Karen Snetselaar
Ph.D., University of Georgia

Research Interests:  Pathogenic fungi, especially Ustilago maydis, the corn smut fungus.

We are taking several different approaches to learn more about how the pathogenic fungus U. maydis infects corn plants.  One project focuses on events that happen just before infection begins.  The individual fungal cells cannot cause infection by themselves; each one must "mate" with a compatible cell first.  This involves an interesting developmental series of events that includes cell-cell recognition, following a pheromone gradient toward a compatible partner (see picture below), and cell-cell fusion.  We have developed a number of microscopic assays to study these events, and, in collaboration with Dr. McCann, we are combining the assays with molecular methods to learn about the genetic basis for mating behavior. 

A second focus of the lab is on understanding the host-parasite interaction between the corn plant and the parasitic fungus.  Smut fungi are extremely interesting because during most of the infection process, they do not kill or even seriously damage the host plant from which they receive all nutrients.  Through microscopic studies, we know a bit about the morphology of the host and fungal cells where they contact each other during the infection process (see pictures below).  At the present time, however, we know little about the biochemistry or physiology of the disease interaction.  For example, which nutrients are passed from plant to fungus?  Students in the lab are currently infecting corn plants with a variety of Ustilago mutants to try and answer this and related questions.  For example, they infect young corn seedlings with a mutant that is auxotrophic for a particular amino acid, and then assess the ability of the mutant strain to cause disease.  If the mutant fungal strain causes normal disease, then it must be receiving the amino acid for which it is auxotrophic from the host plant.  These studies will help us learn more about how fungi cause disease. 

Figure 1: An ear of corn infected with Ustilago makes tumors instead of normal corn kernels. 

Figures 2 - 3: When drops of compatible Ustilago cells are placed close together, the cells form filaments that follow a pheromone gradient from one drop to the other (top).   This scanning electron micrograph shows a fungal filament that has just entered into the epidermal cell of a corn leaf.  The arrow points out a specialized infection structure called an appressorium (bottom)