Simulated Herbivory Leads Grasses to Pass Fungal Symbionts to Their Offspring

Many grasses host symbiotic fungi, Epichloë spp. These fungi can help grasses tolerate herbivory, or help repel herbivores. But how do these fungi pass between plant hosts? They could be passed horizontally between plants, or vertically from plants to their seed offspring. Pedro Gundel and colleagues examined how Epichloë interacts with autumn bluegrass, Poa autumnalis.

P. autumnalis is found in south-eastern quarter of the USA, mainly, though the USDA also lists it as native as far north as Nunavut. The authors were interested in the plant in a Texan context, where it is common in mesic, hardwood forest understoreys.

Over three years, the team clipped the plants to simulate herbivory. After chopping the grass, they examined if Epichloë increased tolerance to herbivory. Then they asked if herbivory increased vertical transmission of fungi. If this was the case, then they expected to find more hyphae in the grass seeds.

“Clipped plants with the endophyte survived better than endophyte-free plants,” write Gundel and colleagues, “suggesting that improved tolerance to herbivory can be an additional fitness benefit of symbiosis beyond the herbivore resistance reported in our previous study.”

“In addition to modulating the fitness benefits of symbiosis, simulated folivory also increased the amount of vertical transmission of the endophyte to both seeds and seedlings. Together with the increase in the density of endophyte hyphae in the seeds, these results suggest, for the first time, that symbiont vertical transmission can function as an induced response to herbivory.”

The results indicate that when herbivores chew on a plant host, they trigger a response to help protect the next generation. The new seedlings grow already inoculated with their defensive symbiont. It is this frequent exposure to damage that helps promote plant-fungi partnerships in the long-term. This partnership may also explain the low incidence of pathogenic fungi in wild populations of autumn bluegrass.