Submitted to: Mycological Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/25/1999
Publication Date: N/A
Interpretive Summary: Rusts can cause serious damage to crops of barley, wheat and other cereal grains. As new varieties with resistance to rust disease are introduced by plant breeders, the rust fungus pathogen develops new races that overcome the newly introduced resistance. A big factor in the development of new races is the rust pathogen's ability to go through its sexual life cycle. The purpose of the research reported here is to understand processes of th sexual cycle toward the goal of eliminating the ability of the pathogen to produce new races. As part of the sexual cycle, the rust fungus produces spores in a sugary nectar. Insects feed on this nectar, carrying the spores from one infection site to another, initiating sexual crosses. We have reported recently the finding that nectar of one infection site induces a special cap on spores of other infection sites. The cap is suspected to be essential for sexual fusion of spores to the fungus at a second infection site. Here we report that cap induction occurs between different rust fungus species, i.e., nectar of one species induces caps on spores of other species, even though the two species cannot undergo sexual fusion(cannot cross). This was an unexpected surprise. It tells us that different rust fungus species share some processes in common leading to sexual fusion. The results highlight the importance of the spore capping phenomenon and form the basis for further research by plant pathologists on the precise role of caps in sexual processes. By understanding these processes, we may someday be able to manipulate cap induction to interfere with the ability of the fungus to develop new and dangerous races.
Technical Abstract: Pycnial nectar of one mating type is known to induce cap formation on pycniospores of opposite mating type within several species of Puccinia and Uromyces. To learn if caps are induced by nectar transfers between species, we used interspecific pairings involving six species of Puccinia and three of Uromyces. Overall, caps were induced in 14 pairings between different species involving all tested species, except P. helianthi which has no intraspecific cap induction. Nectar (with pycniospores) exchanged in reciprocal transfers between individual pycnial clusters of two different species gave pycniospore caps in 9 of 16 cluster pairings, comparable to rates within species. Spore-free nectar combined from five or more pycnial clusters of one species (to ensure that nectar of two mating types was present) usually induced caps in pycniospores from single pycnial clusters of a second species. This occurred in all tested pairings sof species except pairings involving P. helianthi. In experiments with pycniospore-free nectar of one capping type specificity from P. recondita, caps were induced in about 50% of pycnial clusters of unknown capping type from P. triticina or P. hordei and only in pycnial clusters of one capping type from P. triticina or P. reichertii in experiments in which type within species was determined. Coupled with the fact that capping type specificity and mating types are coincident within species, the results indicate that mating type-specific induction of pycniospore caps by nectar extends across species boundaries. Although aecia were never produced in interspecific pairings, cap induction occurred as it does in intraspecific pairings where it precedes aecium formation in species exhibiting the capping phenomenon.