GENETIC-ANALYSIS OF BEET CURLY TOP VIRUS - EXAMINATION OF THE ROLES OF L2 AND L3 GENES IN VIRAL PATHOGENESIS

The monapartite DNA genome of beet curly top geminivirus (BCTV, strain Logan) contains four leftward (complementary sense) open reading fram es (ORFs) designated L1, L2, L3, and L4. We investigated the functions of the L2 and L3 ORFs by mutational analysis. We found that in Nicoti ana benthamiana and sugarbeet plants, neither a functional L2 nor a fu nctional L3 gene is required for infectivity. Double mutants were also infectious, and no evidence for a synergistic effect of these genes w as evident. However, while sugarbeet plants inoculated with L2 or L3 m utants showed symptoms that were indistinguishable from those elicited by wild type virus, mutant-inoculated N. benthamiana plants displayed a navel phenotype in which recovery of the plant from initially sever e disease symptoms was greatly enhanced. Enhanced recovery was associa ted with a large reduction in viral DNA levels. Our studies did not pr ovide evidence for functional homology between the BCTV L2 gene and it s presumed homologue (AL2) in the bipartite geminiviruses, In contrast , mutants with lesions in the L3 ORF accumulated three- to five-fold l ess DNA than wild type virus in a protoplast replication assay, consis tent with the interpretation that the BCTV L3 gene is a homologue of t he bipartite geminivirus AL3 gene which is known to function as a repl ication enhancer. Functional homology was directly confirmed in experi ments which demonstrated that the BCTV L3 gene can complement a tomato golden mosaic virus AL3 mutant, and Vice versa. (C) 1995 Academic Pre ss, Inc.