Effects of Helminthosporium victoriae Toxin on Germination and Aleurone Secretion by Resistant and Susceptible Seeds

Abstract

Oat seeds of cultivars susceptible and resistant to Helminthosporium victoriae were held for various times in pathogen-produced, host-specific toxin solutions; control seeds were in water. Seeds were then washed thoroughly and incubated on moist paper, or dried and stored for 2-3 weeks before germination was attempted. In both cases, germination of susceptible seeds was prevented by previous exposure to toxin for 1 hour or more. Control seeds and treated resistant seeds grew normally. Toxin Several lines of evidence show that resistance of oats (Avena sativa L.) to infection by Helminthosporium victoriae Meehan and Murphy is the same as resistance to the pathogen-produced, host-specific toxin (HV-toxin) (12, 18). Many of the data behind this conclusion were obtained from experiments with germinating seeds and young seedlings. The validity of the conclusion is further supported by the fact that toxin rather than the fungus can be used to screen for disease resistance (16). Experiments to determine action of toxin have relied heavily on use of young seedling shoot and root tissue (12). Even the standard bioassays for toxin depend on use of germinating seeds (7). Nevertheless, there is little detailed information on the effects of toxin on seed germination.We have attempted in this study to determine the effects of toxin on resting seeds, on the germination process, and on certain aleurone activities. Aided by National Science Foundation Grant GB-6560. enzymes (2). These enzymes make the carbon and nitrogen stored in the endosperm available to the developing seedlings. Aleurone cells separated from the embryo secrete no amylase until they are activated by an exogenous supply of GA (14).Results of the study should be considered in relation to several hypothetical concepts of disease and toxin resistance. According to one hypothesis, disease resistance is based on synthesis of antifungal substances by the host after contact with the pathogen. There is strong support for this hypothesis (4), but to date there is no conclusive evidence that it can account for disease resistance in general. An alternative hypothesis is that disease resistance is a constitutive, possibly passive, characteristic of certain plants. Constitutive rather than induced resistance would seem more plausible if seeds and aleurone cells in a resting condition would express resistance. In either case, the aleurone layer is a simplified system for resistance studies, and for studies on the effects of toxins on plant metabolism. A further advantage of the model is that susceptibility or resistance to HV-toxin (and to H. victoriae) is determined at a single gene locus (7,12). An abstract describing some of this work has been published (10). MATERIALS AND METHODSOat seeds susceptible (cv. Park) and resistant (cv. Clinton) to H. victoriae and to HV-toxin were used in all experiments. Aleurone cells were obtained by cutting seeds into two parts and discarding the part containing the embryo (14). For germination tests, the glumes were removed and...

Mechanisms by Which Genetically Controlled Resistance and Virulence Influence Host Colonization by Fungal and Bacterial Parasites

“…Work by Strobel (1973a;1973b;1974a;1974b; purports to show that sugarcane clones sensitive to HS-toxin produced by Helminthosporium saccharri carry a specific binding protein associated with the plasmalemma. victoriae and HV toxin by Samaddar & Scheffer (1970). The protein was said to be responsible for the transport of Gc-galactosides but its interaction with HS-toxins was considered to disrupt ion transport.…”

Amylase activity in dormant and germinating seeds of Polygonum pensylvanicum

“…In a study related to chemical regulation of amylase production, Sammaddar and Scheffer (1970) showed that Helminthosporium victoriae toxin blocked gibberellic acid-induced synthesis and secretion of alpha amylase by embryoless seeds of susceptible oats, but not of resistant oats or barley. Their data indicated that the toxin was not a competitive inhibitor of gibbftrpllic acid, but that the blockage of enzyme production may have been an indirect effect.…”