GENE SILENCING FROM PLANT DNA CARRIED BY A GEMINIVIRUS

The geminivirus tomato golden mosaic virus (TGMV) replicates in nuclei and expresses genes from high copy number DNA episomes. The authors u sed TGMV as a vector to determine whether episomal DNA can cause silen cing of homologous, chromosomal genes. Two markers were used to assess silencing: (1) the sulfur allele (su) Of magnesium chelatase, an enzy me required for chlorophyll formation; and (2) the firefly luciferase gene (luc). Various portions of both marker genes were inserted into T GMV in place of the coat protein open-reading frame and the constructs were introduced into intact plants using particle bombardment. When T GMV vectors carrying fragments of su (TGMV::su) were introduced into l eaves of wild-type Nicotiana benthamiana, circular, yellow spots with an area of several hundred cells formed after 3-5 days. Systemic movem ent of TGMV::su subsequently produced variegated 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' f ragment. TGMV::su-induced silencing was propagated through tissue cult ure, along with the viral episome, but was not retained through meiosi s. Systemic downregulation of a constitutively expressed luciferase tr ansgene in plants was achieved following infection with TGMV vectors c arrying a 623 bp portion of luc in sense or anti-sense orientation. Th ese results establish that homologous DNA sequences localized in nucle ar episomes can modulate the expression of active chromosomal genes.