J.M. Daly

The structure of the toxin from helminthosporium carbonum

Using Fast Atom Bombardment Mass Spectrometry and Mass Spectrometry/Mass Spectrometry, the structure of HC-toxin a metabolite of Helminthosporium carbonum, is postulated to be structure 3.

Hypersensitive response of near-isogenic wheat carrying the temperature-sensitive Sr6 allele for resistance to stem rust

Abstract The hypersensitive response of near-isogenic wheat lines with either the Sr6 allele for compatibility or the temperature-sensitive Sr6 allele for incompatibility toward Puccinia graminis tritici, race 56, was studied. The response was determined at daily intervals during infection by counting with the aid of a fluorescence microscope the number of size of fluorescent sites in standardized segments of leaf tissue randomly selected from minimum populations of 10 heavily infected leaves. When grown continuously at 20 to 21 °C, the resistant Sr6 line was characterized by a progressive increase in the number of fluorescent sites during the critical period (through the fourth day after inoculation) when compatibility or incompatibility is established. At 26 °C, the Sr6 line is susceptible in terms of infection type, number and time of appearance of lesions and increase in glucosamine content during sporulation. Statistically, the development of the hypersensitive response in the Sr6 line at 20 °C could not be distinguished from the response at 26°C, yet at the latter temperature there were no visible signs of incompatibility between host and parasite. When treated with 80 parts/106 of ethylene at 20°C, the Sr6 line developed even more fluorescent sites than untreated plants despite the induction of susceptibility by ethylene. Pre-inoculation temperature appears important in determining the extent of the hypersensitive response, but not in determining disease reaction. The data provides less ambiguous evidence in support of previous contentions that the hypersensitive response is not a determinant of resistance in rust disease. It is possible that the response is a result only of a general stress occasioned by the presence of an organism to which the host normally does not accomodate or to which it accomodates with difficulty.

Biological activity of purified host-specific pathotoxin produced by Bipolaris (Helminthosporium) maydis, race T☆

Abstract A purified, chemically characterized toxin from Helminthosporium maydis, race T, was examined for its host-specific effect on Texas male sterile (T) cytoplasm corn. This preparation consists of several nearly identical linear polyketols and it is shown that each component has the same specific toxicity for oxidation by mitochondria from susceptible corn. The stability of the dry preparation and knowledge of its mol. wts enabled a quantitative comparison to the dosage required to affect ion balance, dark CO2 fixation, coleoptile elongation, and mitochondrial oxidation. Unexpectedly, the toxin was active at concentrations between 5 and 50 ng ml−1 (6·5 × 10−9 to 6·5 × 10−8 m on all these processes in T corn, despite potentially different barriers to toxin penetration. N cytoplasm corn was not affected at concentrations 1000 times greater. These results are discussed in relation to possible modes of action. The quantitative specific effects of this preparation are compared to data on other race T toxin preparations for which chemical structures have been postulated.

Structure of the host-specific pathotoxins produced by Helminthosporium maydis, race T

Abstract Polyketo-polyalcohol structures of the host-specific pathotoxins (Band 1- and Band 2-toxins) from Helminthosporium maydis , race T were elucidated by NMR analysis and chemical degradations.

Dominance at the Tox1 locus controlling T-toxin production by Cochliobolus heterostrophus

Abstract Heterokaryons of Cochliobolus heterostrophus were constructed which contained approximately equal numbers of nuclei bearing either the tox1+ allele for presence of T-toxin or the tox1− allele for absence of T-toxin in culture filtrates. The heterokaryons were grown in a liquid medium known to support production of T-toxin; the resulting culture filtrates were assayed for T-toxin activity using 2 different bioassays: seedling root growth and dark CO2 fixation. Both assay procedures detected the presence of T-toxin in culture filtrates of tox1 + tox1 − heterokaryons; levels of activity were slightly less than those found in filtrates from tox1 + tox1 + heterokaryons. Since these heterokaryons are known to function as diploids, the simplest interpretation of the results is that the tox1+ allele, which determines high virulence as well as T-toxin production, is either dominant or semidominant.

A comparison of the development of Puccinia graminis tritici in resistant and susceptible wheat based on glucosamine content

Abstract Procedures based on ligand exchange chromatography of complexes of Zn2+ with amino sugars are described for the determination of glucosamine in 6 n -HCl hydrolyzates of samples equivalent to a single wheat leaf. Leaves incubated in a dew chamber for 14–15 h after inoculation with Puccinia graminis tritici showed an increase of about 6 nmol of glucosamine per leaf resulting from development of germ tubes on the leaf surface. No additional increase was observed in either compatible or incompatible hosts through the third day (68–72 h) after the deposition of inoculum. Just prior to symptom development and sporulation, the fungus appeared to have a more rapid rate of growth in compatible hosts as indicated by significant increments in glucosamine. Smaller but definite increases in glucosamine also were observed in incompatible hosts at this time, suggesting persistent growth of the pathogen. The results are discussed in connection with literature reports of the histological events during development of stem rust fungi in compatible and incompatible hosts.

Structure of the host-specific pathotoxins produced by Phyllosticta maydis

Abstract Linear polyketol structures of two host-specific pathotoxins from Phyllosticta maydis (PM-toxin B and PM-toxin C) were elucidated by MS fragmentation patterns of the toxins and their derivatives, and by NMR analysis.

Biochemical comparisons of resistance to wheat stem rust disease controlled by the Sr6 or Sr11 alleles

Abstract Near-isogenic lines of wheat carrying the Sr11 and sr11 alleles for resistance and susceptibility to race 56 of Puccinia graminis tritici were examined for total phenolic compounds and total peroxidase activity during infection. As in the case of the Sr6 allele, no significant changes in phenolic components were detected. Increases in total peroxidase with resistant reactions controlled by the Sr11 allele were similar to those found previously for the Sr6 allele and the same isozyme was responsible for the increase. Previous evidence obtained with the Sr6 allele indicated that this isozyme is not a causal factor in resistance. Because the genetic and physiological basis for resistance controlled by the Sr6 and Sr11 alleles is distinct, it is concluded that increased activity of the same isozyme in both instances is a result of a non-specific event analogous to wounding. Infected plants carrying the sr6 allele, with low peroxidase activity, produced much more ethylene than resistant infected plants. The relationships among ethylene production, disease reaction and peroxidase activity are not easily resolved.

BIOLOGICAL ACTIVITIES AND STRUCTURES OF HOST-SELECTIVE PATHOTOXINS

Abstract Examples of host-specific or -selective toxins (HST) in plant diseases are now well-documented by an increasing number of reports. Knowledge of them is important in formulating theories of disease resistance and its genetics. This knowledge has potential importance to pesticide research because selective toxicity with minimal environmental impact is a major goal in the effective use of pesticide. A limiting factor in studies of HST has been lack of knowledge of their structure. In the last 6 years, elucidation of structures of several HST provides an oppotunity for determining mode of action and the basis of specificity.

Synthesis and biological activity of mimics of pm-toxins the host-specific cornpathotoxin produced by Phyllosticta maydis

Abstract To establish structure-activity relationships, 12 mimics of PM-toxin A, a component of the host-specific corn pathotoxin produced by Phyllosticta maydis , have been synthesized as stereoisomeric mixtures. All the mimics synthesized have four β-ketol groups spaced by varying lengths of methylene chains or by a 1,3-diene chain. Mimics with the shorter methylene side-spacers or with the diene side-spacers are 30- to 300-fold less toxic than the native toxin, but the remaining compounds are equally or more toxic than the native toxin. These results can be accounted for by postulating that intramolecular associations at the β-ketol groups may yield two types of cage structure with active and less active conformations.