C端亮氨酸富集区可结合水稻Pi-ta，并参与稻瘟病菌整个致病过程。Pi-ta蛋白催化区域的突变会减弱二者之间的相互作用，说明Pi-ta很可能是Avr-Pita蛋白的一个底物[108-109]。相比之下，无毒基因AVR-Pii与水稻抗性基因Pii[91, 110]、AvrPiz-t与Piz-t[90, 111-113]不直接发生互作，而是需借助其他蛋白来完成互作。
在AVR-Pii与Pii的互作模式中，二者的成功识别需要其他水稻基因的参与，如囊泡运输相关蛋白、氧化还原相关的酶。AVR-Pii与水稻胞吐相关蛋白OsExo70-F2、OsExo70-F3发生直接的物理互作[91, 110]。在Pii背景水稻下，对OsExo70-F3基因进行沉默，转基因水稻则丧失了对AVR-Pii菌株的抵抗能力，但仍对亲和菌株的表现出感病性，这说明了OsExo70-F3特异参与Pii介导的抗病反应[91]。此外，苹果酸酶(NADP-ME 2-3)与AVR-Pii蛋白发生特异相互作用[110]。NADP-MEs催化氧化脱羧反应，将苹果酸可逆转变成丙酮酸，并伴随着NADP向NADPH的转化。NADPH是NADPH氧化还原酶的的电子供体，为细胞防御性氧爆发的一个重要源泉[114-115]。在非Pii水稻中，AVR Pii蛋白专一性地抑制OsNADP-ME 2-3的酶活力，阻止水稻细胞氧爆发，进而抑制水稻免疫防御反应[110]。在Pii水稻中，OsNADP-ME 2-3基因的沉默则导致了Pii水稻丧失了对AVR-Pii稻瘟病菌的抵抗力[110]。综上，OsExo70-F3和OsNADP-ME 2-3均参与AVR-Pii与Pii介导的稻瘟病菌-水稻的相互作用过程。然而OsExo70-F3和OsNADP-ME 2-3是否与Pii蛋白发生互作或是形成复合体则有待于进一步研究。
无毒基因AvrPiz-t与抗病基因Piz-t的作用模式需要E3连接酶、转录因子以及核孔蛋白的参与[90, 111-113]。当侵染非Piz-t水稻的时候，稻瘟病菌无毒基因AvrPiz-t执行有毒效应因子的功能，抑制宿主免疫反应。AvrPiz-t转基因水稻中的PTI反应受到不同程度的抑制，最终削弱了水稻的抗病性。AvrPiz-t蛋白不与Piz-t直接互作，而是与水稻蛋白APIP6、APIP10、APIP5和APIP12相互作用(互作模式如图 2所示)[90, 111-113]。在非Piz-t水稻中，AvrPiz-t通过诱导E3连接酶APIP6、APIP10泛素化降解来阻断信号传导，以抑制水稻PTI免疫反应，达到感病的目的[90, 111]。反之，APIP6/10亦可泛素化降解AvrPiz-t。在Piz-t水稻中，APIP10负调控Piz-t基因的表达，使其蛋白产物维持在较低的水平。当含有AvrPiz-t的稻瘟病菌侵染水稻后，AvrPiz-t蛋白进入水稻细胞，结合APIP10蛋白，解除了APIP10对Piz-t的抑制。随后，Piz-t蛋白迅速积累，导致HR爆发，引发下游抗病反应。APIP10基因的沉默导致水稻出现细胞程序性死亡，同时诱导Piz-t大量积累，可见AvrPiz-t蛋白通过抑制APIP10来稳定Piz-t表达[111]。
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| 图 2 稻瘟病菌AvrPiz-t与水稻Piz-t介导的ETI信号转导 |
APIP5编码一个bZIP转录因子，与AvrPiz-t、Piz-t发生直接相互作用[112]。APIP5以二聚体的形式进入细胞核中，负调控细胞坏死相关基因表达，抑制细胞程序性死亡。APIP5基因沉默导致了水稻自发细胞坏死症状，而AvrPiz-t的存在则加剧了坏死症状的发生。在非Piz-t水稻中，AvrPiz-t蛋白可在细胞质中结合并降解APIP5，进而解除了APIP5对细胞坏死的抑制，最终诱导稻瘟病菌侵染病斑的形成。在Piz-t水稻中，Piz-t蛋白的存在有利于维持APIP5蛋白的稳定性，抑制稻瘟病菌侵染后期坏死斑的形成。反过来，APIP5蛋白亦促进Piz-t蛋白的积累，最终激活ETI抗病反应[112]。
AvrPiz-t与APIP12的作用模式不同于上述3个水稻蛋白[113]。APIP12编码一个核孔蛋白，与Nup98同源。APIP12蛋白与AvrPiz-t、APIP6发生直接相互作用。在非Piz-t水稻背景下，APIP12基因的沉默或敲除均抑制了防御相关基因的表达，进而降低了水稻对稻瘟病菌的抵抗力。然而，在Piz-t水稻中对APIP12进行过表达或沉默，由Piz-t介导的ETI反应却不受影响，可见APIP12主要参与水稻基础免疫反应，该蛋白与AvrPiz-t的互作独立于ETI反应[113]。
除了无毒基因对宿主免疫相关基因进行修饰之外，病原菌还存在一类非无毒基因的效应因子，通过干扰PTI反应来抑制宿主抗病能力。植物病原菌编码的一些核心效应因子含LysM结构域效应蛋白，在致病过程中起重要作用。几丁质结合蛋白番茄叶霉病菌(Cladosporiu fluvum)ECP6[116-117]、稻瘟病菌Slp1[92]、油菜炭疽病菌(Colletotrichum higginsianum)ChELP1和ChELP2[118]蛋白结构保守，功能相似，均可抑制宿主PTI反应。稻瘟病菌Slp1与水稻细胞膜几丁质受体CEBiP蛋白竞争结合几丁质，以切断几丁质信号转导，最终抑制宿主防御反应(图 1)[92]。深入研究发现，Slp1受多个稻瘟病菌蛋白调控。首先内质网膜转运蛋白MoSec62决定了Slp1的正常分泌[119]。其次，α-1，3-甘露糖转移酶ALG3催化Slp1的糖基化修饰过程，糖基化的Slp1才能抑制宿主PTI反应[120]。MoSec62或ALG3基因缺失突变体菌体均可快速激活水稻防御相关基因的转录、活性氧爆发，导致无法顺利侵染水稻细胞，丧失致病能力[119-120]。与Slp1类似，稻瘟病菌分泌蛋白MC69基因的缺失则限制了侵染菌丝的扩展，导致稻瘟病菌对感病水稻和大麦的致病力下降[121]。同样，西瓜炭疽病菌(Colletotrichum orbiculare)中该同源基因CoMC69的敲除，则削弱了该菌对黄瓜和本氏烟草的致病力[121]，这说明MC69基因可能在单、双子叶病原菌中都起着致病的功能。
3.2 非效应因子型蛋白介导的水稻感病反应为保证顺利侵染水稻，稻瘟病菌通过加固侵染菌丝细胞壁来避开宿主细胞的识别。据研究，稻瘟病菌细胞壁成分α-1，3-葡聚糖可干扰水稻防御反应，并为病程所需[122]。当α-1，3-葡聚糖合成基因MgAGS1发生缺失或α-1，3-葡聚糖的合成受到抑制，稻瘟病菌丝对植物几丁质酶的敏感性则明显提高。MgAGS1的缺失激活了水稻防御相关基因表达，导致稻瘟病菌致病力下降。α-1，3-葡聚糖酶可水解α-1，3-葡聚糖，然而水稻基因组尚无该酶的编码基因。异源表达细菌α-1，3-葡聚糖酶编码基因可激活水稻防御相关基因表达，增强水稻广谱抗病能力。由此可见，α-1，3-葡聚糖可保护稻瘟病菌细胞壁，防止被水稻相关水解酶所降解，以阻止PAMP物质的释放，进而抑制宿主PTI的发生[122]。
PTI和ETI介导的宿主免疫防御反应常常伴随着活性氧的爆发，稻瘟病菌还可通过调节宿主细胞氧化还原环境来加速侵染过程。稻瘟病菌DES1编码一个富含丝氨酸的蛋白，该基因的缺失提高了稻瘟病菌对过氧化物胁迫的敏感性，并抑制了过氧化物酶和漆酶编码基因的正常转录活动。在侵染感病水稻初期，DES1缺失突变体可引起水稻细胞ROS爆发，同时也激活了PR防御基因的表达，使得侵染菌丝扩展受限，最终降低了稻瘟病菌的致病性。进一步研究发现，NADPH氧化还原酶抑制剂DPI可回补des1突变体的致病性[123]。与之类似，谷胱甘肽过氧化物酶编码基因MoHYR1参与清除体内活性氧，维持稳定的氧化还原环境，促进稻瘟病菌成功侵染水稻[124]。
除此之外，稻瘟病菌还面临另外一种胁迫，即一氧化氮(NO)介导的植物氧化反应。NO是植物免疫反应中的一个组成部分，与ROS一类化合物相互作用，并衍生出具有高度氧化活性的硝基类化合物，即活性氮(Reactive nitrogen species，RNS)[125]。活性氧和活性氮可阻止病原菌的进一步侵染。然而，稻瘟病菌的一些酶可清除活性氮积累，如氮酸酯单加氧酶NMO。在稻瘟病菌营养生长过程中，NMO2催化硝基烷的脱硝基化反应，缓解硝基氧化胁迫给菌体细胞带来的脂质硝化。NMO2基因的缺失抑制了侵染菌丝的扩展，并且引起水稻细胞氧爆发。NMO2基因对水稻氧化还原环境的调节也影响了稻瘟病菌效应蛋白的分泌，这种情况下效应蛋白无法抑制宿主PTI反应[126]。然而，提前用DPI处理水稻组织，使水稻细胞处于还原状态，阻止氧爆发，mno2突变体则可正常地侵染水稻组织。这从侧面反映了，当水稻失去活性氧这一抵御防线后，其自身产生的活性氮或是由NO介导的信号传导不足以抵抗稻瘟病菌的侵染。在对抗稻瘟病菌的侵染，水稻细胞内氧化环境的变化影响到抗病进程。以活性氧引起的ROS爆发起主导作用，而以活性氮引起的RNS起辅助作用，但二者均在水稻抗病过程中起重要作用，缺一不可。稻瘟病菌则通过释放一系列效应蛋白，调控水稻相关基因的表达，阻止抗病信号的传导，抑制ROS或是RNS的爆发，保证菌丝的成功增殖。
4 展望水稻与稻瘟病菌之间的相互作用是一项持久的“军事装备战”。为抵抗稻瘟病菌的侵害，在自然或是人工选育的条件下，水稻基因组进化出一些抗性相关基因，然而随着种植年限的延长或其他的气候因素，稻瘟病菌小种不断发生突变以攻克水稻防御体系。这些如此往复的相互进化事件推动了水稻-稻瘟病菌相互作用的发展。近20年来，水稻抗病基因、稻瘟病菌无毒基因的克隆为二者相互作用机理的解析提供了大量的实验基础。但从水稻PTI基础免疫反应到ETI高级防御体系，这中间包含极其复杂的互作网络，目前的研究只是掀开了该互作网络的一角。为了进一步揭示水稻与稻瘟病菌互作的机理，今后可以从以下几个方面开展研究:(1)新型水稻PRR受体的鉴定以及与PAMP的识别机制；(2)新型PAMP的发现，明确它们激发水稻免疫反应的途径；(3)新的效应蛋白和无毒蛋白基因的克隆，解析它们介导水稻感病和抗病反应的作用机理；(4)通过各种组学技术，进一步寻找水稻与稻瘟病菌互作网络中的关键节点蛋白，明确它们的功能。
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