12.0.9 12.15: Most plants have fungal symbionts.

If you examine the roots of a plant with a microscope, you will find round structures and fibers closely associated with the fine rootlets and root hairs. These are root fungi, or mycorrhizae (pronounced my-koh-rye-zee). Some mycorrhizae have hyphae that press closely against the walls of root hair cells. Others send hyphae through the root cell walls and into the space between the cell wall and the plasma membrane (FIGURE 12-34).

Beneficial associations between roots and fungi are ancient—they have been found in 400-million-year-old fossils of early land plants—and nearly all species of modern plants have them. Lumber companies have found that they have greater success when replanting areas they have clear-cut if the conifer seedlings are first grown in soil that has been inoculated with mycorrhizal fungi. Home gardeners can buy potting soil with mycorrhizal spores or buy spores compressed into tablets that can be placed in the soil when seedlings are transplanted. The association between mycorrhizal fungi and plants is beneficial to both partners. The mycorrhizal fungus benefits by drawing sugar from the plant, which it uses to support its own cellular respiration. The fungus extracts phosphorus and nitrogen from the soil and releases them inside the roots of the plant, allowing it to grow faster and larger. Research on the impact of root fungi on their plant hosts even demonstrated that the offspring of parental plants with mycorrhizae had significantly better survival than the offspring of parental plants without mycorrhizae (FIGURE 12-35). (In this case, unfortunately, the fungi were enhancing the growth and survival of an invasive plant called velvetleaf, which causes hundreds of millions of dollars of damage each year to corn crops in the United States.)

Not all plants play fair, however. Some take nutrients from the fungus and give nothing in return. One of the best known of these mycorrhizal parasites is the ghost pipe (named for its ghostly white appearance), which grows in forests in the northern hemisphere (FIGURE 12-36). The ghost pipe is an angiosperm related to heaths and heathers. Its roots are associated with mycorrhizal fungi that are also connected to the roots of trees. The ghost pipe obtains all its nutrients from or through the mycorrhizae: nitrogen and phosphorus are provided by the mycorrhizal fungus, and sugar travels through the fungus from tree roots to the ghost pipe.

Some plants manipulate mycorrhizae to gain a competitive advantage over other plants. Garlic mustard, for example, was brought to North America by European colonists to add a tangy flavor to salads. Initially it was planted in kitchen gardens, but it escaped into the wild, where it has proved to be an extremely successful invasive species. Garlic mustard excretes compounds from its roots that interfere with the partnership between mycorrhizal fungi and local native plants. Because the native plants do need mycorrhizal fungi to prosper, the destruction of these fungi by garlic mustard weakens the plants. This chemical warfare is so successful that garlic mustard has wiped out entire populations of native woodland plants and is threatening many forests in the central United States.

Fungi also form mutually beneficial relationships with chlorophyll-containing bacteria and/or algae. These two-way or three-way partnerships are called lichens, and they grow on surfaces such as tree trunks and rocks. The fungus is fed by its photosynthetic partner and helps it absorb water and nutrients. In addition, the fungus provides nutrients for its partners by excreting enzymes that dissolve organic material and acid that dissolves rock.