Basidiomycota : Ustilaginomycotina and Pucciniomycotina (Smuts and Rusts)

Basidiomycota, commonly known as the club fungi. Currently, within Basidiomycota, there are three subphyla. Those are Ustilaginomycotina, Pucciniomycotina, and Agaricomycotina. Let’s first examine fungi within the subphylum, Ustilaginomycotina. Generally, fungi within this subphylum are known as smuts, or fungi that cause diseases in plants. These smuts fall into the class Ustilaginomycetes, then the order Ustilaginales. These fungi mainly infect plants in the Poaceae family, also known as the grass family. This means that some smuts can adversely affect economically important cash crops such as barley, wheat, sugar cane, or potatoes.

Smuts attack plant fruits and seeds causing them to swell into structures called galls. These galls are full of spores and burst open when fully matured. Although many smuts cause issues for us, the corn smut, Ustilago maydis, is an edible smut that infects corn. It causes the kernels of corn to swell into galls. In Mexico, these galls from Ustilago maydis are used for fillings in tacos, quesadillas, and several other dishes. Corn smut has an exciting life cycle consisting of two phases: a single-celled haploid yeast phase and a dikaryotic mycelia phase. As we recall from our study of biology, haploid means the cells each have one nucleus and one set of chromosomes. Dikaryotic means there are two nuclei in each cell, one from each of the parent fungi, a feature that is unique to the fungi kingdom. We must not confuse dikaryotic with diploid.

In diploid organisms the nuclei from parents fuse into one, whereas dikaryotic fungi have two separate nuclei that do not fuse into one. Starting at the yeast phase, the cells have two genetic alleles that determine if another yeast cell is a compatible mate, the “a” locus and “b” locus. If the two yeasts have different alleles at the “a” locus, conjugation tubes form on the cells and grow toward each other, fusing at the conjugation tube tips. This connection will allow the cells’ respective nuclei to be transported from one to the other. However, this is conditional upon the “b” locus. If the “b” locus differs from one cell to the other, the conjugation tubes will transport the nuclei and form a dikaryotic cell, which can grow into a filamentous dikaryotic mycelium.

This dikaryotic mycelium will insert itself into the plant tissue and form galls that release the spores. Originally, species of fungi within the subphylum Ustilaginomycotina were generally known to be plant pathogens called smuts and fall into the class Ustilaginomycetes. Ustilago maydis is considered a Ustilaginomycete. Recently, there have been other unique divisions of the fungal lineage that fall into this subphylum. These are subject to change due to the ever-expanding knowledge of fungal taxonomy, but along with ustilaginomycetes, there are three other classes: Exobasidiomycetes, Moniliellomycetes, and Malasseziomycetes. Species within the class Exobasidiomycetes also act as plant pathogens on grass species and thus are considered smuts.

A notable species is Tilletia indica, or Karnal bunt which parasitizes wheat, causing kernels of wheat to produce a putrid odor, and making the products of wheat such as flour taste off-putting. Species within the Malasseziomycetes are known to cause dermatomycoses or fungal skin infections. A genus within the class Malasseziomycetes, known as Malassezia, lives on human skin and some species cause problems such as dandruff. Next, we’ll discuss the subphylum, Pucciniomycotina. Most species within this subphylum are plant pathogens in the form of rusts, but many species act as parasites that act on mushrooms, also called mycoparasites.

We’ll discuss the rust fungi, which generally constitute the class Pucciniomycetes and fall into the order Pucciniales. Rusts get their name from the powdery red fruiting bodies that form on parts of plants. Wherever there are plants, there are also rusts which can infect a diverse number of plants from a wide range of ecosystems. Many rust fungi have coevolved with their host plants, which has led the order to become quite diverse, with about 7,000 described species. A notable rust fungus is Gymnosporangium virginiana, or Cedar Apple rust, which infects both Juniper trees and Apple trees to complete its life cycle. When a fungus requires two host organisms to complete its life cycle, it is considered heteroecious. Rust fungi are known for having a heteroecious life cycle.

Fungi that need only one organism to complete their life cycle are considered autoecious. Rust fungi can have quite an interesting life cycle with up to five stages and five different types of spores. A rust is considered macrocyclic if it produces all spore types. These are urediniospores, teliospores, basidiospores, pycniospores, and aeciospores. Let’s describe the life cycle and spore types of the well studied Puccinia graminis, or stem rust. Puccinia graminis is heteroecious and has a primary host where asexual reproduction occurs and an alternate host where sexual reproduction occurs. Its primary host is wheat and its alternate host is the barberry plant. Barberry is considered the alternate host because the rust can reproduce with or without its presence.

We’ll begin the life cycle during the summer months by focusing on the stem of its primary host, wheat. Hyphae have penetrated the wheat stem, and at the places where the red rust emerges, there are structures called uredinium that are filled with urediniospores. The urediniospores allow for the rust to asexually reproduce and infect other plants. As the temperature gets colder, and the fungus recognizes that winter is coming, the fungus forms a telium instead of a uredinium. The telium houses the production of teliospores, which are black, thick-walled, and diploid. These teliospores are quite durable and are stable until spring, when they germinate and transition from a diploid spore to a haploid spore through meiosis. These new haploid spores are known as basidiospores, and they allow the fungus to infect its alternate host, barberry leaves.

The basidiospores germinate inside the leaves and produce a haploid mycelium. From this mycelium, structures called pycnia produce receptive mycelium and haploid pycniospores, which are found in a sticky substance that attracts insects. The insects will transport pycniospores from one pycnia to another, allowing sexual reproduction. Once a pycniospore fuses with receptive hyphae, new hyphae produce a mycelium having both nuclei instead of just one. This new, double nucleus mycelium, known as a dikaryotic mycelium, grows through the barberry leaf and produces a structure called an aecium which houses dikaryotic aeciospores. These spores can only infect the primary host, wheat. Aeciospores are separated from urediniospores because the spore shape and production process is quite different.

Once aeciospores reach the wheat plant, they penetrate and begin producing a mycelium. Soon, that mycelium will produce dikaryotic urediniospores, and the cycle starts over again. So that covers two subphyla of the phylum basidiomycota. But there are many more organisms in there to discuss, so let’s move forward and examine some more.

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