Infection of Plants by Tobacco Mosaic Virus

Abstract

Plant virus diseases cause an estimated $15 billion crop loss worldwide (Matthews 1993). Plant viruses may infect organisms as diverse as fungi and maple trees. They exist in several different sizes and shapes, contain single or double-stranded RNA or DNA, and are transmitted mechanically and/or by an insect or arthropod (Figure 1). Disease symptoms arising from virus infection may be invisible (latent), small localized lesions (Figure 2A), or quite dramatic, such as the mosaic disease of tobacco (Figure 2B). While plant viruses are often economically destructive, they are otherwise harmless to humans and animals. In fact, information gleaned from the study of tobacco mosaic virus (TMV) has led to landmark discoveries in the field of science. Although many scientists and educators are aware of the events surrounding the discovery of the ubiquitous TMV, few are aware that the study of the biological and physicochemical properties of TMV provided a strong foundation in the basic sciences for later development of the field of molecular biology (Fraenkel-Conrat 1986). Tobacco mosaic virus, the type member (standard for comparison) of the tobamovirus group, is one of the world's most thoroughly studied viruses. The rod-shaped TMV particle (Figure 3) is 300 nanometers in length (0.000012 inch). Nearly 1700 particles could fit end-to-end to span the diameter of the period at the end of this sentence. It is composed of subunits of one coat protein that protectively en-