Maize response to endophytic Metarhizium robertsii is altered by water stress.

To defend against damage from environmental stress, plants have evolved strategies to respond to stress efficiently. One such strategy includes forming mutualist relationships with endophytes which confer stress-alleviating plant defensive and growth promoting effects. Metarhizium robertsii is an entomopathogen and plant-protective and growth-promoting endophyte. To determine the context dependency of the relationship between M. robertsii and maize, we conducted a greenhouse experiment that imposed stress as deficit and excess soil moisture on maize plants which were inoculated or not inoculated with M. robertsii and measured plant growth and defense indicators. Maize height and endophytic root colonization by M. robertsii were positively correlated in the deficit water treatment, but not in the adequate or excess water treatments. The relative expression of ZmLOX1 in the jasmonic acid (JA) biosynthesis pathway was significantly greater in M. robertsii-inoculated than in non-inoculated plants, but water treatment had no effect. There was significant interaction between M. robertsii and water treatments on foliar concentrations of JA and jasmonoyl isoleucine (JA-ILE), suggesting that water stress impacts M. robertsii as a modulator of plant defense. Water stress, but not inoculation with M. robertsii, had a significant effect on the expression of MYB (p = 0.021) and foliar concentrations of abscisic acid (p<0.001), two signaling molecules associated with abiotic stress response. This study contributes toward understanding the highly sophisticated stress response signaling network and context dependency of endophytic mutualisms in crops.