Epichloë endophyte infection enhances the tolerance of Stipa purpurea to parasitic stress through the regulation of antioxidants and phytohormones

Aims Root hemiparasite infection is considered a potential biotic stress that affects the growth of cool-season grasses, and the damage caused by a hemiparasitic plant to host grasses is largely determined by the density of the hemiparasite. Epichloë endophytes have been shown to be mutualistic symbionts of cool-season grasses under most environmental conditions. Although cool-season grasses can be host to both Epichloë endophytes and hemiparasites, little attention has been given to how Epichloë infection regulates the physiological and phytohormone responses of host grasses under parasitic stress. Methods We performed a pot experiment with an endophyte-infected and endophyte-free host grass, Stipa purpurea , and a facultative hemiparasite, Pedicularis kansuensis . The grasses were grown together under different densities of the hemiparasite (control, one or three plants per pot). Grass growth as well as physiological and phytohormone parameters were determined following parasitic stress. Results Successful parasitism increased the biomass and survival of P . kansuensis while suppressing the growth of S . purpurea. The reduction in the growth of S. purpurea was dependent on the density of P . kansuensis . Epichloë infection of S. purpurea improved the tolerance of this grass to parasitic stress, resulting in more tillers, longer roots, higher shoot and root biomasses and a higher root:shoot ratio compared with those of the endophyte-free grass. Interestingly, Epichloë infection increased proline, indole-3-acetic acid and abscisic acid production as well as superoxide dismutase, peroxidase and catalase activities and reduced the malondialdehyde content, root vitality and zeatin riboside content under root hemiparasitic stress. Conclusions Our results demonstrate that the growth of the host grass was severely reduced under parasitic stress; the negative effects of hemiparasitic plants on the host grass were largely determined by the density of the hemiparasite. The findings also indicate that the Epichloë endophytes greatly alleviated the damage caused by the hemiparasite to the cool-season grass by producing more antioxidant enzymes, enhancing root viability, synthesizing certain phytohormones and suppressing haustorium formation.