Mycorrhizae - a flora / fungi partnership

When we were looking at fungi we had a lot of ground to cover in just one lab -- Ascos, Basidios, Zygos -- we learned about an entire KINGDOM in three hours! So obviously there were fascinating aspects of fungi that we had to leave out. But one especially important interaction, the mycorrhizal symbiosis, is just too cool to go unexplored.

Certain species of fungi form symbioses with plants underground, setting up a "trading network" where plant photosynthate (carbon) is exchanged via roots and hyphae for soil nutrients obtained by the fungus. This is beneficial for both the plant and the fungus -- the fungus (a heterotroph) gets a reliable source of carbon while the plant can take advantage of the fungus' extensive mycelial network to gain access to nutrient resources (e.g., phosphorus, nitrogen) its own root system can't access. Fungal hyphae can extend much farther afield than most plant root systems, and can fit through very small gaps (pores) in the soil matrix to get at nutrients.

This ancient interaction, the mycorrhizal symbiosis, is found in over 80% of plant species worldwide, and is thought to have facilitated the rapid spread of land plants ~400 million years ago (fungi were on land before plants!). Though the symbiosis can at times be parasitic (especially in human-altered systems), mycorrhizas in natural systems are thought to be generally mutualistic (i.e., both partners benefit from the trading). But only some species of fungi engage in this symbiosis; they are called (not surprisingly), mycorrhizal fungi. There are two main groups of mycorrhizal fungi that we'll explore here.

Ectomycorrhizal fungi

The first are ectomycorrhizal fungi -- "ecto" meaning "outside," which is in reference to how the fungus interacts with its plant host. We'll see later that other mycorrhizal fungi actually penetrate plant cells with their hyphae, but ectos (as they're colloquially called) keep their hyphae outside the plant root cells. As you see in the illustration below, fungal hyphae grow between root epidermal and cortical cells to form what's known as a Hartig net; this network of hyphae is where nutrients are exchanged between the plant and fungal symbionts. Hyphae also often envelop root tips in what are called mantles, or fungal sheaths, seen at left below.

Diagram 1

But we see the real benefit of associating with mycorrhizal fungi when we look at the mycelial network of hyphae extending through the soil. In the photo below, you see a young pine seedling colonized by an ectomycorrhizal fungus -- the tree roots are brown, with thousands of white fungal hyphae extending into the soil around them. These mycelial networks can increase a plant's absorptive area by orders of magnitude.

Many of our most common forest mushrooms are the result of sexual reproduction in ectomycorrhizal fungal species, like chanterelles and many boletes (below). Though there are a few exceptions, most ectomycorrhizal fungi are Ascomycetes or Basidomycetes.

Cantharellus cibarius

Boletus reticulatus

Vesicular-arbuscular endomycorrhizal fungi

The other main class of mycorrhizal interactions involve fungi that actually penetrate their host plant's cell walls when forming symbioses. We call these vesicular-arbuscular endomycorrhizal fungi, and often abbreviate using VAM fungi (Vesicular Arbuscular Mycorrhizal) or AMF (Arbuscular Mycorrhizal Fungi). Endo refers to the fact that the fungi actually take up residence inside plant cell walls (as opposed to ectomycorrhizal fungi). We'll soon see where the rest of their (very long) name comes from.

Diagram 2 - Arbuscular mycorrhizal fungi hyphae, vesicles and spores

All AM fungi are included in the phylum Glomeromycota. Like ectomycorrhizal fungi, AMF have huge mycelial networks running throughout the soil matrix. But when AMF colonize plant root tissue, their hyphae actually go through the cell walls of root cortex cells and form specialized structures called arbuscules that exchange nutrients with the plant (see details below); note that though the hyphae penetrate the plant cell wall, they cannot get past the plasma membrane and do not invade the cytoplasm (that would be very messy indeed!).

Here's a photo of some heavily colonized Clarkia xantiana ssp. parviflora roots that I sampled out in Southern California, showing lots of AMF hyphae and arbuscules:

Many AMF also form vesicles (see Diagram 2 above) that act as storage organs for the fungus, somewhat analogous to vacuoles in plant cells. AMF have been shown to be important not only in supplying limiting nutrients like N and P to plants, but also protecting against pathogens and mediating water stress. There's a great Nature Review on AMF here if you'd like more info.

We just scratched the surface of the mycorrhizal symbiosis here (we didn't even get into ecology!); there are even other mycorrhizal fungal groups such as ericoid fungi and orchid mycorrhizal fungi, but I'll leave it up to you to explore further!

-j