The role of repellents in the lifecycle of Ustilago maydis

Wieke Teertstra, Han Wösten

Ustilago maydis is a dimorphic phytopathogenic fungus. The plants it infects are Zea mays(maize) and its close relative (or anchestor) Euchlena mexicana (teosinte). Infection commences when two compatible yeast-like sporidia meet and subsequently mate. The infection itself proceeds through the use of apressoria or by growth through stomata. Once inside the plant, dikaryotic hyphae grow in the plant and give rise to tumor formation; probably by secreting hormones (steroids) or by affecting the plants hormone production (indole acetic acid; auxin). Inside the tumors plant cells and fungal structures proliferate and thick walled, single celled, diploid teliospores are produced. Upon maturation of these spores the tumor attains its typical black color that gave Ustilago (meaning burned) it’s name. The teliospores are spread by wind and upon germination haploid sporidia are formed. Compatibility of the yeast-like sporidia is based on two loci descriptively called a and b. Two strains are compatible when they differ in both a and b. Two alleles of a are known (1 and 2), while 33 alleles of b (1,2,3…33) have been isolated. 

Thus, strain a1b2 is compatible with a2b3 but incompatible with a2b2 (same b) or a1b2 and a1b3 (same a). 

In 1996 a novel protein (Rep1) was discovered (Wösten et al., 1996) that is expressed in the filamentous stage of Ustilago maydis. In crosses of compatible strains with disrupted Rep1 genes, formation of hyphae in a dry environment (e.g. the air) was affected and the colony surface was no longer hydrophobic. This phenotype is very similar to that of a strain of Schizophyllum commune in which the SC3 hydrophobin gene was inactivated. However, Rep1 is not homologous to the fungal hydrophobins (see link fungal hydrophobins). Rep1 is a secreted protein that is processed in the endoplasmic reticulum into twelve small peptides each 34-55 amino aicds in length. These peptides are secreted in the cell wall. We are studying how repellents fulfill their role in formation of hyphae in a dry environment and whether they are involved in the infection process of maize. It may be that repellents in some way interact with hydrophobins.