No abstract

A new member of the Plasmodiophorales with rather unusual characteristics has been found parasitizing the roots of wheat grown in soil from three different localities in Ontario. In addition to spore clusters of the Ligniera type, large, septate zoosporangia with conspicuous tubes for zoospore discharge are present. These multinucleate zoosporangia are produced by progressive lobular outgrowths from uninucleate amoebae and from the beginning are always surrounded by a thin wall. In the formation of resting spores, naked multinucleate myxamoebae develop first, then segment to form spore clusters without formation of a soral membrane. Both zoosporangia and resting spores produce identical zoospores with two flagella of unequal length.The somatic nuclear divisions in the growing myxamoebae are characterized by the simultaneous division of both the nucleolus and chromatin within a persistent nuclear membrane. During the transitional phase which follows, the nucleolus disappears and at the same time there is an intensification of the staining properties of the surrounding cytoplasm. Prior to segmentation of the myxamoebae to form the spore clusters, and in all divisions during growth of the zoosporangia, the nucleoli and nuclear membranes disappear, and divisions are of ordinary mitotic type.Relationship with the Plasmodiophorales is indicated by the form of the resting spore clusters, the method of nuclear division during growth of the myxamoebae, and the characteristic flagellation of the zoospore. Since the zoosporangial characteristics of this fungus differ from those of other genera within this order, it is considered desirable to place it in a new genus for which the binomial Polymyxa graminis is proposed.

The fatty acid compositions of oils extracted from the spores of 21 species of fungi, mainly rusts and smuts, have been determined by gas liquid phase chromatography. Cis-9,10-epoxyoctadecanoic acid was present in the spore oil of six species of Puccinia, two species of Melampsora, and one species each of Cronartium and Phragmidium, but not in one species each of Uromyces, Gymnosporangium, and Ravenelia. Palmitic acid was the principal saturated acid in all the fats, except those from the mildews, and the unsaturated C18 acids generally made up the remainder. Rusts of the same genus with hosts in the same plant family had similar fatty acids but other rusts of the same genus with hosts in other families had a quite different pattern. The fats of smuts and mildews are quite distinct from those found in the rusts.

Chemical analyses of wheat stem rust uredospores before and after germination indicated that the lipid fraction was the major endogenous substrate used up during the germination. Some nitrogenous material, presumably protein, was also utilized and increase in chitin was observed during the germination. A number of commercially available metabolic intermediary compounds did not affect the respiration of germinated spores. Heavy metal enzyme inhibitors, such as cyanide and azide, strongly retarded the respiration of spores and of germinated spores. This, together with the findings that the spore material contained hematin and reacted positively to the Nadi reagent, suggested that a cytochrome system was operative in the respiration process. Compounds such as malonate, maleate, fluoride, and fluoroacetate, known to be the inhibitors of succinic dehydrogenase and related enzymes, did not show significant inhibition of the respiration of rust spores. However, pyrophosphate, another specific succinic acid dehydrogenase inhibitor, retarded carbon dioxide production. Other inhibitors, such as iodoacetate, benzoate, and arsenate, inhibited respiration appreciably.

Studies were made on the oxidative metabolism of stem rust uredospores under self-inhibited conditions and when self-inhibition was removed. The oxidative metabolism of self-inhibited spores is characterized by the following features: the oxygen uptake declines rapidly; the respiration is mainly based on a fatty acid oxidation apparently bypassing the Krebs cycle (malonate insensitivity); and the enzymes of carbohydrate breakdown are more or less inactive. Compounds that are able to overcome self-inhibition stimulated oxygen consumption. Simultaneously an intensified utilization of endogenous fatty acids was initiated and also an increased malonate sensitivity. Longer incubation periods with stimulants resulted in germination and in a concomitant activation of carbohydrate metabolism. The catalytic effect of pelargonaldehyde or other stimulants in overcoming self-inhibition was reproduced by the administration in substrate concentrations of suitable respiratory material such as butyrate and propionate. The stimulated respiration induced by these substances appears to be a prerequisite for germination.The possible significance of the above findings from the point of view of host–parasite relations is discussed.