Multistrain infections of the grass Brachypodium sylvaticum by its fungal endophyte Epichloë sylvatica

Abstract

Endophytes of the genus EpichloeW (Clavicipitaceae, Ascomycota) are systemic symbionts of cool-season grasses. Their interactions with grass hosts may vary between mutualistic and pathogenic depending on the mode of endophyte reproduction. Sexual strains prevent flowering and seed set (choke disease) of the host and can be horizontally transmitted by ascospores, while asexual strains remain asymptomatic and are vertically transmitted through seeds. In Switzerland nearly all plants of Brachypodium sylvaticum (Huds.) P.B. are infected by EpichloeW sylvatica Leuchtmann & Schardl, but choke symptoms are formed very rarely, and are restricted to particular locations and to a minority of plants at those locations. Earlier research has revealed that E. sylvatica is genetically differentiated into sexual and asexual subpopulations. Given the high level of infection and assuming horizontal transmission of sexual strains, multiple host infections have been predicted. In this study, 25 plants out of 63 examined by isozyme analysis were found to be infected by two or three different endophyte genotypes. In most cases endophyte genotypes appeared to be correlated with the symptom type of a particular tiller, suggesting that the fungal genome controls choke formation and that the sexual and asexual subpopulations are separated at the ramet (tiller) level rather than at the genet (plant) level. These conclusions were further supported by analyses with log-linear models of the population structure of E. sylvatica at four locations where choke symptoms were present. These analyses also revealed a geographic structure in the asexual subpopulation but not in the sexual subpopulation which could be caused by the different dispersal ranges of their propagules. The rare occurrence of sexually reproducing strains and the dominance of a single genotype in asymptomatic plant populations may be explained by the colonization history of B. sylvaticum and its endophyte in Switzerland.

Epichloë grass endophytes increase herbivore resistance in the woodland grass Brachypodium sylvaticum

“…To explain this finding, it has been suggested that seed-transmitted, asexual endophytes should invest more in host protection than sexual endophytes, because asexual endophytes depend on host fitness and seed production for dispersal, while horizontally transmitted, sexual endophytes do not ). Our observations on B. sylvaticum would be consistent with such a hypothesis, assuming that flowering and stromata-forming tillers harbour genetically distinct endophyte genotypes, for which there is evidence (Meijer and Leuchtmann 1999). Of course, alkaloid content may vary in endophyte-infected wild grasses, depending on genetic, geographic or environmental factors (Agee and Hill 1994;Saikkonen et al 1998) and further studies are needed to test this hypothesis in other natural grass-endophyte associations.…”

“…Surveys in Switzerland and other European countries suggest that probably almost all plants of B. sylvaticum are infected with the strictly host-specific fungus E. sylvatica (Bucheli and Leuchtmann 1996;Leuchtmann and Schardl 1998). The association is characterised by a high rate of asymptomatic, seed-transmitted infections, and only rare sexual reproduction of the fungus (Bucheli and Leuchtmann 1996;Meijer and Leuchtmann 1999). The wide distribution of E. sylvatica and the consistent infection of host plants suggest that infected plants have a substantial fitness advantage.…”

“…Therefore, natural selection will favour endophytes (and their hosts), which invest resources into products that enhance host fitness. This could explain in part the wide distribution of this association and the high infection rate with asexual genotypes of the endophyte in populations of B. sylvaticum (Bucheli and Leuchtmann 1996;Meijer and Leuchtmann 1999). Alternatively, endophyte infection could also affect seedling establishment and competitive abilities of plants at natural sites and thus may have altered host plant demography as found for other grass/endophyte associations (e.g.…”

“…This genotype has distinct isozyme patterns and is predominantly found in flowering tillers of B. sylvaticum (Meijer and Leuchtmann 1999). After 2 weeks on agar, the seedlings were transferred to trays with standard potting substrate for another 6 weeks.…”

Preliminary study of endophytic fungi in timothy (Phleum pratense) in Estonia

“…Sometimes impact from the environment can make these fungi parasitic or pathogenic to host plants (Schardl et al 1997;Meijer, Leuchtmann 1999;Schulz et al 1999;Kogel et al 2006;Johnson, Oelmüller 2009). Most of them are non-specific regarding their host preference (Petrini 1986;Saikkonen et al 1998;Arnold et al 2000) and can live together with other endophytes in the same host (An et al 1992;Saikkonen et al 1998;Arnold et al 2000), holding greater promise for plant growth promotion and increased yield in agriculturally important grasses.…”