A paper in Science shows that certain grasses found near hotsprings in Yellowstone can only persist thanks to a three-way symbiosis. Márquez et al. (2007) showed that the grasses could not survive near hotsprings unless a symbiotic fungus was growing in them. No, not a true mushroom, I only put that in the title to befuddle internet perverts.

These sorts of interactions between plants and fungi are common. There are stories of plants so interlaced with their fungal symbiote that samples of the plant taken for DNA analysis wound up being classified as fungae in systematic analyses. These fungi help gather nutrients and water from the soil, and also seem to capture free radicals in the plant. In exchange, the fungus sips some of the sugars that the plant produces. This sort of arrangement is called a mutualism, because the species get a mutual benefit from living together, a special sort of symbiosis ? literally “together (sym) + living (biosis)”.

The fungi from the grasses in Yellowstone don’t confer heat resistance on their own. When the grasses are grown without their fungal mutualist, they die in high temperature soils, but when a viral infection of the fungus is killed, the plants still die in those high temperatures, as shown in the figure reproduced here. The wildtype (Wt) and plants grown after being sterilized and reinfected with fungi (An) both grow normally. Plants grown without the symbiotic fungus (NS) and with the fungus but not the virus (VF) both do poorly in high temperature soils.

As if that weren’t interesting enough, the researchers found that they could get similar results with tomato plants. Adding the same fungus infected with the same virus induced heat resistance in tomatoes, while the fungus alone did not improve survival of the plants. This is especially interesting given that grasses and tomatoes come from distant parts of the plant family tree. This suggests that whatever mechanism the fungus and virus interact with in plants is widespread, and this discovery might be useful in a wide range of circumstances.

The researchers believe this is the first three-way mutualism in the plant kingdom, though mutualisms between viruses, bacteria and insect hosts have been documented before.