BOTH BIPARTITE GEMINIVIRUS MOVEMENT PROTEINS DEFINE VIRAL HOST-RANGE,BUT ONLY BL1 DETERMINES VIRAL PATHOGENICITY

Bipartite geminiviruses such as squash leaf curl virus (SqLCV) encode two movement proteins (MPs), BR1 and BL1, that are essential for virus movement and systemic infection of host plants. BR1 has been implicat ed in the host range properties of the virus, and BL1 in viral pathoge nic properties. To more precisely examine the roles of each MP, we hav e introduced missense and deletion mutations into the coding sequence of both BR1 and BL1, and examined the effects of these mutations on vi ral infectivity and the production of disease symptoms in pumpkin, squ ash, and Nicotiana benthamiana. For each MP, a range of mutant phenoty pes from partially to fully defective was observed that affected the o verall level and rate of infectivity. However, only mutations in BL1, and not BR1, affected the severity of disease symptoms, confirming our earlier finding that BL1 is responsible for the production of disease symptoms. For all mutants, the cucurbit hosts were found to be more p ermissive for viral movement than was N. benthamiana, and several muta tions in both BL1 and BR1 produced host-specific phenotypes, retaining high levels of infectivity in pumpkin and squash, but abolishing infe ctivity for N. benthamiana. Unexpectedly, functional SqLCV coat protei n (AR1) was found to specifically mask the phenotypes of certain BR1 m utations, suggesting some redundancy of function between coat protein and the BR1 MP and an interaction of AR1 with the viral movement pathw ay. AR1 and BR1 have similar nucleic acid binding affinities, suggesti ng a possible mechanism for the observed effects of the viral coat pro tein on viral movement. (C) 1995 Academic Press, Inc.