A symptom variant of beet curly top geminivirus produced by mutation of open reading frame C4

Two beet curly top virus (BCTV) mutants have been constructed in vitro that contain G-to-T transversions either at nucleotide 2682 or at nucleotide 2802 within the overlapping open reading frames (ORFs) C1 and C4. The mutations introduce termination codons in ORF C4 without affecting the amino acids encoded by ORF C1. When agroinoculated into Nicotiana benthamiana the mutants caused stunting and yellowing of the plant and downward leaf curl but not the vein swelling and upward leaf curl symptoms that are characteristic of wild-type BCTV infection in this host. Levels of viral single- and double-stranded DNA forms were similar in mutant and wild-type infections. Symptoms induced by one such mutant in Nicotiana clevelandii and Datura stramonium were less severe than those in wild-type infections and were again qualitatively distinct. The mutants caused symptomless infections in Beta vulgaris, contrasting with stunting, severe leaf curl, and vein swelling symptoms associated with wild-type infection of this host. The levels of mutant DNA in newly expanding asymptomatic leaves frequently reached those of wild-type virus in leaves showing severe symptoms. The results suggest that ORF C4 encodes a protein that is a major determinant of pathogenesis that might affect the hyperplastic response of the host to BCTV infection.