李振轮李鑫强杨水英
(1. 西南大学资源环境学院 土壤多尺度界面过程与调控重庆市重点实验室,重庆 400715;2. 西南大学植物保护学院,重庆 400715)
土壤因子对绿僵菌生命活动的影响研究进展
李振轮1李鑫强1杨水英2
(1. 西南大学资源环境学院 土壤多尺度界面过程与调控重庆市重点实验室,重庆 400715;2. 西南大学植物保护学院,重庆 400715)
绿僵菌是一种重要的昆虫病原真菌,它广泛分布于世界各地的土壤之中,在调节和控制土壤中有害昆虫方面发挥着重要作用。但在农业实际应用过程中,绿僵菌杀虫剂的施用效果很不稳定,原因之一是受到了土壤因子对其生命活动的影响。从土壤温度和水分、土壤pH、土壤微生物和土壤质地4个方面归纳总结了土壤因子对绿僵菌生命活动影响的研究进展,并对土壤因子影响绿僵菌生命活动的新研究方向进行了初步讨论,以期为进一步研究土壤因子影响绿僵菌生命活动的作用机理及绿僵菌在实际生物防控地下害虫领域的广泛和高效应用研究提供理论参考。
土壤因子 昆虫病原真菌 绿僵菌 土壤温度和水分 土壤pH 土壤质地
绿僵菌(Metarhiziumspp.)属半知菌亚门(Deutero-mycotina)、丝孢纲、丝孢目、绿僵菌属(Metarhiziumsorokin),是一种昆虫病原真菌。它能寄生8个目200余种昆虫及一些螨类和线虫。因其易于培养和寄主范围广的优点,使绿僵菌成为了当今世界应用最广泛的生物杀虫剂之一[1,2]。
绿僵菌广泛分布于世界各地土壤之中,尤其集中分布在根际土壤之中[3,4],是一种常见的土著昆虫病原真菌[5,6]。适宜的土壤环境能增加绿僵菌分生孢子的含量和菌的活力,有助于绿僵菌的生物防控功能[7]。 研究显示,绿僵菌对持续控制地下害虫的数量等方面,发挥着举足轻重的作用[4,6,8]。目前,人们对绿僵菌的研究主要集中在绿僵菌与寄主相互作用的分子机制方面,绿僵菌分生孢子对寄主的识别、萌发和侵入寄主的机理,逃避寄主免疫、分泌胞外水解酶降解消耗寄主血腔营养以及分泌毒素等分子机理已经有了深入的认识。
尽管利用绿僵菌制成的微生物杀虫剂在防治地下害虫方面具有重要的意义[9,10],但在实际应用过程中,其施用效果常常不稳定[11],原因之一是忽视了土壤因子对于绿僵菌生命活动的影响。土壤是一个复杂的生态系统,绿僵菌一旦进入土壤这个带电多孔介质后,其生命活动必然会受到土壤中有生命和无生命因子的影响[12]。目前研究结果表明土壤类型、质地、温度、水分和pH值等土壤宏观性质会直接或者间接影响绿僵菌在土壤中的移动、数量、侵染率等[6],但是这些研究都集中在现象的描述,其作用机理并不清楚。基于目前国内外这方面的研究成果,本文从土壤温度和水分、土壤pH、土壤微生物、土壤质地等4个方面归纳总结了土壤因子对绿僵菌生命活动的影响,以期为进一步研究土壤因子影响绿僵菌生命活动的作用机理及绿僵菌在实际生物防控地下害虫领域的广泛和高效应用研究提供理论参考。
1.1 土壤温度、水分
温度和水分是土壤因子的重要组成部分,同时也是影响绿僵菌数量和分布的最主要的因子。它们通过共同作用来影响绿僵菌的生存和侵染率。目前,土壤温度和水分对于土壤中昆虫病原真菌及其分生孢子影响的研究比较全面。绿僵菌集中分布在20-30℃的土壤中[13],且它的最适生长温度为30℃[14];在水分适量的情况下,绿僵菌分生孢子的最适萌发温度为33-34℃[15];15℃以下时,绿僵菌在土壤中的数量较少[16]。绿僵菌最适生长的水分活度值在0.97-0.99之间[14]。在水分活度值小于0.333时,绿僵菌分生孢子特别容易受到吸胀损伤的影响[17]。Ekesi[13]等用土水势研究的结果显示,在适宜温度下,土水势在-0.1和-0.01 Mpa时绿僵菌分生孢子对地中海果蝇蛹的侵染率最高,而-0.0055 MPa和-0.0035 Mpa时侵染率最低。这与其他人的研究结果一致,当土壤中水分含量长时间处于高水平时,如遭遇雨季等,绿僵菌的数量会大幅度的减少甚至消失[18];短时间内大规模的降雨和洪水会突然增加土壤中水分的含量,同样会加速减少绿僵菌孢子的数量和分布[6]。
但是,土壤温度对于绿僵菌生命活动的影响往往受限于其他环境因子,如光、水分、化学药品等[18]。其中,水分的影响比较显著。如Ekesi等[13]研究发现,在干土中,低温(15℃)条件下对绿僵菌孢子的生存没有影响;在湿土中,最适温度(30℃)下,绿僵菌菌落数量也会急剧降低。即使是在适宜的温度范围,干燥绿僵菌分生孢子的萌发率也会降低44%[19]。
1.2 土壤pH
土壤pH对绿僵菌的分布和生理活动具有重要影响。Hallsworth和Magan[20]的研究表明,包括绿僵菌在内的3种土著昆虫病原真菌的最适生长pH范围为5-8,并且指出pH显著影响分生孢子的基因表达,这可能是参与诱导了水的应激反应有关。绿僵菌在pH<7和pH 8-8.5的土壤中分布数量相似,并且都显著高于其它pH值土壤中绿僵菌的数量[12]。St Leger等[21]研究发现,周围环境pH对于土壤中绿僵菌分泌昆虫体壁分解酶和疏水蛋白起到了决定性作用,如合成并且分泌天冬氨酰磷酸酶的最适pH为3,分泌几丁质酶的最适pH为5;pH 5-8时,疏水蛋白的分泌量没有显著差异,但是pH降为 3时,疏水蛋白的分泌量为零。尽管土壤pH对绿僵菌毒力基因表达能产生强烈的影响,但是绿僵菌的变异菌株也会通过调节草酸的分泌量来改变周围土壤环境的pH,以此保证分泌的胞外蛋白酶的产量和活性[19]。所以,土壤pH既可以影响土壤中绿僵菌的生命活动,也可以被绿僵菌的代谢产物调节和影响。
1.3 土壤微生物
土壤是由不同种类的无生命的、有生命的物质所组成的复杂生态系统。土著昆虫病原真菌与广泛存在于土壤中的真菌、细菌、原生动物、线虫、节肢动物等存在着寄生、捕食和拮抗等相互作用[17,22-24]。其中,土壤微生物通过在营养物质和生存空间方面的竞争,以及寄生和拮抗作用等对土著昆虫病原真菌产生直接或间接的显著影响[25,26]。Zou[27]和Popowska-Nowak等[28]报道了多种细菌和放线菌等可以合成抗真菌的物质(乙酰胺、苯甲醛等),这些物质能够显著的抑制多种昆虫病原真菌孢子的萌发和菌丝的生长。而且,越来越多的证据证明了土壤中微生物能够合成分泌一类很广范围的抗真菌有机物质[25,26],这些有机物可能对绿僵菌的生命活动产生显著影响。
土壤中最重要的、广泛分布的3种昆虫病原真菌:虫生真菌粉质拟青霉(Paecilomyces farinosus)、白僵菌(Beauveria bassiana)及绿僵菌(M. anisopliae)[29],它们之间也存在着全面且长远的竞争关系,尤其是白僵菌和绿僵菌之间。研究表明,在土壤生态环境中,特别是在种植小麦和棉花的土壤中[30],白僵菌数量显著高于其他两种昆虫病原真菌[29,31]。白僵菌在高黏粒含量、高土壤pH和低有机质含量土壤中的数量远大于绿僵菌[12]。Sun等[32]研究发现,白僵菌对一种雌性璃眼蜱的致死率显著高于绿僵菌,表现出更强的毒性[33]。得出一致结果的还有Hussein等[30],他们的研究结果显示,在昆虫病原真菌致死的大蜡螟幼虫中,白僵菌占了85.71%,而绿僵菌只占14.29%。这些研究结果表明,绿僵菌在土壤中的分布和生命活动会受到白僵菌显著的影响。
土壤中也存在着许多微生物和绿僵菌友好的共生在土壤生态系统中。Krauss等[34]和Lopez 等[35]报道了一种寄生真菌(Trichoderma),不仅对绿僵菌的生命活动没有影响,而且它们能够长期共存,共同有效地致力于对害虫的生物防控。Pava-Ripoll等[36]研究发现在含有豆科植物根分泌物的根际土壤中,绿僵菌分生孢子的萌发率与一种土壤腐生微生物(Trichoderma harzianum)孢子的发芽率相同,暗示绿僵菌和这种腐生微生物可以共生。与此同时,不同种类的绿僵菌之间也会分泌不同的蛋白质,与土壤中的不同因子发生复杂的相互作用来维持生存[37]。
1.4 土壤质地
土壤质地对绿僵菌及其分生孢子在土壤中的数量、分布以及生命活动的影响受到越来越多的关注。Roberts[38]研究表明,绿僵菌分泌的能够引起大蜡螟幼虫强制性麻痹的外毒素,大量集中的分布在土壤表层的中下部,而在表层向下10 cm范围分布较少。这表明土壤的不同分层会影响绿僵菌的分布,绿僵菌的生命活动也会受到显著的影响。Salazar等[39]试验证明了绿僵菌分生孢子在不同土壤中垂直移动与分布不同。在某些土壤中,分生孢子能快速的移动到整个土壤剖面均匀分布,而在有的土壤中移动缓慢,且分布不均。这表明土壤的质地和组成也会影响绿僵菌及其分生孢子的生命活动。
为了更加深入的解释土壤对绿僵菌生命活动影响的原因,人们进行了多方面的研究。Klingen等[40]研究表明,绿僵菌在黏粒含量低的土壤上分布数量较多,在相同土壤基质势时,绿僵菌在黏土中30 d后数量下降显著,而在沙壤土上要60 d时数量才开始减少。这一结果与Quesada-Moraga等[12]研究结果一致,试验表明,在低含量或者中等含量黏粒(<10%)的土壤中,绿僵菌的菌落数量达到最大。并且,土壤孔隙的大小和分布会显著影响绿僵菌的致病性[41]。
同时,土壤中的有机质的含量也会影响绿僵菌的分布。有机质含量越高,绿僵菌数量越多;绿僵菌几乎不存在于有机质含量低(<2%)的碱性沙土中。然而Scheepmaker等[6]的研究发现泥炭含量越高的土壤,越不利于绿僵菌的生存,但是越有利于绿僵菌分生孢子在土壤中的移动。泥炭也是有机质,这种结论出现矛盾的原因还有待进一步研究。绿僵菌在含有离子(KCl)的培养基中的生长速率显著高于在非离子(甘油)和惰性溶液(PEG600)培养基中的生长速率[14];但Garrido-Jurado 等[42]研究显示,土壤中添加不同离子强度的CaCl2溶液,却不影响绿僵菌分生孢子对一种地中海果蝇围蛹的的侵染率。尽管作用的机制尚不清楚,但由此可以推测土壤中的某些电解质溶液会显著影响绿僵菌的生长,但是可能对侵染力影响不显著。
土壤为包括绿僵菌在内的许多昆虫病原真菌提供了适宜的栖息场所[43],同时,昆虫病原真菌也在为控制土著害虫的数量上发挥着非常重要的作用。
土壤温度、水分对昆虫病原真菌的影响,往往受到地理位置、气候、季节交替等多方面综合因素的影响,从而在宏观上影响着绿僵菌的整体分布,也使得土壤温度和水分成为土壤影响绿僵菌生命活动的主导因素。土壤pH值、微生物种群结构和土壤质地,从微观角度调节绿僵菌的数量以及各项生理功能,它们3个因子也构成了相互影响、共同作用的综合性影响因子。土壤pH和土壤质地,会显著影响微生物的群落结构;而各种微生物的不同新陈代谢产物,也会反作用于土壤pH和土壤质地,从而产生显著的影响。同时,人类的农业生产活动,如耕作、种植不同作物[44]、施加化学农药、添加硅藻土[45]等,会局部地、间接地改变土壤水分状况、土壤pH值、微生物种群结构和土壤质地组成,从而影响了昆虫病原真菌在土壤中的分布和其生命活动。所以,在绿僵菌实际生防应用过程当中,不能单一考虑某种土壤因子的影响,而是要综合性分析各个土壤因子之间的关系,从中找出影响因子的主次关系,这可以为绿僵菌在实际农业生产中生物防控领域的广泛和高效地应用提供理论基础。
虽然一些化学农药、杀虫剂、除草剂和杀菌剂在纯培养基上的试验结果表明这些化学药剂对绿僵菌等昆虫病原真菌有抑制甚至致死的作用,但这些化学药剂进入土壤这个多空介质后对绿僵菌等昆虫病原真菌的作用会反生很大变化,有的甚至变成促进病原真菌对寄主的侵染[46,47]。然而目前这方面的研究结果很有限,因此值得更多的去研究和积累。研究成果将为我们在实际的生产过程中合理利用化学农药和生物农药提供理论依据,可以极大地提高土著害虫的综合防治效率。
尽管目前研究结果已经证明,土壤pH等宏观性质会显著影响绿僵菌及其分生孢子的生 命活动,但鲜有深入研究这些土壤宏观性质对绿僵菌等昆虫病原真菌影响的作用机制,更没有从土壤宏观性质形成基础——土壤胶体方面来研究其作用机理。因此土壤胶体的类型、表面电场强度、电荷密度等方面是否对绿僵菌生命活动产生影响值得关注。我们相信,随着研究的深入,土壤因子影响昆虫病原真菌的作用机制将越来越明朗化,这将为提高昆虫病原真菌防控地下害虫的应用效果奠定理论基础。
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(责任编辑 狄艳红)
Edaphic Factor that Affect the Activity of Entomopathogenic Fungus Metarhizium anisopliae
Li Zhenlun1Li Xinqiang1Yang Shuiying2
(1. Chongqing Key Laboratory of Soil Multi-scale Interfacial Processes,College of Resources and Environment,Southwest University,Chongqing 400715;2. College of Plant Pretection,Southwest University,Chongqing 400715)
Metarhizium anisopliae is an important entomopathogenic fungus which is common and widely distributed in soil throughout the world. M. anisopliae plays an essential role in the regulation and control of soil pests and it is one of the most widely used biological insecticides. However, the efficiency of controlling soil pests is not steady, and one of the reasons is the activity of M. anisopliae affected by the complexity edaphic factors. The present article reviewed 5 edaphic factors that influence the activity of M. anisopliae, such as soil temperature and moisture, soil pH, edaphon and soil texture. New research direction about edaphic factor influencing on the activity of M. anisopliae was also discussed. The review would provide reference for further study of mechanism about the edaphic factors influencing life activities of M. anisopliae, which would be benefic to regulate and control soil pests using the M. anisopliae.
Edaphic factor Entomopathogenic fungus Metarhizium anisopliae Edaphic temperature and moisture Soil pH Soil texture
2013-08-14
国家自然科学基金项目(30871630)
李振轮,男,博士,副教授,研究方向:土壤生物学;E-mail:lizhlun4740@sina.com