TOMATO BUSHY STUNT VIRUS SPREAD IS REGULATED BY 2 NESTED GENES THAT FUNCTION IN CELL-TO-CELL MOVEMENT AND HOST-DEPENDENT SYSTEMIC INVASION

We have investigated the importance of two small nested genes (p19 and p22) located near the 3' end of the genome of tomato bushy stunt viru s (TBSV) for infectivity in several hosts. Our results show that both genes are dispensable for replication and transcription and that the p 19 gene encodes a soluble protein, whereas the p22 gene specifies a me mbrane-associated protein. Assays using TBSV derivatives that have the beta-glucuronidase gene substituted for the capsid protein gene demon strate that p22 is required for cell-to-cell movement in all plants te sted. Mutations inactivating p19 ameliorate the severe necrotic system ic symptoms elicited by wild-type TBSV in Nicotiana benthamiana and Ni cotiana clevelandii, but pig does not obviously affect movement in the se hosts. However, in some local lesion hosts p19 influences the lesio n diameter, which suggests that it has an auxiliary host-dependent rol e in movement. This notion is supported by the observation that p19 is required for long-distance spread of TBSV in spinach and for systemic infection of pepper plants. Thus, movement of TBSV is regulated by tw o nested genes; p22 governs cell-to-cell movement and p19 has a host-s pecific role in systemic invasion. (C) 1995 Academic Press, Inc.