Highlights and prospects of potyvirus molecular biology.

The potyvirus group [named after its type member, potato virus Y (PVY)] is the largest of the 34 plant virus groups and families currently recognized (Ward & Shukla, 1991). It contains at least 180 definitive and possible members (or 30% of all known plant viruses) which cause significant losses in agricultural, pasture, horticultural and ornamental crops (Ward & Shukla, 1991). These viruses are unique in the diversity of inclusion bodies that are formed during the infection cycle (see Lesemann, 1988). A feature shared by all potyviruses is the induction of characteristic pinwheel or scroll-shaped inclusion bodies in the cytoplasm of the infected cells (Edwardson, 1974). These cylindrical inclusion (CI) bodies are formed by a virus-encoded protein and can be considered as the most important phenotypic criterion for assigning viruses to the potyvirus group (Milne, 1988; Shukla et al., 1989; Ward & Shukla, 1991). Many potyviruses also induce cytoplasmic amorphous inclusion bodies and some form nuclear inclusions. Virions are flexuous and rod-shaped, 680 to 900 nm long and 11 to 15 nm wide, made up of about 2000 units of a single structural protein surrounding one molecule of ssRNA of approximately 10000 nucleotides and messenger polarity (Dougherty & Carrington, 1988). Although some of these viruses are transmitted by mites, and possibly by whiteflies, the predominant transmission of potyviruses is by aphids. In addition, there are viruses that share many characteristics with the typical potyviruses but have some important peculiarities. They are transmitted by fungi, have bipartite ssRNA genomes that are encapsidated in two different rod-shaped virions (approximately 275 and 550 nm long), and are not serologically related to potyviruses, differences that would set them apart from the potyvirus group (Kashiwazaki et al., 1990). On the other hand, their cistron order, strategy of genome expression and amino acid sequence homologies as well as the formation of similar cylindrical inclusions in infected cells indicate a common ancestry and a close evolutionary relationship between these viruses [whose type member is barley yellow mosaic virus (BaYMV)] and typical potyviruses (Kashiwazaki et al., 1990). The taxonomy of the potyvirus group, which has been studied extensively, may therefore be quite complex (reviewed in Milne, 1988; Ward & Shukla, 1991 ; for a proposal on potyvirus taxonomy, see Barnett, 1991). A breakthrough in potyvirus studies was achieved in 1986 when the complete genome sequences of two members of this group, tobacco etch virus (TEV; Allison et al., 1986) and tobacco vein mottling virus (TVMV; Domier et al., 1986) were reported for the first time. Intensive research during the following years led to a greater understanding of the potyviral genome structure and expression. The complete genome sequences of two more potyviruses, PVY (Robaglia et al., 1989) and plum pox virus (PPV; Lain et al., 1989a; Maiss et al., 1989; Teycheney et al., 1989) are now available. Unsuspected relationships between potyviruses and other groups of plant and animal viruses arose from sequence comparisons (Domier et al., 1987; Lain et al., 1989b), allowing functions to be proposed for most of the potyviral gene products. However, actual knowledge about these functions is still scarce, and not all of the gene products have been identified in vivo. A second breakthrough in potyvirus research is probably near, given the recent isolation of full-length cDNA clones of three potyviruses from which infectious transcripts can be synthesized: TVMV (Domier et al., 1989), PPV (Riechmann et al., 1990) and zucchini yellow mosaic virus (Gal-On et al., 1991). The ability to generate virus infection from cloned cDNA of potyviruses opens the possibility of applying genetic engineering techniques for the study of their biology at the molecular level. Aspects of the potyvirus life cycle such as translation, replication, symptom induction, and cell-to-cell and plant-to-plant propagation are now amenable to a new experimental approach, and the functions that have been proposed for potyviral gene products can now be tested in vivo. Also, discoveries have been made working with potyviruses that will influence our knowledge of the biology of most plus-stranded RNA viruses; on the other hand, results obtained by working with other viruses are