Studies on the life-cycle of Polymyxa betae in sugar beet roots

Abstract

Light and electron microscopy was used to study the development of Polymyxa betae , the plasmodiophoromycete vector of beet necrotic yellow vein virus, in Beta vulgaris seedling roots. A triple staining technique (methylene blue — azure II — basic fuchsin) discriminated host-plant and fungal material and revealed changes in fungal constituents during development. Initially, infection was restricted to root epidermal cells. In later samples all stages in the fungal life-cycle were found throughout the cortex, although P. betae was never observed in the root endodermis or stele. Granular pink-stained material, associated with vacuoles, increased to become an important constituent of plasmodia as they matured and differentiated into zoosporangia, but decreased as zoospores developed. In electron micrographs the pink colour corresponded to an unidentified material of complex structure which may be of host origin or a novel storage compound. Electron micrographs also revealed that cysts are differentiated within the cystosorus by the formation of a membranous bi-layer. This contrasts with zoospores, which arise through the division of the zoosporangial cytoplasm by cleavage vacuoles. Peripheral vesicles, characteristic of some other plasmodiophoromycetes, were observed in mature zoospores.