Form and Function
As a species of arbuscular mycorhizzal fungi, Glomus intraradices has a long history of co-evolution with most land plants. This has led to intense morphological specialization to better capture groundwater and nutrients as well as maintaining connection to the host plant’s root hairs. Some examples of these plants can range from the cotton tree to wild rice. Furthermore, throughout its lifecycle, the morphology of this fungus varies greatly and is highly dependent upon environmental factors such as host allelochemicals.
Glomus intraradices, being a member of the Glomeromycota, have no visible mushrooms. They are however constructed of hyphal networks organized into groupings of mycelium, which when it comes time to reproduce, produce spores in a budding fashion from the hyphae. Spores may appear singly, in clusters, or in large groups called sporocarps (Redecker and Raab 2006). These spores may contain thousands of individual nuclei, and are characterized by their three layered cell wall encapsulating the spore. Each of which is slightly larger in diameter than the last. The spores range in color from white, to cream, tan and brown, sometimes with a green tint (INVAM, 2013). They tend to be elliptical, perhaps due to pressure whilst in the root system, however in free soil the spores are spherical to irregularly shaped.
The following picture was taken from the INVAM website and shows Glomus intraradices spores in the root of a corn plant.
These spores are produced when environmental conditions are favorable and aid in movement of the fungus. Spores may either germinate, and extend a new hyphal network from a single start point called a subtending hypha (INVAM 2013) from which additional spores will be produced asexually and the hyphae will continue to extend and branch through the soil in search of nutrients or the individual spores may in fact be physically moved by animals, water, and wind from one local to the next.
Because of co-evolution with most land plants the Glomus intraradices mycelium network can only remain alive for a few days without a host before aborting. After which the hyphae die off and the spores produced go dormant until more favorable conditions allow them to spread their own mycelium. (Kloppholz et al. 2011). To combat a lack of host, the rate of hyphal spreading and branching will be elongated and multiplied after sensation of allelochemicals. These include the presence of CO2 flavenoids (a secondary plant metabolite) (Nadal and Paszkowski 2013) and other plant hormones.
After contact with root hairs has been made, the fungus will penetrate the cell wall by use of hyphopodium (Nadal and Paszkowski 2013) and implant one or many spores. These spores, together with the hyphae extending from it, form a branching pattern both inside the root and out. This drastically increases the surface area available to the host for nutrient absorption.
If you want to learn more about different fungi and their interactions with other organisms, please visit the International Culture Collection of (Vesicular) Arbuscular Mycorrhizal Fungi website.