STUDIES ON POLYMYXA GRAMINIS, N. GEN. N. SP., A PLASMODIOPHORACEOUS ROOT PARASITE OF WHEAT

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.

Response of UK winter wheat cultivars to Soil-borne cereal mosaic and Wheat spindle streak mosaic viruses across Europe.

“…Optimal activity of P. graminis from temperate locations has been shown to occur at or near 15°C (Legreve et al 1998). Cycling wet and dry periods, as seen in 2000, have been shown to favour the growth of P. graminis (Adams et al 1986), whereas prolonged periods of wetness have been shown to reduce spread (Ledingham 1939). In addition, Cadle-Davidson et al (2003) have shown no significant transmission of Soil-borne wheat mosaic virus (SBWMV), a close relative of Cultivars are ordered by increasing yield.…”

“…Both SBCMV and WSSMV are vectored by Polymyxa graminis (Ledingham 1939), a soil dwelling protist able to survive and remain infective in soil for over a decade (Kendall and Lommel 1988). Cultivars vary in susceptibility to each virus; some cultivars have been shown to have low virus titre in leaf material, mild or no symptoms, and little or no yield penalty when grown on virus-infected land, and therefore offer a practical method for disease management.…”

Management of plant health risks associated with processing of plant-based wastes: A review

“…Polymyxa graminis (Ledingham, 1939) and P. betae (Keskin, 1964) are examples of species which can act as vectors of significant pathogenic soil-borne viruses, for example, Soil borne cereal mosaic virus (Brakke, 1971) and BNYVV (Fujisawa and Sugimoto, 1976). Although these particular viruses are no longer of statutory concern to the UK, their vectors are still likely to be of quarantine concern in the future as resistance-breaking strains of current viruses or new viruses appear.…”

Scanning electron microscopic observation of resting spore clusters of Polymyxa spp.

“…Several researchers have carried out morphological observations of these fungi with light microscopes or transmission electron microscopes (TEM) (Barr, 1979;Barr and Asher, 1996;D'Ambra and Mutto, 1977;Keskin, 1964; Keskin and Fuchs, 1969;Langenberg and Giunchedi, 1982;Ledingham, 1939). However, scanning electron microscopy (SEM) was employed in only a few studies Marotta, 1988, 1989) due to the difficulty in preparing suitable specimens.…”