Genetic Determinants of Symptoms on Viral DNA Satellites

Begomovirus-DNA-beta disease complexes induce different symptom phenotypes in their hosts. To investigate the genetic determinants of the phenotypic differences, Nicotiana spp. and tomato plants were inoculated with infectious clones of Tobacco curly shoot virus (TbCSV)/TbCSV DNA-beta (TbCSB) and Tomato yellow leaf curl China virus (TYLCCNV)/TYLCCNV DNA-beta (TYLCCNB) pseudorecombinants and showed that TYLCCNB induced characteristic vein-thickening and enation symptoms, while TbCSB only slightly exacerbated the leaf-curling symptoms, regardless of the helper virus being used. The roles of DNA-beta-encoded beta C1 and a 430-nucleotide fragment containing the A-rich region and the putative beta C1 promoter region of the beta C1 gene (referred to as AP) in symptom development were further investigated by constructing hybrid satellites in which the beta C1 coding region or AP was exchanged between the two satellite molecules. A TYLCCNB hybrid with TbCSB beta C1 lost the ability to elicit the vein-thickening and enation phenotypes. TbCSB hybrids containing the TYLCCNB beta C1 or AP fragment failed to induce the characteristic vein thickening and enations. A TYLCCNB hybrid having the TbCSB AP fragment produced the enations, but the number of enations was less and their sizes were reduced. Differently from the phloem-specific pattern of the TYLCCNB promoter, a full-length fragment upstream of the TbCSB beta C1 gene confers a constitutive beta-glucuronidase expression pattern in transgenic tobacco plants. The above results indicate that the DNA-beta-encoded beta C1 protein is the symptom determinant, but the promoter of the beta C1 gene has influence on symptom production.