Fungal endophytes in herbaceous plants: mutualists or antagonists?

“内生菌”这个术语用来描述一种真菌，它生活在植物的活组织中，而不会引起明显的感染迹象。在草和树中，内生菌可以通过产生化学物质来保护寄主植物免受脊椎动物和无脊椎食草动物以及病原真菌的侵害。然而，在草本植物中，内生真菌的数量可能是草或树的十倍。此外，已知草本内生菌在实验室琼脂上生长时会产生多种有毒化学物质。值得注意的是，这些真菌没有引起生态学家的注意。它们是否与寄主存在一种互惠关系，通过它们的化学物质生产来保护植物免受攻击？或者，如果它们没有引起感染症状，这是否意味着它们抑制了植物的防御系统，使其更容易受到食草动物的攻击？本研究的目的是利用匍匐蓟（Cirsium arvense）及其相关内生菌和昆虫的模型系统来回答这些问题。在这个项目中，我们将研究食草昆虫是否可以将这些真菌从植物传播到植物之间，要么直接在它们的身体上传播，要么通过它们进食造成的植物伤口让真菌进入组织。我们将看到，已知会增加植物大小的土壤中的生物和非生物因素（共生真菌和营养物）是否会影响感染植物的内生菌的数量。我们还将研究反向相互作用：如果内生菌消耗植物资源，那么它们可能会在植物中竞争碳，从而减少共生真菌的生长。最后，当我们发现影响内生菌发生的因素后，我们将种植无内生菌或被感染的植物，看看真菌感染对吸吮、咀嚼和叮咬昆虫的生长和存活有什么影响。我们还将使用最现代的化学分析技术来检测和鉴定受感染植物中产生的有毒化学物质，并将其与未受感染植物和真菌本身的有毒化学物质进行比较。为什么这项工作很重要？因为它有可能改变我们对植物-昆虫相互作用的看法，并可能导致许多新形式的害虫或杂草控制。如果植物中至少有一些化学物质是由这些真菌产生的，而不是由植物本身产生的，那么这可能对利用内生菌作为昆虫生物防治剂产生深远的影响。我们可以将真菌接种到农作物中，这样就可以从内到外保护植物。此外，黄花蓟马是世界上危害最严重的杂草之一，所有使用昆虫或病原真菌作为生物防治剂的尝试都失败了。我们认为存在的看不见的，迄今为止未被考虑的内生菌可能是原因。如果我们证明它们可以保护植物免受昆虫侵害，那么在杂草中，我们就可以开始控制真菌的发生，这样生物控制剂就有更大的成功机会。

'Endophyte' is the term used to describe a fungus that lives inside the living tissues of a plant, while causing no visible signs of infection. In grasses and trees, endophytes can protect the host plant against vertebrate and invertebrate herbivores and pathogenic fungi, by the production of chemicals, active against the attacker. However, in herbaceous plants, the number of endophyte fungi may be ten times that found in a grass or a tree. Furthermore, herbaceous endophytes are known to produce a wide variety of toxic chemicals when grown on agar in the laboratory. Remarkably, these fungi have escaped attention by ecologists. Do they exist in a mutualistic relationship with their hosts, protecting plants against attack through their chemical production? Alternatively, if they cause no symptoms of infection, does this mean they suppress the plant's defence system, rendering it more susceptible to herbivore attack? It is the aim of this research to answer these questions, using a model system of creeping thistle (Cirsium arvense), and its associated endophytes and insects. In this project, we will examine whether insect herbivores can transmit these fungi from plant to plant, either directly on their bodies, or by allowing fungal entry to tissues, through wounds in the plant created by their feeding. We will see if biotic and abiotic factors in the soil that are known to increase plant size (symbiotic fungi and nutrients) can affect the numbers of endophytes that infect a plant. We will also examine the reverse interaction: if endophytes consume plant resources then they might compete for carbon in the plant, thereby reducing the growth of the symbiotic fungi. Finally, when we have discovered the factors that affect endophyte occurrence, we will grow plants that are endophyte-free or infected to see what effects fungal infection has on the growth and survival of a sucking, chewing and a galling insect. We will also use the most modern techniques of chemical analysis to detect and identify the toxic chemicals produced in infected plants and compare them to those in uninfected plants and the fungi themselves. Why is this work important? Because it has the potential to change the way in which we think about plant-insect interactions and could lead to many novel forms of pest or weed control. If at least some of the chemicals in plants are produced by these fungi, rather than the plants themselves, this could have profound implications for the use of endophytes as insect biological control agents. We could inoculate the fungi into crop plants and so protect the plant from the inside out. Also, C. arvense is one of the world's worst weeds and all attempts to use insects or pathogenic fungi as biological control agents of it have failed. We think that the presence of invisible, and hitherto unconsidered, endophytes may be the reason. If we show that they protect the plant against insects, then in weeds, we can start to manipulate fungal occurrence, so that biological control agents stand a better chance of success.