Multiplication of Rice Black-Streaked Dwarf Virus in its Plant and Insect Hosts

Abstract

In a serial transfer from insect to insect by means of injecting inoculation of plant-hopper, Laodelphax striatellus Fallen, rice black-streaked dwarf virus (RBSV) was successfully carried to the 6th group of insects. The calculated dilution of the final inoculum would be 10-17, but the dilution end-point of RBSV in viruliferous insect juices was found to be below 10-5. The serial passage provided direct evidence for multiplication of RBSV in its insect vectors. The infectivity of RBSV was detected in salivary glands, intestine, fat body, hemolymph and ovarian tubules in viruliferous insects, but not in brain, Malpighian tubules, and testis. The virus was recovered first from the intestine 10 days after acquisition feeding on diseased rice plants, from aslivary glands and hemolymph after 13 days, and from fat body after 16 days.Two kinds of spherical particles were found mixed in the virus fractions isolated from viruliferous insects. One of which, the isometric particles of 70nm in diam., was also isolated from healthy insects and seemed to be latent virus of this insect vector. The other was 60nm in diam. being morphologically identical with RBSV isolated from the diseased plants.The virus was recovered from roots as well as from leaves of the diseased rice plants. Infectivity of the expressed juice from vein portions of the diseased rice leaves was much higher than from interveinal portions. When rice plants were inoculated with this virus, the infectivity of the expressed juice of the infected plants rapidly increased within 2 weeks and reached a maximum in 22 to 30 days, and was stabilized at high level for a period from 40 to 110 days after inoculation. In contrast, the amount of virus particles isolated from infected rice plants increased gradually to reach a maximum in 63 to 90 days after inoculation. Rice and corn plants were highly susceptible to RBSV, but the virus contents, when purified from diseased rice or corn plants, were extremely low. With same purification procedures, the contents in diseased barley, wheat, and oat plants were much higher than those in diseased rice and corn plants.