Info

phenomenon of dimorphism—it switches from a yeast-like saprophytic form to a filamentous, pathogenic growth. Regine Kahmann's group in Germany is attempting to explain mating and fusion of conjugant cells (sporidia) in biochemical terms and the switch into a pathogenic filamentous mycelium.

U. maydis causes the abnormal growth of kernels or leaves into galls or tumors that are filled with countless numbers of brownish-black spores. These spores are formed by the separation of binucleate hyphal cells and are called chlamydospores. The chlamy-dospores (Figure 8.3) are dormant structures that can survive in crop refuse and in soil for many years. These cells, in which the two haploid nuclei fuse, are also referred to as teliospores. The teliospore germinates to form a short tube that becomes divided by septa— a distinguishing structure of fungi belonging to the order Ustilaginales in Phylum Basid-iomycotina of the Kingdom Eumycota. This structure is called a basidium, in which the diploid nucleus undergoes meiosis and produces four haploid nuclei that are abstricted as basidiospores (also called sporidia). The sporidia can be grown on artificial media where they multiply by yeast-like budding. The characteristics of the colonies formed from the sporidia of a single basidium differ in color, topography (smooth or rough colonies) and mating type, demonstrating that they are recombinant progeny.

In the first step in sexual reproduction, the cells must distinguish self from non-self and choose a potential mating partner in the environment. The partners must coordinate their choices by making contact, but is the contact based on chance or some physiological mechanism? The sporidia secrete small size polypeptides called pheromones that are sensed by sporidia of the opposite mating type in the vicinity, reorient their growth and fuse via conjugation tube. The formation of conjugation tubes using cell culture superna-tants is a biological assay for secreted pheromone. An example of visible evidence of cell-cell signaling in a fungus is given in Figure 5.3.

Induction of — Cell/cell

Induction of — Cell/cell

Sporidia Structure

Figure 8.3 Diagram of life cycle of Ustilago maydis. The haploid (n) sporidia multiply to form secondary sporidia by budding. Sporidia of compatible mating type fuse to form a dikaryotic mycelium (n + n) that infects a corn plant, the kernels of which are filled with powdery mass of spores (chlamydospores). Nuclear fusion occurs in chlamydospores (teliospores) that are 7-10 ^m in diameter. These germinate to form a tube-like basidium in which the diploid (2n) nucleus divides by meiosis. The four haploid nuclei are abstricted in four cells (sporidia) which can grow saprophytically on refuse and multiply by budding. (From The Mycota I, Mating-type genes in heter-obasidiomycetes, Kämper, K., Bölker, M. and Kahmann, R., p. 326, Fig. 2, 1994, © Springer-Verlag Berlin Heidelberg. With permission.)

Figure 8.3 Diagram of life cycle of Ustilago maydis. The haploid (n) sporidia multiply to form secondary sporidia by budding. Sporidia of compatible mating type fuse to form a dikaryotic mycelium (n + n) that infects a corn plant, the kernels of which are filled with powdery mass of spores (chlamydospores). Nuclear fusion occurs in chlamydospores (teliospores) that are 7-10 ^m in diameter. These germinate to form a tube-like basidium in which the diploid (2n) nucleus divides by meiosis. The four haploid nuclei are abstricted in four cells (sporidia) which can grow saprophytically on refuse and multiply by budding. (From The Mycota I, Mating-type genes in heter-obasidiomycetes, Kämper, K., Bölker, M. and Kahmann, R., p. 326, Fig. 2, 1994, © Springer-Verlag Berlin Heidelberg. With permission.)

A simple test of mating compatibility is to co-spot sporidia on a nutrient agar medium containing charcoal where the fused sporidia show filamentous growth (the "fuz" reaction). However, the sustained growth of the hypha can occur only as a parasite in the corn plant (Zea mays). Only the haploid cells (sporidia) that differ at two mating type loci can mate, resulting in hypha (dikaryon) in which the two nuclei divide several times but maintain their nuclear identity. Fertilization is postponed in favor of dikaryosis. This mycelium is a special type of heterokaryon in which each cellular compartment contains two nuclei of opposite mating type in close proximity without fusion. The dikaryotic hypha recognizes and enters into the corn plant, which responds to infection by forming galls (tumors) filled with chlamydospores (teliospores).

0 0

Post a comment