Plant use of endophytic fungi in defense occurs when endophytic fungi, which live symbiotically with the majority of plants by entering their cells, are utilized as an indirect defense against herbivores. In exchange for carbohydrate energy resources, the fungus provides benefits to the plant which can include increased water or nutrient uptake and protection from phytophagous insects, birds or mammals. Once associated, the fungi alter nutrient content of the plant and enhance or begin production of secondary metabolites. The change in chemical composition acts to deter herbivory by insects, grazing by ungulates and/or oviposition by adult insects. Endophyte-mediated defense can also be effective against pathogens and non-herbivory damage.
This differs from other forms of indirect defense in that the fungi live within the plant cells and directly alter their physiology. In contrast, other biotic defenses such as predators or parasites of the herbivores consuming a plant are normally attracted by volatile organic compounds (known as semiochemicals) released following damage or by food rewards and shelter produced by the plant. These defenders vary in the time spent with the plant: from long enough to oviposit to remaining there for numerous generations, as in the ant-acacia mutualism. Endophytic fungi tend to live with the plant over its entire life. The endophytic fungi grow in the intercellular spaces of the plants, parallel to the leaves and stems, as elongated and thinly-dispersed branched hyphae. The fungal hyphae penetrates the host plant's embryo and grows along the seeds to infect the new plants that will grow from the seeds, which is a process of transmission that is known as vertical transmission.
Diversity of endophytic associations
The fungal endophytes are a diverse group of organisms forming associations almost ubiquitously throughout the plant kingdom. The endophytes which provide indirect defense against herbivores may have come from a number of origins, including mutualistic root endophyte associations and the evolution of entomopathogenic fungi into plant-associated endophytes. The endomycorrhiza, which live in plant roots, are made up of five groups: arbuscular, arbutoid, ericoid, monotropoid, and orchid mycorrhizae. The majority of species are from the phylum Glomeromycota with the ericoid species coming from the Ascomycota, while the arbutoid, monotropoid and orchid mycorrhizae are classified as Basidiomycota. The entomopathogenic view has gained support from observations of increased fungal growth in response to induced plant defenses and colonization of plant tissues.
Examples of host specialists are numerous – especially in temperate environments – with multiple specialist fungi frequently infecting one plant individual simultaneously. These specialists demonstrate high levels of specificity for their host species and may form physiologically adapted host-races on closely related congeners. Piriformospora indica is an interesting endophytic fungus of the order Sebacinales, the fungus is capable of colonising roots and forming symbiotic relationship with every possible plant on earth . P. indica has also been shown to increase both crop yield and plant defence of a variety of crops(barley, tomato, maize etc.) against root-pathogens. However, there are also many examples of generalist fungi which may occur on different hosts at different frequencies (e.g. Acremonium endophytes from five subgenera of Festuca) and as part of a variety of fungal assemblages. They may even spread to novel, introduced plant species. Endophytic mutualists associate with species representative of every growth form and life history strategy in the grasses and many other groups of plants. The effects of associating with multiple strains or species of fungus at once can vary, but in general, one type of fungus will be providing the majority of benefit to the plant.