Abstract:

Endophytes, especially asexual and systemic endophytes in grasses, are generally viewed as plant mutualists based on the action of their alkaloids. Enhanced drought tolerance is a well-known benefit of endophytic infection in tall fescue (Festuca arundinacea) and perennial ryegrass (Lolium perenne), and increased tolerance to other environmental stresses like heat, low light and low soil fertility has also been reported. Three endophyte life histories have been recognized: symptomatic of life cycle where the fungus horizontally transmits by meiotic ascospores which induce sterilization of the host, asymptomatic life cycle where the fungus remains internal and there is vertical transmission by plant seeds throughout the season and a mixed mechanism of life cycle, which can be plastic. Neotyphodium endophytes are closely related to sexual Epichloı species, which are the grass choke pathogen, and likely evolved either directly from sexual Epichloı species or by interspecific hybridization of distinct lineages of Epichloı and Neotyphodium. In vertical transmission, only one fungal genotype is transmitted to the seed progeny, which are usually produced by outcrossing in the host. The same fungal genotype is present in seeds that are genetically variable, and the high level of genetic specificity is probably tied to genetic incompatibility constraining the diversity of successful genotype-genotype combinations of the systemic seed-borne endophytes and the host grasses. The defensive mutualism depends on a certain grass-endophyte genotype combination and environmental conditions. Recent studies have suggested that there is a mutualism-parasitism continuum for the symbiosis between asexual endophytes and grasses and that the symbiosis existing in native grass-endophyte symbionts has a more complex mechanism than in agricultural ecosystems. The host-specific endophyte, with negligible biomass, may alter plant community structure, reduce plant diversity and control food-web structure by disrupting the transfer of energy from plants to upper trophic levels. Future studies should focus on how ecology and genetics interact to shift fungal life history traits between the extremes of sexuality and asexuality and antagonism and mutualism. These questions require a more comprehensive understanding of the genetic basis and phenotypic plasticity of traits of the grass-endophyte interactions.

Key words: grass, endophyte, genetic diversity, genetic combination, symbiotic interactions