The effects of genetic and environmental factors on disease expression (stroma formation) and plant growth in Brachypodium sylvaticum infected by Epichloe sylvatica

Fungi in the genus Epichloe (Clavicipitaceae, Ascomycota) are endophytic an d often mutualistic symbionts of many grasses in temperate areas. Species w ith a sexual cycle suppress host flowering and seed formation, whereas asex ual fungi remain asymptomatic and transmit vertically by seed. Thus, the mo de of reproduction may determine whether the symbiosis is mutualistic or pa rasitic. The level of sexual reproduction (disease expression) varies among different endophytes and on different grass hosts, but factors responsible for this variation, and evolutionary mechanisms leading to one or the othe r life strategy are not understood. As experimental system, we chose Brachy podium sylvaticum in which the endophyte E. sylvatica can express both repr oduction modes. A field experiment was done in plots of a free air carbon d ioxide enrichment (FACE) facility. Wt: investigated the effects of three en vironmental factors (elevated CO2 concentration, shading and fertilisation) and one genetic factor (plant and fungal genotype combination), on plant g rowth and disease expression. Variation in plant growth was mainly dependen t on the genotype and was increased by fertilisation. Elevated CO2 and shad ing slightly stimulated plant growth, but only in fertilised plants. Diseas e expression was overwhelmingly dependent on the genotype. hence genetic: f actors. Fertilisation slightly stimulated disease expression in some genoty pes, while the effect of elevated CO2 was negligible, and by interaction wi th fertilisation inconsistent in the two years. Horizontal transmissions du ring the experiment, presumably mediated by ascospores, confounded the orig inal infection status of the plants. Contagious infections occurred more fr equently in the shade, and in endophyte-free host plants. The latter sugges ts that pre-existing infections render host plants less susceptible to supe rinfection by choke forming strains. Although our results clearly indicate that disease expression of E, sylvatica has a genetic basis, it is still un clear whether selection on the plants or the fungi is driving the evolution of this association.