Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/13/1999
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 report here that the nectar of one infection site can induce a cap on spores of other infection sites. Our results indicate that this cap may be essential for sexual fusion of the spores with the fungus at a second infection site. This new finding sets the stage for further research by plant pathologists on the sexual cycle and opens the possibility of someday being able to manipulate the cap-inducing factor to interfere with the ability of the fungus to complete the sexual cycle.
Technical Abstract: Pycnial nectar transferred between pycnia of opposite mating type induced formation of a cap on one end of pycniospores. The caps developed with seven species of Puccinia and three of Uromyces, but not in P. helianthi, Tranzschelia pruni-spinosae, or U. vignae. The caps stained with colloidal gold, India ink, or wheat germ agglutinin (WGA) conjugated with FITC, but were not visible in unstained preparations. Cap formation started within 10 min of nectar transfer and was completed in 20-60 min. Pycniospores that were removed from a pycnium and induced to form caps by pycniospore-free nectar of opposite mating type did not induce aecia when returned to the original pycnium, showing that cap formation alone was not sufficient for completion of mating processes. Caps were removed by treatment with Proteinase K, SDS, or 0.01N HC1 and partially removed with trypsin, indicating the presence of protein. The cap-inducing activity of nectar was lost if the nectar was boiled or autoclaved. Nectar run on native PAGE gels had principal protein bands near 132 kDa, which in turn had cap-inducing activity. In SDS gels, multiple polypeptide bands ranging from 14-62 kDa were observed, but bioassays of these polypeptides for cap induction were not successful. The results indicate that pycnial nectar contains cap-inducing proteins which are mating type-specific and induce pycniospore cap formation as an essential initial event preceding fusion of pycniospores with receptive hyphae.