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Asexual Epichloë are obligate fungal mutualists that form symbiosis with many temperate grass species, providing several advantages to the host. These advantages include protection against vertebrate and invertebrate herbivores (i.e., grazing livestock and invertebrate pests, respectively), improved resistance to phytopathogens, increased adaptation to drought stress, nutrient deficiency, and heavy metal-containing soils. Selected Epichloë strains are utilised in agriculture mainly for their pest resistance traits, which are moderated via the production of Epichloë-derived secondary metabolites. For pastoral agriculture, the use of these endophyte infected grasses requires the balancing of protection against insect pests with reduced impacts on animal health and welfare.
Microbial endophytes, primarily comprising archaea, bacteria, fungi, or viruses, are associated with most plant species . The term ‘endophyte’ was derived from the Greek words ‘endon’ (within) and ‘phyton’ (plant) , and initially included both pathogenic and beneficial microorganisms . However, the term endophyte has now become synonymous with mutualism in reference to microbes that spend all or part of their life cycle within the plant host while causing no apparent disease symptoms , and provides a net benefit outcome to both itself and the host plant .
Asexual Epichloë endophytes (previously belonging to the taxonomic genus Neotyphodium ) were identified in the 1980/90s as the cause of two economically important diseases that affected livestock that grazed fescue in the USA and perennial ryegrass in New Zealand, namely fescue toxicosis  and ryegrass staggers , respectively (Figure 1). These obligate symbionts are mutualistic, relying on the host plant for their growth, survival, and transmission through hyphal colonisation of the host’s seed . These endophytes exhibit a degree of host-specificity within the cool-season grasses of the Pooideae, whereby Epichloë species are naturally restricted to a host grass genus or closely related genera within a grass tribe . Asexual Epichloë spend their entire life cycle within the plant host growing systemically within shoot tissues between plant cells  (Figure 2). However, their bioactivity towards certain pests in the rhizosphere  can be attributed to the mobility of fungal secondary metabolites in the roots, produced during the symbiosis, within the plant vascular system .
Epichloë-derived secondary metabolites protect the host plant from herbivores—both vertebrates and invertebrates. However, the effect on ruminants and several non-ruminants including horses, camels, white rhinoceros, and alpacas  can be detrimental and, when first discovered, removal of these endophytes from grasses was considered the best solution. However, in many temperate regions of the world, such as New Zealand, these Epichloë endophytes are essential for pasture persistence. Novel endophyte strains have now been identified and commercialised that provide the host grass with tolerance/resistance to pests, diseases, and some abiotic stresses while reducing or eliminating the debilitating animal health and welfare issues . These endophytes can be transferred between plants through artificial infection .