Endophyte
An endophyte is an endosymbiont, often a bacterium or fungus, that lives within a plant for at least part of its life cycle without causing apparent disease.
[4] Vertically transmitted fungal endophytes are typically considered clonal and transmit via fungal hyphae penetrating the embryo within the host's seeds, while reproduction of the fungi through asexual conidia or sexual spores leads to horizontal transmission, where endophytes may spread between plants in a population or community.
[16] Endophytic fungal hyphae appear to grow at the same rate as their host leaves, within the intercellular spaces of the plant tissue.
This enhanced survivability is largely attributed to endophytic production of secondary metabolites which protect against herbivory as well as increased uptake of nutrients.
This supports the hypothesis that plant signaling is required in order to induce expression of endophytic secondary metabolites.
Through association with fungal endophytes, the root and shoot structures of Pseudotsuga menziesii (Douglas-fir) saplings in low-nutrient conditions have been shown to be elongated, as well as undergo overall biomass increases.
[21] Additionally, various reports on endophyte interactions have shown increased photosynthetic capacities of host plants as well as improved water relations.
[22] Specifically, evidence points to endophytes producing ABA to affect stomatal conductance as well as microbial respiration and plants recycling CO2.
Inoculation saw an upregulation in the genes relevant to jasmonate and ethylene production in the plant roots, although the mechanism to this is still unknown.
[28] One group of fungal endophytes are the arbuscular mycorrhizal fungi involving biotrophic Glomeromycota associated with various plant species.
Bacterial endophytes are polyphyletic, belonging to broad range of taxa, including α-Proteobacteria, β-Proteobacteria, γ-Proteobacteria, Firmicutes, Actinobacteria.
[32] A diazotrophic bacterium isolated in lodgepole pines (Pinus contorta) in British Columbia, Canada, is Paenibacillus polymyxa, which may help its host by fixing nitrogen.
[40] Non-systemic or transient endophytes on the other hand vary in number and diversity within their plant hosts under changing environmental conditions.
Non-systemic endophytes have also been shown to become pathogenic to their host plants under stressful or resource limited growing conditions.
This class of non-clavicipitaceous endophytes has been the most extensively researched and has been shown to enhance fitness benefits of their plant host as a result of habitat-specific stresses such as pH, temperature and salinity.
[51] Turfgrass seed of Festuca and Lolium perenne infected with fungal inoculants, Acremonium coenophialum and A. lolii, is commercially available for use in growing lawns which might require less pesticide use -the grasses are poisonous to cattle and more resistant to some insect damage.
[52] A 2008 experiment with an isolate of a fungus called NRRL 50072 found that this strain can produce a small amount of fuel-like hydrocarbon compounds which was promoted as "myco-diesel".
[53][54] A strain of endophytic fungi which appeared to be closely related to Nigrograna mackinnonii which was isolated from a stem of the plant Guazuma ulmifolia collected in Ecuador was found to produce a variety of volatile organic compounds including terpenes and odd chain polyenes.
In another experiment Burkholderia bacteria with both the TOM plasmid as well as nickel resistance genes was inoculated into yellow lupine; this increased the root mass of the plants, but the amounts of TCE transpired was not statistically significant.
[58][59] Endophyte bioprospecting has already yielded compounds with antibacterial,[60][61] antifungal,[62] antiviral,[62] antiparasitic,[63] cytotoxic,[60][64] neuroprotective,[63] antioxidant,[63] insulin-mimetic,[63] α-glucosidase inhibitory,[62] and immunosuppressant[63] properties.
[11] Studies have shown endophytic fungi are able to produce secondary metabolites previously thought to be manufactured by their plant hosts.
[63] A well known example of the discovery of chemicals derived from endophytic fungi is from the fungus Taxomyces andreanae isolated from the pacific yew Taxus brevifolia.
[60] Three triterpenes were found in the endophyte Xylarialean sp., all three of these compounds displayed mild cytotoxic effects on tumor cells.
including secondary metabolite-methyl ether of fusarubin have shown antibacterial activity against Staphylococcus aureus, E. coli, P. aeruginosa, and Bacillus megaterium.
Endophytes also provide their hosts with an increased resilience to both abiotic and biotic stressors such as drought, poor soils and herbivory.
The increased growth and resilience is likely caused by the endophytes ability to improve plant nutrition or secondary metabolite production, as in the case of Phoma eupatorii's inhibition of the phytopathogen Phytophthora infestans.
[44] Many endophytes protect plants from herbivory from both insects and animals by producing secondary metabolites that are either unappetizing or toxic to the herbivore.
One example of an endophyte-plant-insect interaction is located in the New Zealand grasslands, where endophytes, known as AR1 and AR37 are utilized to protect valuable ryegrass from the Argentine stem weevil but remain palatable to another important food source, livestock.
Indole diterpenes, known as nodulisporic acids, have been harvested from this endophyte which have effective insecticidal properties against the blowfly larvae.
[74] Despite this, the interest and use of bio-insecticides and using endophytes to aid in plant growth is increasing as organic and sustainable agriculture is considered more important.
