Endophytic fungi and drought tolerance: Ecophysiological adjustment in shoot and root of an annual mesophytic host grass

Abstract

Epichloid endophytic fungi, vertically transmitted symbionts of grasses, can increase plant tolerance to biotic and abiotic stress. Our aim was to identify ecophysiological mechanisms by which the endophyte Epichloë occultans confers drought tolerance to the annual grass Lolium multiflorum Lam. Endophyte-Associated or endophyte-free plants were either well-watered or subjected to water deficit. We evaluated plant biomass, root length and nitrogen concentration, and we assessed intrinsic water use efficiency (iWUE) and its components net photosynthesis and stomatal conductance, by carbon and oxygen isotope analysis of shoot tissues. Endophyte-free plants produced more biomass than endophyte-Associated ones at field capacity, while water deficit strongly reduced endophyte-free plants biomass. As a result, both types of plants produced similar biomass under water restriction. Based on oxygen isotope composition of plant cellulose, stomatal conductance decreased with water deficit in both endophyte-Associated and endophyte-free plants. Meanwhile, carbon isotope composition indicated that iWUE increased with water deficit only in endophyte-Associated plants. Thus, the isotope data indicated that net photosynthesis decreased more strongly in endophyte-free plants under water deficit. Additionally, endophyte presence reduced root length but increased its hydraulic conductivity. In conclusion, endophytic fungi confer drought tolerance to the host grass by adjusting shoot and root physiology.