謝成建
(重慶師范大學(xué)生命科學(xué)學(xué)院,重慶 401331)
大麗輪枝菌(Verticillium dahliaeKleb.)是一種土傳維管束真菌病害,嚴(yán)重影響農(nóng)業(yè)生產(chǎn)[1]。大麗輪枝菌能造成對農(nóng)業(yè)生產(chǎn)極大的危害,一方面原因是其致病力強、寄主范圍廣[2],變異性強,大麗輪枝菌基因組中存在大量轉(zhuǎn)座子以及頻繁的基因重組現(xiàn)象[3],轉(zhuǎn)座子與基因重組導(dǎo)致大麗輪枝菌產(chǎn)生不同的生理小種,對宿主很快產(chǎn)生抗性,生產(chǎn)上很難培育出長期抗大麗輪枝菌的作物品種。大麗輪枝菌防治困難的另一個原因是它能產(chǎn)生休眠結(jié)構(gòu)——微菌核[4],該結(jié)構(gòu)能讓大麗輪枝菌在土壤中存活20年[2];當(dāng)有合適寄主存在時,微菌核就成為土壤中大麗輪枝菌的初侵染源,目前還很難找到有效的手段清除田間的微菌核以控制大麗輪枝菌為害[2]。因此闡明大麗輪枝菌致病及微菌核形成的分子機理對于設(shè)計新的大麗輪枝菌防治技術(shù)具有重要意義,本文對近幾年大麗輪枝菌致病及微菌核形成相關(guān)基因的研究進行了總結(jié),旨為大麗輪枝菌致病及微菌核形成機理的進一步研究奠定理論基礎(chǔ)。
現(xiàn)在大多數(shù)研究者都認為大麗輪枝菌分泌到細胞外的毒素(包括細胞壁降解酶與效應(yīng)因子等)在大麗輪枝菌致病過程中發(fā)揮重要功能,研究發(fā)現(xiàn)大麗輪枝菌上清液能直接導(dǎo)致植物葉片萎蔫,可能與植物蛋白質(zhì)的翻譯調(diào)控過程相關(guān)[5]。
1.1.1 細胞壁降解酶 細胞壁降解類酶(Cell-walldegrading enzymes,CWDEs)被認為是決定大麗輪枝菌能否成功侵入植物的重要因素,在大麗輪枝菌(V.dahliae)和黑白輪枝菌(V. albo-atrum)基因組中存在大量植物細胞壁降解酶類基因[6]。大麗輪枝菌細胞壁降解酶在大麗輪枝菌侵染植物過程中主要發(fā)揮兩方面作用,一方面是通過降解植物細胞壁輔助大麗輪枝菌侵染;另一方面降解的細胞壁多糖供給大麗輪枝菌有利于其在植物內(nèi)低營養(yǎng)環(huán)境下的生長[7]。有研究表明細胞壁降解相關(guān)酶(果膠與纖維素)合成代謝通路的基因在大麗輪枝菌侵染植物時表達量明顯上升;與果膠代謝通路相關(guān)的兩個基因VdPL3.1與VdPL3.3被敲除后,大麗輪枝菌致病力降低[8]。細胞壁降解相關(guān)基因(Sucrose nonfermenting 1 gene,VdSNF1)[9]與內(nèi)葡聚醣酶基因(Endoglucanase,EG-1)被T-DNA插入突變后導(dǎo)致大麗輪枝菌致病力降低[1],研究也發(fā)現(xiàn) β-1,6-endoglucanase gene(VegB)被敲除后,大麗輪枝菌降解纖維素的能力降低[10];VdSSP1是從高致病力大麗輪枝菌VDG1分離到的分泌蛋白,VdSSP1突變體在以果膠(Pectin)或淀粉(Starch)為碳源的培養(yǎng)基上生長減緩,致病力也降低,當(dāng)VdSSP1被互補到突變體或低致病力大麗輪枝菌VDG2后回復(fù)或提高了大麗輪枝菌的致病力,推測該基因與植物細胞壁降解相關(guān)[7]??梢姡毎诮到庀嚓P(guān)酶在大麗輪枝菌侵染過程中發(fā)揮重要的功能。許多植物的表皮覆蓋著一層角質(zhì),既可以防止植物水分流失,也可以防止病原菌入侵[11],研究發(fā)現(xiàn)大麗輪枝菌角質(zhì)酶(Cutinase)VdCUT11能夠誘導(dǎo)植物細胞死亡以及植物的的防御反應(yīng),而具有糖基(Carbohydrate)結(jié)合域的蛋白VdCBM1可抑制VdCUT11的功能,可見角質(zhì)酶降解植物角質(zhì)層的活性是其功能的關(guān)鍵[12]。
1.1.2 效應(yīng)因子相關(guān)基因 效應(yīng)因子在真菌與植物的互作過程中發(fā)揮重要作用[13],一些大麗輪枝菌的效應(yīng)因子被發(fā)現(xiàn),如Ave1是大麗輪枝菌生理小種race 1中分離到的一個效應(yīng)因子,其基因位于大麗輪枝菌遺傳特異性變異基因區(qū)[Highly dynamic lineage-specific(LS)genomic regions][14],Ave1 被番茄細胞表面受體Ve1識別后,介導(dǎo)番茄對race 1的抗性[15-16];在煙草中穩(wěn)定表達的Ave1甚至不需要Ve1存在就能誘導(dǎo)植物防御相關(guān)基因的表達[17]。
真菌在侵染植物時一方面要通過植物細胞壁降解酶輔助侵染,另外一方面也要防止自身細胞壁被植物真菌細胞壁降解酶降解,LysM是一類可以結(jié)合在真菌細胞壁幾丁質(zhì)上的效應(yīng)因子,在真菌侵染植物時LysM結(jié)合在真菌細胞壁上防止真菌細胞壁被植物幾丁質(zhì)酶降解,Vd2LysM是在大麗輪枝菌中發(fā)現(xiàn)的一類LysM效應(yīng)因子,Vd2LysM敲除后,大麗輪枝菌對番茄的致病力降低[18]。VdCP1屬于SnodProt類分泌蛋白,在大麗輪枝菌侵染植物時表達量升高,推測VdCP1在大麗輪枝菌侵染植物時保護其細胞壁免受植物細胞壁降解相關(guān)酶類的破壞,VdCP1敲除后影響大麗輪枝菌致病力[19]。
PevD1是大麗輪枝菌中分離到的一個能引起植物防御反應(yīng)的效應(yīng)蛋白,植物中表達的PevD1提升了植物對灰葡萄孢菌(Botrytis cinerea)、丁香假單胞菌(Pseudomonas syringaepv. Tomato DC3000)與煙草花葉病毒TMV的抗性[20-21],甚至參與了植物開花周期的調(diào)控[21],進一步研究表明PevD1通過拮抗NRP蛋白的功能間接提升植物細胞質(zhì)內(nèi)CRY2的量來發(fā)揮調(diào)控功能[22]。
VdIsc1是在大麗輪枝菌中發(fā)現(xiàn)的一類缺乏信號肽的效應(yīng)因子,進入植物細胞后抑制植物SA的產(chǎn)生,輔助大麗輪枝菌的侵染[23];效應(yīng)因子VdSCP7被大麗輪枝菌分泌后進入植物細胞核提升植物的免疫反應(yīng),VdSCP7被敲除后大麗輪枝菌致病力增強[24]。
壞死與乙烯誘導(dǎo)相關(guān)蛋白(Necrosis- and ethyleneinducing-like proteins,NLPs)是被廣泛研究的大麗輪枝菌毒素蛋白,瞬時表達或體外表達的VdNEP蛋白能夠使煙草、棉花[25]及向日葵葉片枯萎[26],研究發(fā)現(xiàn)8個同源的NLPs基因(NLP1-7,9),其中NLP2、NLP3、NLP4與NLP5包含一個保守功能域GHRHDWE;根據(jù)氨基酸數(shù)量的不同NLPs被分為含兩個半胱氨酸的I類(NLP1,NLP2、NLP3與NLP6)與含4個半胱氨酸的II類(NLP4,NLP5、NLP7與NLP9)。研究發(fā)現(xiàn)僅NLP1與NLP2能誘導(dǎo)植物壞死,被敲除后影響大麗輪枝菌的產(chǎn)孢能力且大麗輪枝菌菌絲生長更加旺盛,但僅輕微影響大麗輪枝菌致病力[27-28]。
研究發(fā)現(xiàn)四跨膜蛋白VdPls1通過與NADPH氧化酶VdNoxB互作激活VdNoxB產(chǎn)生活性氧并提升大麗輪枝菌細胞內(nèi)Ca2+濃度,最終激活轉(zhuǎn)錄因子VdCrz1調(diào)控大麗輪枝菌形成侵入釘(Penetration pegs)結(jié)構(gòu)輔助侵染,VdPls1與VdNoxB被敲除后大麗輪枝菌無法侵入植物[29]。侵入釘與菌絲之間的頸環(huán)結(jié)構(gòu)處會分泌大量效應(yīng)因子輔助大麗輪枝菌的侵染,septin蛋白VsSep5、F-actin、Exocyst相關(guān)蛋白VdExo70與VdSec8、ER-Golgi轉(zhuǎn)運蛋白VdSec22以及endosome介導(dǎo)的轉(zhuǎn)運相關(guān)蛋白VdSyn8在頸環(huán)結(jié)構(gòu)的形成與效應(yīng)因子分泌過程中扮演了重要角色,效應(yīng) 因子 VdSCP5、VdSCP8、VdSCP9與 VdSCP10都定位于頸環(huán)處[30]。
cAMP-dependent protein kinase A是大麗輪枝菌中乙烯合成相關(guān)的基因,其突變導(dǎo)致大麗輪枝菌乙烯合成減少,相應(yīng)的大麗輪枝菌致病力也降低[31],G蛋白亞基(G protein beta subunit)基因的敲除影響大麗輪枝菌侵染后植物葉片乙烯的合成[32],大麗輪枝菌侵染后植物乙烯代謝相關(guān)通路的基因明顯上調(diào)[33]。擬南芥乙烯相關(guān)通路基因突變后,植物在遭受黃萎病侵染時癥狀會減輕[34],這些研究都表明大麗輪枝菌與宿主植物乙烯的合成與大麗輪枝菌致病緊密相關(guān)。
一些與大麗輪枝菌營養(yǎng)代謝調(diào)控與傳遞相關(guān)的基因被敲除后也影響大麗輪枝菌的致病力,如NUC-2是phosphate(Pi)代謝通路的關(guān)鍵基因[35],VdNRS/ER參與大麗輪枝菌細胞壁鼠李糖(Rhamnose)的合成[36],它們突變后影響大麗輪枝菌的致病力。大麗輪枝菌VdTHI4參與噻唑(Thiazole)合成,而VdThit參與維生素B1(Thiamine)的運輸,VdTHI4與VdThit被敲除后影響大麗輪枝菌的致病力[37-38]。在侵染植物的過程中大麗輪枝菌也面臨低氨基酸營養(yǎng)的情況,這時真菌啟動自身氨基酸的合成通路,大麗輪枝菌與長孢輪枝菌中發(fā)現(xiàn)的轉(zhuǎn)錄因子CPC1參與氨基酸的合成,VlCPC1突變后大麗輪枝菌在氨基酸缺乏的環(huán)境中生長被抑制且致病力降低,長孢輪枝菌氨基酸合成相關(guān)基因Vlaro2突變后影響其侵染也印證了氨基酸的合成對輪枝菌屬致病菌致病力非常重要[39]。
一些大麗輪枝菌生長發(fā)育相關(guān)基因也與大麗輪枝菌致病力相關(guān),如細胞自噬在生物的發(fā)育抗逆等方面發(fā)揮了重要作用,研究發(fā)現(xiàn)大麗輪枝菌細胞自噬相關(guān)基因VdATG8與VdATG12被敲除后影響大麗輪枝菌的致病力[40]。VdQase是具有cupin保守功能域及槲皮素酶(Quercetinase)活性的蛋白[41-42];VdRac1是一個小的GTPase,該蛋白與VdCla4互作調(diào)控大麗輪枝菌的極性生長[43];而Vdsho1具有4個跨膜功能域,在HOG-MAPK調(diào)控通路上游發(fā)揮功能[44],VdQase、VdRac1、Vdsho1被敲除后都影響了大麗輪枝菌的致病力。轉(zhuǎn)錄調(diào)節(jié)子VdSge1與大麗輪枝菌菌絲生長與孢子產(chǎn)生相關(guān),它同時參與了多個候選效應(yīng)因子的調(diào)控[45],而bZIP轉(zhuǎn)錄因子VDAG_08676參與大麗輪枝菌孢子形成及氧化脅迫防御等[46],它們被敲除后影響大麗輪枝菌的致病力。
微菌核不僅是大麗輪枝菌的休眠結(jié)構(gòu),而產(chǎn)微菌核能力常常與大麗輪枝菌致病力相關(guān)[47],如果能闡明微菌核形成的分子機理對揭示大麗輪枝菌的致病機理以及設(shè)計新的大麗輪枝菌防治方案都十分重要。一些研究發(fā)現(xiàn)在微菌核形成過程中,外層菌絲細胞膨大,分泌出黑色素粒子,填充細胞間隙,大量黑色素顆粒成纖維狀包裹在菌絲的細胞壁上,最外層菌絲細胞自溶(Autolysis)形成緊密結(jié)構(gòu)[48-50],這個緊密結(jié)構(gòu)能保護大麗輪枝菌度過低溫等嚴(yán)酷的外界環(huán)境[2]。有關(guān)微菌核形成機理的研究主要包括黑色素合成與微菌核形成兩個方面。
研究發(fā)現(xiàn)大麗輪枝菌微菌核的黑色素屬于DHN-melanin類黑色素[51-52]。在微菌核的形成過程中,黑色素合成相關(guān)基因的表達明顯上調(diào)[53];DHN-melanin 合成通路基因VdPKS1[54]與Vayg1[52]被敲除后,大麗輪枝菌不能形成黑色素及微菌核;大麗輪枝菌真菌特異性轉(zhuǎn)錄因子Vdpf調(diào)控黑色素代謝通路相關(guān)基因的表達,Vdpf基因被敲除后大麗輪枝菌黑色素及微菌核形成能力受阻[55]。盡管黑色素與微菌核形成緊密相關(guān),但研究發(fā)現(xiàn)黑色素并不是微菌核形成的必須因素,一些黑色素合成突變體也可形成白化或棕色微菌核[48,56],但黑色素對大麗輪枝菌形成具有完整功能的微菌核非常重要[57]。
在微菌核形成過程中許多基因被調(diào)控,研究發(fā)現(xiàn)許多轉(zhuǎn)錄因子與大麗輪枝菌微菌核的形成相關(guān),如 MADS-Box類轉(zhuǎn)錄因子VdMcm1[58]、APSES類轉(zhuǎn)錄因子Vst1[59]、真菌特異性轉(zhuǎn)錄因子Vdpf[55]及鈣調(diào)控轉(zhuǎn)錄因子VdCrz1[60]等,這些轉(zhuǎn)錄因子對應(yīng)基因被敲除后大麗輪枝菌微菌核形成受阻;而粘合轉(zhuǎn)錄激活子Vta2通過調(diào)控基因的表達而抑制大麗輪枝菌微菌核的形成,在微菌核形成過程中發(fā)揮負調(diào)控的功能[61]。
活性氧被發(fā)現(xiàn)誘導(dǎo)絲狀真菌產(chǎn)生菌核或微菌核[62],研究發(fā)現(xiàn) Nox蛋白(Flavinoxidore ductase/NADH oxidase)在微菌核產(chǎn)生過程中發(fā)揮功能,該基因被敲除后影響活性氧誘導(dǎo)下綠僵菌微菌核的形成[63-64],大麗輪枝菌同源基因VdNoxB被敲除后也影響其微菌核形成,同時該基因也參與大麗輪枝菌侵染植物過程中侵入釘(Penetration pegs)的形成[29]。
此外一些分泌蛋白、細胞壁蛋白、跨膜蛋白及疏水蛋白被發(fā)現(xiàn)與微菌核或菌核形成相關(guān),它們參與把外界環(huán)境壓力信號傳遞到真菌細胞內(nèi),誘導(dǎo)真菌產(chǎn)生微菌核或菌核。如核盤菌的分泌蛋白基因Ss-Caf1[65]與細胞壁蛋白基因Ss-Sl2[66]被沉默后影響核盤菌菌核的形成,進一步研究發(fā)現(xiàn)Ss-Sl2與 GAPDH、Hex1及 elongation factor 1-alpha互作并調(diào)控核盤菌菌核形成[66],而綠僵菌跨膜蛋白Sho1p與Sln1p傳遞細胞外壓力信號到細胞內(nèi)部,激活MrHog1激酶(MAPK)[67-68],從而誘導(dǎo)微菌核形成[69],而真菌 Hog1 被 Pbs2(MAPKK)所調(diào)控[70]。大麗輪枝菌跨膜蛋白VdMsb是一種跨膜黏液素,研究發(fā)現(xiàn)其參與MAPK通路信號傳遞,VdMsb被敲除后大麗輪枝菌無法形成微菌核[71]。MAPK(Mitogenactivated protein kinase)相關(guān)基因與許多真菌微菌核或菌核形成的相關(guān)性被深入研究,如大麗輪枝菌的VdHog1[72]、VdPbs2[73]、VMK1[4];與大麗輪枝菌類似,核盤菌(Sclerotinia sclerotiorum)的Smk1[74]和Smk3[75]及綠僵菌(Nomuraea rileyi)的Mrhog1與Mrslt2[76]等也屬于MAPK通路基因,這些基因被敲除或被沉默后影響真菌微菌核或菌核的形成。大麗輪枝菌cerevisin基因被敲除后影響微菌核的形成,進一步研究發(fā)現(xiàn)大麗輪枝菌分泌蛋白減少,減少的分泌蛋白就包括VdASP F2[77],當(dāng)VdASP F2被敲除后,大麗輪枝菌在外界環(huán)境壓力下(低溫或低營養(yǎng))的微菌核形成明顯延遲[78]。大麗輪枝菌壓力脅迫調(diào)節(jié)子VdSkn7突變后影響大麗輪枝菌壓力耐受、產(chǎn)微菌核及侵染植物等方面的能力[79]。富含谷氨酸蛋白VdGARP1被敲除后,大麗輪枝菌微菌核形成延遲,推測VdGARP1在大麗輪枝菌感受外界脅迫方面發(fā)揮著重要的作用[80]。
一些與生長發(fā)育相關(guān)的基因突變后也影響大麗輪枝菌微菌核的形成,大麗輪枝菌疏水蛋白VDH1被發(fā)現(xiàn)與微菌核形成相關(guān),但VDH1被敲除后并不影響大麗輪枝菌產(chǎn)孢及致病能力,分析其可能參與微菌核形成中菌絲的黏附過程[81-82]。此外一些其它基因如核糖體相關(guān)基因VdRACK1被敲除后影響大麗輪枝菌生長相關(guān)性狀包括產(chǎn)微菌核能力[83];葡萄糖阻遏介導(dǎo)蛋白VdCYC8[84]、糖代謝調(diào)控蛋白VdSNF1[9]、尿嘧啶 DNA 糖基化酶 VdUDG[85]及病程相關(guān)蛋白類似蛋白 VdPR1[86]與 VdPR3[87]也與微菌核形成相關(guān),相關(guān)基因被敲除后影響大麗輪枝菌微菌核的形成及致病力。VdRACK1屬于Gbetalike/RACK1類蛋白,擁有WD40保守功能域,該基因被敲除后影響大麗輪枝菌孢子形成與萌發(fā)、菌絲生長及產(chǎn)微菌核的能力,同時也影響大麗輪枝菌對具有完整根系的棉花的侵染[88]。
從目前的研究結(jié)果來看,黑色素DHN-melanin的合成決定了大麗輪枝菌能否形成完整成熟的微菌核,同時微菌核形成相關(guān)的基因往往也與大麗輪枝菌致病力相關(guān),微菌核形成機理的研究有助于解析大麗輪枝菌致病分子機理。
此外,大麗輪枝菌侵入釘結(jié)構(gòu)的發(fā)現(xiàn)對于闡明大麗輪枝菌致病分子機理具有重要的意義,特別是一些參與大麗輪枝菌與植物互作的效應(yīng)因子正是從該侵染結(jié)構(gòu)分泌,隨后進入植物細胞調(diào)控植物的免疫系統(tǒng)。但目前發(fā)現(xiàn)的大部分效應(yīng)因子在植物表達后往往能增強植物的免疫反應(yīng),提高了植物對真菌、細菌與病毒的抗性。效應(yīng)因子激發(fā)植物免疫反應(yīng)與大麗輪枝菌這種半活體營養(yǎng)型致病菌之間的關(guān)系并不清楚,同時多個效應(yīng)因子在植物中的靶標(biāo)或者互作蛋白尚不清楚,效應(yīng)因子在大麗輪枝菌與植物早期互作過程中的作用也需進一步闡明。
目前的結(jié)果大部分只是對單個基因的研究,未能系統(tǒng)的揭示大麗輪枝菌致病及微菌核形成的分子機理。在進一步的研究中,大麗輪枝菌微菌核的形成機理及大麗輪枝菌效應(yīng)蛋白作用機理將是重要研究方向。特別是近期的一項研究預(yù)測大麗輪枝菌中具有潛在效應(yīng)因子可能的基因有181個[89],這表明大麗輪枝菌中的效應(yīng)蛋白還需要進一步的深入探索。
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