Plants can increase their access to soil nutrients by forming partnerships with fungi. Mycorrhizae are symbioses between roots and fungi that enhance nutrient uptake: The plant provides carbohydrates produced by photosynthesis, and the fungus provides nutrients they have obtained from the soil.

Mycorrhizae are of two main types (Fig. 29.18). Ectomycorrhizae produce a thick sheath of fungal cells that surrounds the root tip. Some of the fungus extends into the interior of the root, producing a dense network of filaments that surrounds individual root cells. Endomycorrhizae do not form structures that are visible on the outside of the root. Instead, their networks form highly branched structures, called arbuscules, which grow inside root cells.

Why do fungi enhance the ability of roots to obtain nutrients from the soil? One reason is that fungal networks can extend more than 10 cm from the root surface, greatly increasing the absorptive surface area in contact with the soil. And because fungal filaments are so thin, they can penetrate tiny spaces between soil particles and thus access nutrients that would otherwise be unavailable to roots. Another reason is that fungi secrete enzymes that make soil nutrients more available. Endomycorhizae are important in enhancing phosphorus uptake, while ecotomycorrhizal fungi provide host plants with access to nitrogen that would be otherwise unavailable. Both kinds of mycorrhizae may also offer other benefits to plants: For example, mycorrhizal roots may be less susceptible to infection by pathogens.

616

Mycorrhizal associations are costly to the plant, consuming between 4% and 20% of its total carbohydrate production. The plant has some control over whether or not the roots become infected: When nutrients are readily available, the percentage of mycorrhizal roots decreases. Nevertheless, mutualistic associations between roots and fungi are a characteristic feature of most groups of plants. Mycorrhizal roots are thus the norm rather than an exception. Fossils from more than 400 million years ago show that interactions between plants and fungi are ancient. Perhaps fungi helped plants gain a foothold on land.