Two Types of Virus-like Particles Isolated from Downy Mildew Diseased Rice Plants

Abstract

Two morphologically distinct types of small, spherical virus-like particles (VLPs), tentatively designated as type A and B, were purified from rice plants infected by downy mildew in field. Type A shows a diameter of 30nm with phosphotungstic acid (PTA) staining or 32nm with uranyl acetate (UA) staining, bearing spikes of 4nm on its periphery. Type B has a diameter of 32nm with PTA staining or 35nm with UA staining and no spikes. Purified preparations of type A and B showed an ultraviolet absorption spectrum with maximum at 260nm and minimum at 244nm, indicating their nucleoproteinaceous nature. The A₂₆₀/A₂₈₀ ratios of type A and B were 1.44 and 1.60, respectively. Antisera prepared against type A and B individually did not react with heterologous antigens. Out of 46 samples collected from the fields throughout Miyagi Prefecture, 11 samples contained both types, 9 samples contained only type A, 21 samples contained only type B and 5 samples contained none. Healthy rice plants collected near the two diseased fields contained neither type A nor B.