Epichloë − a lifeline for temperate grasses under combined drought and insect pressure

Fungal Epichloë endophytes form symbiotic associations with many temperate grasses, such as Lolium and Festuca, giving their host grasses an ecological advantage. The importance of specific Epichloë endophytes in providing varying levels of protection against invertebrate pests has been well documented. Similarly, but with fewer studies, the benefits of Epichloë to host grasses in drought events has been shown. Endophyteinfected grasses show an improved persistence against herbivore insect attack as well as resilience under drought. However, there are relatively few studies that investigate the interaction between drought and insect pressure, and yet it is these combined pressures that can prove detrimental for a ryegrass or fescue crop. This review examines the current state of knowledge on the effects of Epichloë on the interactions of insects and drought in temperate grasses. Citation: Hewitt KG, Popay AJ, Hofmann RW, Caradus JR. 2021. Epichloë − a lifeline for temperate grasses under combined drought and insect pressure. Grass Research 1: 7 https://doi.org/10.48130/GR-2021-0007 Introduction Worldwide, around 26% of the world land area is dominated by forage grass species for animal production systems[1]. Poaceae grasses (subfamily Pooidea), such as Lolium and Festuca, are often used in managed pasture systems throughout temperate zones[2,3]. Along with other cool-season grasses, they are capable of forming both sexual and asexual associations with fungal Epichloë endophytes that range from mutualistic to antagonistic. The mutalistic outcomes of the relationship for their Lolium and Festuca hosts are considered to be a major factor in the evolution of maternal transmission of the endophyte[4]. These endophytes are critical constitutents of agricultural ecosystems and likely have an important role in regulating communities in natural grasslands although these have been less well studied. Living only in the above-ground tissues, endophyte infection in these temperate grasses can increase the plants' ability to tolerate herbivory attack through the production of a large range of secondary metabolites such as alkaloids[5]. Four main classes of alkaloids are recognised as being produced by Epichloë endophytes; peramine, indole diterpenes, ergot alkaloid, and lolines[6]. All of these alkaloid classes are involved in invertebrate deterrence and/or toxicity[7], while ergot alkaloids as well as indole diterpenes can also impair grazing animal performance[8]. Removal of the fungal endophyte from the host grass to counter the toxic effect on grazing livestock is a logical approach. However, such endophyte-free plants were not viable in New Zealand and other countries that suffer from high invertebrate pest pressure[9]. The economic loss and reduced animal performance of livestock grazing pastures containing a toxic endophyte led to the identification of endophyte strains that retain insect-active alkaloids while minimising the production of the mammalian active toxins[10,11]. The anti-herbivory properties have been utilised and commercialised in agriculturally managed agroecosystems that use perennial ryegrass (Lolium perenne L.) and tall fescue (Festuca arundinacea Schreb.), two dominant plant species sown for ruminant livestock production[12−15] (Table 1 and Table 2). In addition to the invertebrates affected by endophyte, in the USA, mammalian-toxic E. coenophiala also affects grazing and reproduction of the prairie vole (Microtus orchrogaster) which in turn modifies community structure[16]. Fungal Epichloë endophytes have become a fundamental and essential management tool in integrated pest management in New Zealand, Australia, and the USA to maintain and/or increase pasture production and persistence[17−19]. Farmers are facing an increasingly complex operating environment through changes in climate as well as increased global demand for more sustainable farming systems. Resource limitations, such as drought, can significantly impact managed grassland productivity. Numerous studies have investigated if Epichloë endophyte infection improves the ability of the grass host to withstand abiotic stress factors and resource limitations such as drought[7,41]. Although there is evidence that E. coenophiala promotes drought tolerance in its tall fescue host, the information for this occurring in ryegrass is more equivocal. In addition, there is the likelihood of interactions occurring between invertebrates and abiotic stress which we do not yet fully understand. For example, REVIEW