People often think of fungi as dietary items or as agents of rot and decay. Fair enough, but those are only two small windows into the wonderful world of fungi.
Way back in time, hundreds of millions of years ago, fungi and algae made an association that made it possible for green algae to move onto land, where they evolved into what we know as plants — without which, there might not have been any animals. And those evolving plants kept those types of fungal relationships as they and their fungal associates diversified to live in most earthly habitats. A type of symbiosis essential to life as we know it.
These fungi, known as mycorrhizal fungi, have appeared in my essays several times, so some of this essay will seem familiar to readers, but there is more to the story than previously discussed. Mycorrhizal fungi can be very abundant, making a considerable proportion of soil volume. These fungi make associations with plant roots (hence the name of mycorrhizae or fungus-root), creating vast underground networks with almost all kinds of plants and linking plants of both the same and different species together. The thin fungal hyphae carry soil nutrients (e.g. phosphorus) to plants and from one plant to another. They carry carbohydrates and lipids from the plants’ photosynthetic processes from plant to plant, and the fungi use them for their own use. In some cases those fungal networks may also transfer poisons, hormones, viruses and indicators of insect attack. Orchid seeds are tiny and dust-like, having no stored nutrients; they cannot germinate without associating with mycorrhizal fungi, which bring them essential nutritional material.
The ability to form mycorrhizal associations has evolved many times, in different fungal lineages; some associations are very specific to certain plants, while others are more promiscuous. The ability to associate with many kinds of fungi was one factor that was important in determining which tree species could most quickly colonize land left open by retreating Ice Age glaciers. Trees that were capable of associating with many different fungi were more likely to encounter a compatible fungus when they arrived on newly barren ground and tended to migrate northward more quickly.
Mycorrhizal associations can have significant effects on plant communities. The fungal partners can have differing ability to decompose organic material and can interact (positively or negatively) with free-living decomposers, affecting the chemical composition of soils. They can also affect interactions among the plant partners, making them better (or worse) competitors, with cascading effects on plant community diversity.
Fungal partners can also change characteristics of their plant associates in interesting ways beyond enhance growth. For instance, when basil plants were grown with different mycorrhizal fungi, the amount and array of aromatic oils in the leaves was different. Tomato plants grown with certain mycorrhizae had sweeter fruits with more nitrates and other useful compounds. Lettuces grown with certain fungi accumulated more nutrient minerals and antioxidants than those grown without the fungi. Bread baked with a modern commercial wheat flour tasted different from that made with older varieties of flour, but addition of a mycorrhizal treatment (with accessory microorganisms) to the flour made the modern bread taste more like that made from the old varieties.
Experiments showed that the identity of the fungal partner could affect not only the appearance and flavor of strawberries (to humans), but also the behavior of the pollinating bumblebees foraging for nectar and pollen. Another study found that fungal identity and nutrient levels interacted to increase a particular alkaloid that in turn decreased the intensity of infection by a gut pathogen of bumblebees; presumably healthier bees would do better pollination. Such differences seem rather subtle and were assessed chiefly in terms of human use. But clearly, since bumblebees reacted to some plant features associated with the fungal partners, other animals might do so too. There could be cascading effects through the ecosystem…That remains to be seen.
Such mycorrhizal subtleties as modifying plant characteristics tend to be lost with modern intensive agricultural practices, which generally focus on increasing harvestable yield. The crop plants have been selected strongly for their positive responses to fertilizers (along with other problems), with the reported result that commercial crop varieties have often lost even the capacity to form strong mycorrhizal associations. Is that why grocery-store strawberries usually seem so insipid?
• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Wednesday in the Juneau Empire.
Shaggy mane mushrooms are characterized by tall, white, shaggy caps. (Photo by Deana Barajas)
