Fungal endophytes and a virus confer drought tolerance to Nicotiana bentharniana plants through modulating osmolytes, antioxidant enzymes and expression of host drought responsive genes

Microbial symbionts increase plant growth and eco-physiological performance under abiotic stress. In this study, we evaluated how the colonization of two fungal endophytes isolated from wild Nicotiana species from areas of drought-prone northern Australia, and a plant virus, yellowtail flower mild mottle virus (genus Tobarnovirts), improved water stress tolerance in N. benthamiana plants. Inoculation with both of the two fungal strains used and the virus significantly increased plants tolerance to water stress as manifested by their significant delay in wilting of shoot tips. The water stress tolerance of fungus-inoculated plants was correlated with increases in plant biomass, relative water content, soluble sugar, soluble protein, proline content, increased activities of the antioxidant enzymes catalase, peroxidase and polyphenol oxidase, decreased production of reactive oxygen species, and decreased electrical conductivity. In addition, there was significant upregulation of several genes previously identified as drought induced. The influence of the virus was similar to the fungi in terms of increasing the plant osmolytes, antioxidant enzyme activity and gene expression. Although separate infection of fungi and virus increased plant water stress tolerance responses, their co-infection in plants did not have an additive effect on water stress responses. These findings show that presence of both fungi and viruses can initiate plant responses to water stress in a similar way.