BOTANY WITH THE BASICS OF HYDROBOTANY - 2010

7. MYXOMYCETES, OR SLIME MOLDS (MYXOMYCOTA)

A characteristic feature of these organisms is their vegetative body structured as a plasmodium—a naked, reticulate, shapeless mass of protoplasm devoid of a Cell wall and containing numerous nuclei. Because of this, they are commonly referred to as slime Molds.

The life cycle of myxomycetes includes the following stages: the plasmodium and its sclerotization under certain conditions, The formation of fruiting bodies and spores, their germination, the formation of amoebae, and their aggregation into a plasmodium (Fig. 7.1).

Fig. 7.1. Developmental cycle of slime molds:

1 — plasmodium; 2 — sporulation; 3 — spores; 4 — zoospores; 5 — haploid myxamoebae (either they or the zoospores can copulate); 6 — diploid myxamoeba

The plasmodium moves in an amoeboid manner. Myxomycete plasmodia can often be found as a slimy layer between the bark and wood of trees or their remains, as well as on manure. The plasmodia of different species vary in shape, size, and color. Their sizes range from a few millimeters to 1 m in diameter. The progressive movement of the plasmodium occurs due to the unidirectional flow of protoplasm. The moving protoplasm forms blunt lobes (pseudopodia), which then elongate and branch out to form a new region. The speed of plasmodial movement in various directions can reach up to 1 mm per minute.

At the sporulation stage, the plasmodium emerges onto The surface of the substrate. Here, sporangia with a tough outer wall called the peridium develop as growths On the surface of the plasmodium. In most cases, sporangia are rounded or oval and typically taper into a stalk at the base.

Spores are rounded, featuring a tough, smooth, or ornamented wall, usually uninucleate, and rich in Glycogen. When dry, spores remain viable for long periods.

Upon germination in Water, spores produce uninucleate zoospores, typically elongated with a tapered anterior end bearing two flagella. Germination on a moist substrate yields amoebae. Zoospores reproduce by binary fission. Subsequently, the zoospores lose their flagella and transform into amoebae (myxamoebae). Amoebae also reproduce by fission, copulate in pairs, and fuse into a common mass, forming a plasmodium.

During the copulation of amoebae, their nuclei fuse, so the resulting plasmodium contains amoebae with diploid nuclei. Within the plasmodium, these diploid amoebae multiply; during sporulation, the diploid nuclei divide, and the resulting spores contain haploid nuclei.

Slime molds are widespread in nature and are typically Saprophytes, living on old, decaying wood or other decomposing plant substrates. They represent a quite diverse group comprising 70 genera and over 400 species.

Plasmodiophora brassicae is the most characteristic representative of slime molds. It is the CAUSATIVE AGENT OF a cabbage disease known as clubroot. The fungus as the pathogen of cabbage was first described by the mycologist M. S. Voronin (1875).

The zoospore of the fungus penetrates the plant through ROOT hairs. Then, entering the larger Cells of the root parenchyma, the zoospore grows into amoeboid plasmatic bodies that can fuse to form a plasmodium. The plasmodium subsequently breaks down into several zoosporangia, within which multiple nuclear divisions give rise to haploid, rounded zoospores. These copulate to transform into diploid amoeboids that spread through the cells of the root Tissues. Afterwards, several amoeboids fuse to form a multinucleate plasmodium. Inside the plasmodium, a twofold division of all nuclei takes place, followed by the formation of resting spores. These spores are released when the root decays and germinate the following year, each producing a single zoospore with one forward-directed flagellum. In the soil, they can retain viability for a long time (6–7 years). The disease can reduce cabbage yields by 30–40% or more.

Selection/5.html">Control Questions and Tasks

1. Provide a general characterization of myxomycetes.

2. How do slime molds reproduce?

3. What is The Role of slime molds in nature and human life?