Transgenic tobacco plants expressing the geminivirus BL1 protein exhibit symptoms of viral disease.

Bipartite geminiviruses, such as squash leaf curl virus (SqLCV), encode two movement proteins (MPs), BR1 and BL1, that are essential for viral movement in and subsequent infection of the host plant. To elucidate the biochemical functions of these MPs and define their respective contributions to viral infection, we have generated transgenic Nicotiana benthamiana plants expressing SqLCV BR1 and BL1. Transgenic plants expressing BR1 or a truncated BL1 were phenotypically indistinguishable from wild-type N. benthamiana. In contrast, transgenic plants expressing full-length BL1, alone or in combination with BR1, were strikingly abnormal both in their growth properties and phenotypic appearance, with leaves that were mosaic and curled under, thus mimicking typical SqLCV disease symptoms in this host. BL1 was localized to the cell wall and plasma membrane fractions, whereas BR1 was predominantly in the microsomal membrane fraction. These findings demonstrate that expression of BL1 in transgenic plants is sufficient to produce viral disease symptoms, and they further suggest that BL1 and BR1 carry out distinct and independent functions in viral movement.