title: Gene silencing from plant DNA carried by a Geminivirus.
abstract: The geminivirus tomato golden mosaic virus (TGMV) replicates in nuclei and expresses genes from high copy number DNA episomes. The authors used TGMV as a vector to determine whether episomal DNA can cause silencing of homologous, chromosomal genes. Two markers were used to asses silencing: (1) the sulfur allele (su) of magnesium chelatase, an enzyme required for chlorophyll formation; and (2) the firefly luciferase gene (luc). Various portions of both marker genes were inserted into TGMV in place of the coat protein open-reading frame and the constructs were introduced into intact plants using particle bombardment. When TGMV vectors carrying fragments of su (TGMV::su) were introduced into leaves of wild type Nicotiana benthamiana, circular, yellow spots with an area of several hundred cells formed after 3-5 days. Systemic movement of TGMV::su subsequently produced varigated leaf and stem tissue. Fragments that caused silencing included a 786 bp 5' fragment of the 1392 bp su cDNA in sense and anti-sense orientation, and a 403 bp 3' fragment. TGMV::su-induced silencing was propogated through tissue culture, along with the viral episome, but was not retained through meiosis. Systemic downregulation of a constitutively expresse luciferase transgene in plants was achieved following infection with TGMV vectors carrying a 623 bp portion of luc in sense or anti-sense orientation. These results establish that homologous DNA sequences localized in nuclear episomes can modulate the expression of active chromosomal genes.
authors.affiliation: Department of Botany, North Carolina State University, Raleigh, NC 27695, USA.
authors.forename: S
authors.initials: S
authors.lastname: Kjemtrup
authors.forename: K S
authors.initials: KS
authors.lastname: Sampson
authors.forename: C G
authors.initials: CG
authors.lastname: Peele
authors.forename: L V
authors.initials: LV
authors.lastname: Nguyen
authors.forename: M A
authors.initials: MA
authors.lastname: Conkling
authors.forename: W F
authors.initials: WF
authors.lastname: Thompson
authors.forename: D
authors.initials: D
authors.lastname: Robertson