Molecular biology of the tobraviruses

The genus Tobravirus comprises of three different viruses, tobacco rattle virus (TRV), pea early-browning virus (PEBV) and pepper ringspot virus (PepRSV) (Harrison, 1973; Robinson & Harrison, 1989). The tobravirus genome is divided into two positive-sense, single-stranded RNAs, each of which is encapsidated separately into rod-shaped particles. TRV, in particular, has a widespread distribution, can infect a very large number of plant species and causes economically significant disease in potato, tobacco and ornamental bulbs. PEBV has been found in northern Europe and North Africa, where it infects primarily legumes, including pea and field bean. PepRSV, which has been described only in Brazil, causes disease in pepper, tomato and globe artichoke. Tobraviruses are unusual in being one of only two types of virus that are transmitted from plant to plant by soil-inhabiting nematodes (Taylor & Brown, 1997). There is a highly specific relationship between virus and nematode, so that particular virus isolates are transmitted only by certain vector nematode species. An additional, distinguishing feature of tobraviruses is their ability to cause two types of infection. Plants infected with a so- called NM-type virus isolate contain only the larger viral RNA (RNA1), which can multiply and spread in the complete absence of the second, smaller RNA (RNA2). As RNA2 encodes the coat protein (CP), infection with RNA1 alone does not produce virus particles. Isolates containing both viral RNAs, referred to as M-type isolates, are encapsidated. Several earlier reviews describe in detail the biology and epidemiology of the tobraviruses (Harrison & Robinson, 1978, 1986; Boulton, 1996). This paper aims to compile more recent molecular data obtained from the sequencing of various tobravirus isolates and from site-directed mutagenesis of infectious, cloned tobravirus genomes.