Signal exchange between higher plants and rust fungi

The rust fungi appear to have evolved a sophisticated complex of molecular interactions with their host plants that govern both plant resistance and susceptibility. It is suggested that many of these interactions relate to the maintenance and effective exploitation of biotrophy, and that host specificity and the obligacy of parasitism are a consequence of the resulting interactive molecular control of plant and fungal activities. For the dikaryon, plant signals are required for locating stomata and the formation of infection structures, haustorial mother cells, and haustoria. Host susceptibility to both the monokaryon and the dikaryon appears to involve the suppression of defensive secretory processes, the induction of cellular alterations in invaded cells, and, for the dikaryon at least, changes in nutrient translocation. Parasite-specific resistance involves cultivar-specific fungal signals (elicitors) of defense responses such as cell death and callose deposition. The nature of, and evidence for, the signals involved in these interactions are reviewed. Key words: biotrophy, elicitors, rust fungi, signal exchange.