Basidioporic Fungi

Basidioporic Fungi

Basidiosporic fungi (also known as the Basidiomycota or Basidiomycotina) are fungi that produce sexual spores on a specialized cell called a basidium.

The basidiosporic fungi are the most diverse phylum of the fungi world, with more than 22,300 species described. Some of the fungi in this phylum are microscopic, while the larger members of this group produce fruiting structures that are basketball-sized and weigh in excess of 10 pounds.

This phylum contains fungi that fall into three classes: mushroom, rusts, and smuts—and range widely in appearance, from the common mushroom to weblike fungi with an odor that can be detected at several feet.

Taxonomy

The basidiosporic fungi are divided into three classes: Basidiomycetes (mushrooms); Teliomycetes (rusts); and Ustomycetes (smuts). The Basidiomycetes are the higher basidiosporic fungi, which are normally fleshy. They produce true basidiocarps, and the only spore formed is the basidiospore.


The other two classes both have more than one spore form and do not have extensive mycelium. The Teliomycetes are commonly called rusts and are serious biotrophic parasites of plants. The rusts are able to complete their life cycle only in the presence of living plant host tissue.

The Ustomycetes are commonly called smuts and are mostly minor pathogens of plants, especially monocots. Some smuts have been cultured in axenic culture, where they form a "yeastlike" phase. The yeastlike phase has no true mycelium but rather individual cells.

Basidium

The basidium is a single cell on which basidiospores are produced externally. The basidium forms either as the terminal cell of a dikaryotic mycelium or from a resting spore that initially is dikaryotic.

basidium

The dikaryotic mycelium or spore contains two haploid nuclei, one donated by each of the parent strains. As the basidium begins to form, the two nuclei migrate into the center of the cell and fuse, forming a diploid nucleus.

This nucleus then undergoes meiosis, forming four haploidnuclei. As this is occurring, the cell wall of the basidium begins to produce little extensions called sterigmata, upon which the basidiospores will form. The tips of the sterigmata then inflate, and one nucleus migrates into each forming basidiospore.

The basidiospore is haploid and has a very thin cell wall. The spore is normally transmitted in air currents. Upon germination, the basidiospore produces a haploid mycelium which will fuse with a compatible hyphae, producing a dikaryotic mycelium.

Spore release from the basidiumcan be either active or passive. Passive release occurs when the junction of the sterigma and basidiospore separates, releasing the spore. Active release is more specialized. When the basidiospore is forming, a small segment of the spore wall at the junction with the sterigma loosens and fills with either gas or liquid.

At the time of release, the fluid or gas escapes, propelling the basidiospore away from the basidium. The distance traveled is not great, just enough to make sure that the basidiospore is able to enter into air currents for dissemination.

Hyphal Structure

The hyphae of the Basidiomycetes are septate and have special modifications at the septa. When a cell divides, a cross wall forms between the two daughter cells. With the dikaryotic hyphae of the Basidiomycetes, as the cell divides, the nuclei migrate toward the apex of the hyphae.

The nuclei then undergo mitosis, with one of the nuclei migrating into a small outgrowth of the hyphae and the other migrating backward. Septa form, creating a new dikaryotic cell near the apex and two haploid cells, one in line and the other as the outgrowth.

The outgrowth then turns and fuses with the haploid cell, and the nucleus migrates back to form a dikaryotic cell. The outgrowth remains visible with a microscope and is called a clamp connection.

The reproductive structure of the Ustomycetes is called a sorus. The sorus is a mass of dikaryotic spores that are normally dark brown or black in color. The sorus is formed in meristematic regions of the plants. The spores are called probasidia, because they form basidia when they germinate.

With the Teliomycetes, there are up to five distinct spore forms. The basidiospore lands on a susceptible plant and germinates, producing a haploid mycelium that infects the plant. The infection results in the formation of a haploid spermagonium that produces both spermatia and receptive hyphae.

When a compatible spermatia and receptive hypha combine, a dikaryotic hypha is produced, which initiates formation of an aecium. The aecium produces dikaryotic spores that are transmitted by air currents and infect another plant.

The resultant infection produces a subcuticular or subepidermalmass of thin-walled spores. These dikaryotic spores are called urediniospores and are formed in the uredinium. The urediniospores are blown by air currents and produce reinfection of the same species of plant.

At the end of the growing season, infections by urediniospores will result in the formation of a subcuticular or subepidermal mass of thick-walled spores called teliospores which are formed in the telium. These spores are initially dikaryotic but then become diploid and finally germinate by formation of the basidium.

Basidiocarps

basidiocarp

The basidiocarp is the fruiting body of the higher Basidiomycetes. This structure is multicellular and composed of hyphae. The basidiocarp resembles the familiar image of the mushroom. The mushroom consists of a stalk (stipe) which has a cap (pileus)on top.

The stipe can be as tall as a meter (40 inches), and the pileus as long as ameter in diameter. Alternatively, both parts could be less than a centimeter in size. The pileus has pores or gills on the underside, where the basidia are produced. The layer of basidia is called a hymenium or "fertile layer."

Other kinds of basidiocarpsmay be found in nature. Some are totally enclosed and remain on the ground, looking much like a golf ball. These are called puffballs. As the puffball matures, the other layers begin to crack at the apex.

When drops of rain fall, the force of the impact causes spores to puff out of the opening. Another kind of puffball is the earthstar. In these unique fungi, the outer layers pull away from central part of the puffball and form a starlike pattern on the ground.

Ecological Importance

The basidiosporic fungi all play important roles in ecosystems. The rusts and the smuts are impor- tant plant pathogens, capable of great destruction of crops. These fungi have been known for thou- sands of years and are some of the most devastating fungi around.

The mushrooms are part of the natural cycle of decay. They are found on the ground or on wood and are the later stages of decay of organic matter. Some mushrooms are found on living plants, where they can be serious pathogens. Others are edible and are excellent sources of digestible protein. Still others are toxic or poisonous and can be fatal when eaten.

Stinkhorns and the bird’s nest fungi are unique basidiosporic fungi. The stinkhorns are basidiocarps that form on the soil and produce the basidia in a mass of putrid cells.The stench from the cells draws flies, which walk over the spores and then disseminate them. These can be found in wooded areas and can be detected by smell at distances of up to several meters.

The bird’s nest fungi look like small birds’ nests. The outer part of the basidiocarp resembles a small nest, up to an inch in diameter. On the inside, several small puffball-like structures can be found, with basidia on the inside. These look like small eggs.

When a drop of water enters the nest, the force thrusts the “egg” upward and extends a small cord from the back. The small cord catches hold of a plant and suspends the egg in the air. As the egg dries, it turns into a powdery mass, which is blown about by the wind.