I am interested in the basic molecular, genetic and evolutionary mechanisms of interactions among organisms. The specific problems my students and I are investigating concern host-parasite interactions, especially between the anther smut fungus Ustilago violacea (aka Microbotryum violaceum) and its host plants. This is an excellent system in which to analyze genetic, evolutionary and developmental problems at the molecular and organismal level.

The disease symptoms of U. violacea in its host plant Silene latifolia range from a change in the genetically determined sex expression (XX, XY) to stunted growth of the internodes. The survival of the diseased host, however, is only moderately affected by the parasite, indicating a presumably evolutionary old relationship.

At present, my investigations focus on (1) the fungus-mediated change in sex expression of S. latifolia, (2) the host-mediated change in the growth habit of U. violacea, (3) the analysis of siderophores of U. violacea and (4) the role of pathogenesis-related proteins in resistant and susceptible strains of S. latifolia.

(1) Unlike in other plant species, sex expression in S. alba is not altered by applied chemicals or environmental factors. The fungus may alter the genetic sex determination of the female host plant and initiate stamen growth as well as ovary abortion by inducing specific mRNA synthesis. Our goal is to identify the molecular mechanism involved in the fungus-mediated change in sex expression in S. latifolia.

(2) Monokaryotic sporidia (a-1 or a-2) of U. violacea grow saprophytically and do not produce disease. Heterokaryotic hyphae (a-1, a-2), the disease-producing form, are unstable in vitro. Stabilization of the heterokaryon and promotion of hyphal growth is host-mediated and can be achieved in vitro by incorporating host-specific substances, the hyphal growth factors (HGFs), into the culture medium of the fungus.

Through the use of mutants that do not respond to the HGFs, we try to identify the genes whose products interact with the hyphae-inducing substances. Our goal is to understand the mechanisms involved in the detection and transduction of the signals from the hyphal growth factors as well as the initiation of the switch from saprophytic to parasitic growth, which leads to infectivity and pathogenicity. This aspect of our research is a cooperative effort with Dr. Michael H. Perlin at the University of Louisville.

(3) Siderophores, iron-scavenging molecules, such as rhodotorulic acid, permit U. violacea to grow in iron-poor environments. We study the genetics of rhodotorulic acid production, its uptake mechanism and effect on pathogenicity with a number of UV-induced siderophore mutants that we have isolated.

(4) By selecting for resistance and susceptibility to U. violacea, we have bred resistant and susceptible strains of S. latifolia. The results indicate that multiple genes control these traits. In response to the invasion of S. latifolia by U. violacea, the host plant produces pathogenesis related proteins (PRPs). We are investigating the function of PRPs in susceptible and resistant strains of S. latifolia.

Recent Publications

Ruddat, M., J. Kokontis, L. Birch, E. D. Garber, KÜS. Chiang, J. Campanella, H. Dai. 1991.

Interactions of Microbotryum violaceum (Ustilago violacea) with its host plant Silene alba. Plant Science 80: 157-165

Garber, E. D. and M. Ruddat 1992.

The parasexual cycle in Ustilago scabiosae (Ustilaginales). Inter J. Plant Sci. 153: 98-101

Garber, E. D., M. Ruddat 1994.

Genetics of Ustilago violacea. XXXII. Genetic evidence for transposable elements. Theor. & Appl. Genet. 89: 838-846.

Garber, E. D., M. Ruddat 1996.

Genetics of Ustilago violacea XXXIII. Genetic evidence for insertional mutations inthe magenta locus. Int. J. Plant Sc. 157: 462-467

Birch, L. E., M. Ruddat 1998.

Extracellular accumulation of rhodotorulic acid in strains of Microbotryum violaceum. Int. J. Plant Sci 159. March issue in press