不同种类内生真菌对禾草宿主及其所在群落影响的比较研究

It is well known that asexual endophytes are usually mutualistic with their host grasses such as tall fescue and perennial ryegrass. Recently, endophytic fungi are reported to be widely distributed in native grasses. Unlike most agronomic grasses that are infected by a single epichlo? species, natural populations are usually comprised of plants harboring multiple endophyte species. Studies involving native grasses are relatively scarce and results range from antagonistic to mutualistic. We suggest that this variation is at least partly resulted from the difference of epichlo? species. Achnatherum sibiricum is a perennial, sparse bunch grass and a constant companion species native to the east-central Inner Mongolia Steppe. In the previous study, we found that A. sibiricum is highly infected by epichlo? endophytes. Up to now, A. inebrians and A. robustum, belonging to the same genus with A. sibiricum, are also found to be infected by Neotyphodium endophytes and both are notorious for the narcotic properties to livestock ascribed to endophyte infection. In contrast to A. inebrians and A. robustum, A. sibiricum has no obvious herbivore deterrence according to local records and our own observations. Particularly, A. sibiricum harbors both asexual Neotyphodium and their Epichlo? sexual ancestors, which can be transmitted vertically or horizontally, depending on environmental conditions. In our previous study, A. sibiricum has been sampled from two geographical populations. In each population, at least three genotypes have been obtained. Grass tillers, with identical genotype, have been re-infected with Neotyphodium gansuense and Epichlo? amarillans, separately, by artificial inoculation. The purpose of this study is to compare the effect of different endophyte species on the host grass, and further on the dominant non-host species, microbial community and the associated grassland community. Our object is not only to study the contribution of endophyte species variation to endophyte-grass interactions, but also to provide some basic insights into the development and succession of the endophyte associated grassland.

内生真菌与人工草地禾草之间的互利共生关系已经被大量的实验证据所证实。近年来的研究表明，内生真菌在天然禾草中也广泛存在，并且有比人工禾草中更高的种类多样性，但内生真菌对天然禾草宿主的影响存在明显的不确定性。结合前期的工作，我们推测：内生真菌-天然禾草之间相互作用的不确定性除与共生体所处的环境有关外，与共生体双方特别是内生真菌的种类密切相关。羽茅是内蒙古中东部草原常见的伴生种，同时也是有性和无性型内生真菌的天然宿主，本研究以来自两个不同地理种群的、不同基因型的羽茅为材料，首先通过人工转接的方式，构建宿主植物基因型完全相同的、分别感染有性和无性内生真菌的植物种群，在研究不同种类内生真菌对宿主生长和代谢影响的基础上探讨不同种类内生真菌对宿主及其所在群落中其它植物及土壤微生物等的影响。本研究不仅有助于阐明内生真菌在决定共生体相互作用性质方面所起的作用，而且有助于预测染菌植物所在群落的发展和演替方向。

关于禾草-内生真菌相互作用的研究多是考虑内生真菌感染与否，而关于内生真菌种类对相互作用影响的研究直到近年来才偶见报道。羽茅（Achnatherum sibiricum）是内蒙古中东部草原常见的伴生种，我们前期的研究表明，感染羽茅的内生真菌有2个优势种，一种为Neotyphodium sibiricum (Ns)，另一种具有2个不同的形态型，分别为营垂直传播的Neotyphodium gansuensis (Ng)和营水平传播的Epichloë gansuensis (Eg)。本项目即以感染不同种类内生真菌的羽茅为研究对象，在比较不同种类内生真菌代谢产物的基础上，探讨内生真菌种类对宿主植物生长、代谢以及抵抗非生物胁迫和生物干扰等方面的影响，四年来我们按照预定计划开展研究，在以下6个方面取得了显著进展：①Eg虽然能够在宿主生殖枝上产生子座，但不产生子囊壳或子囊孢子，即不进行有性生殖；从子座分离得到的分生孢子能够在离体条件下长出菌丝，并感染新的不染菌植株，说明子座上的分生孢子具有水平传播的潜力。②内生真菌种类对宿主绿叶生物量具有显著影响，Ng羽茅绿叶生物量显著高于Ns植株；内生真菌传播方式对羽茅生物量无显著影响；植物母本基因型对宿主植物的影响低于内生真菌。③内生真菌对宿主种间竞争力的影响取决于内生真菌感染与否和氮素供给水平，内生真菌感染在高氮条件下提高了宿主的种间竞争力，而在低氮条件下导致宿主竞争力显著降低；内生真菌种类对宿主羽茅的种间竞争力无显著影响。④内生真菌感染对宿主化感作用的影响与内生真菌种类有关，相比于未染菌羽茅，感染Eg可显著缓解宿主对大针茅的抑制作用，而感染Ns和Ng的上述影响不明显。⑤在内生真菌与病原菌的对峙培养实验、内生真菌培养滤液对病原菌的孢子萌发实验、感染内生真菌的离体叶片的感病实验和感染内生真菌的在体叶片的感病实验中均发现：内生真菌Ns、Ng和Eg对新月弯孢、根腐离蠕孢和枝孢霉这3种病原菌都具有抑制作用，贡献大小的顺序依次为Ns > Ng > Eg。⑥内生真菌感染能够显著提高羽茅对东亚飞蝗的抗性。本研究发现内生真菌感染并未导致未经处理的羽茅产生生物碱，却导致宿主植物中与适口性有关的可溶性糖和必需氨基酸含量显著降低、与防御有关的总酚和木质素含量显著升高，染菌羽茅对东亚飞蝗的抗性与上述指标的变化相关而与生物碱含量的关系不大。