Neotyphodiuminteractions with a wild grass are driven mainly by endophyte haplotype

Summary Strong mutualistic associations are expected to arise between host and microbial symbionts when symbionts lose sexuality, and rely strictly on their host for reproduction via vertical transmission. TheNeotyphodiumendophyte is a vertically transmitted, asexual symbiont of pooid grasses. In agronomic grasses,Neotyphodiumtypically interacts mutualistically with its host by increasing drought resistance and other properties. However, the interaction likely depends on host and endophyte genotypic variation, yet little is known how this variation influences host physiological and morphological responses, especially in wild grasses. We used four differentNeotyphodium‐infected maternal lines of a wild grass, Arizona fescue (Festuca arizonica). Two lines harboured one haplotype and two lines harboured a different haplotype. We experimentally removed the endophyte from some ramets of the four lines. We grew infected (E+) and uninfected (E−) plants in a greenhouse under varying water availability. We examined the effect of endophyte infection, endophyte and host plant maternal genotype, and water availability on traditional growth parameters. We also measured leaf net photosynthesis and dark respiration, leaf conductance to water vapour, leaf water potential, leaf rolling and stomatal density to provide explanations for differences in biomass production and relative growth rates (RGR). Our general findings show thatNeotyphodiuminfection,Neotyphodiumhaplotype and its interaction with host maternal genotype, and varying water availability influence Arizona fescue physiology, growth and biomass production. Based only on infection status, the direction of interaction between endophyte and host is not mutualistic in terms of host growth. Overall, endophyte haplotype and its interaction with host maternal genotype is the most critical, and consistent factor in influencing host growth and physiological outcomes. Variation due to endophyte haplotype in terms of growth and physiological parameters is often greater than that between infected and uninfected hosts. Endophyte–host interactions are likely to be enormously complex because of the genetic and environmental variation that exists in natural populations. The outcome of these interactions in natural grass–endophyte systems is exceedingly difficult to predict based simply on the presence or absence of the endophyte.