It's one of those fabulous examples of yet another huge world about which we know nothing: the realm of the symbiotic fungi living on the leaves of plants.

All plants seem to have 'em, but why are they there? They suck up a small amount of the plant's energy, and in the brutal economics of evolution, that which takes away resources must also contribute something, or it'll get whacked soon enough.

In last week's Nature, Keith Clay[1] explains a wonderful bit of experiment by Elizabeth Arnold of the University of Arizona and her colleagues on fungus living on the leaves of our beloved cacao, the plant that brings us chocolate.

What seems to be the best previous research into the issue (“the best-understood case”, Clay calls it) looked at tall fescue grass, which tends to be “infected”, as it were, with a fungus that starts in the seed but eventually infects the whole plant. The fungus produces a toxin that seems to tend to deter grazing animals. That's the benefit that provides the delicate balance need to keep the relationship teetering on the knife's edge of evolutionary survival. But the fescue is an odd case of what Clay calls “vertical transmission” - from seed to host plant. Most fungi seem to be transmitted “horizontally” - from full-grown plant to full-grown plant on the wind.

Arnold and her colleagues were trying to understand how the evolutionary relationship played out in such a horizontal transmission case.

Clay explains the problem thus:

Given that endophytes use their hosts' resources, they must entail some cost to the plant. If the costs outweigh the benefits, why don't plants defend themselves against infection? And if the benefits outweigh the costs, what are these fungi doing to help the plant?

Cacao's one of those plants that, in domestication, has been far removed from its natural setting. Today's agricultural use primarily happens in the Old World, despite the plant being a native of the Americas. That is because disease ravages the plant here.

Arnold et. al. played with Phytophthora, the organism responsible for an affliction in the cacao called “black pod disease”. They protected some cacao leaves from wind-borne “infection” with the symbiotic fungus, while some were allowed to grow normally. Turns out the cacao without the fungus were far more likely to die from black pod disease, suggesting the fungi are acting as a sort of bodyguard for their host.

Let's recall now my initial point about another huge world about which we know nothing. It seems as though essentially all plants have fungi growing on them. This is another enormous layer of complexity in the evolutionary/ecological story that has yet to be worked out.

We know so much, but we know so little.

[1] Clay, Keith, Fungi and the food of the gods, Nature, 427, 401 - 402 (29 Jan 2004)