Abstract: Transgene-mediated gene silencing is a useful technique for elucidating biological function of genes and modifying crops by genetic engineering. In plants，amiRNAs make an effective tool for specific gene silencing. We designed the amiRchs1 for silencing the petunia chalcone synthase genes according to the amiRNA design principles refined from the structure characters of plant microRNA. We used the precursor for Arabidopsis miR319a as backbone for the amiRchs1 expression under the control of 35S promoter，exchanging the natural miR319a sequence with the sequence of amiRchs1 by overlapping PCR. The transgenic petunia plants expressing the amiRchs1 precursor produced flowers with varying amounts of diffused white dots or irregular white patches，and nearly white flowers. Both the contents of anthocyanin and CHS mRNA in corollas of the transgenic plants were reduced significantly. These results showed that the precursor for Arabidopsis miR319a can be used to express amiRNA in petunia，and structure genes can be knocked down effectively by the amiRNA technology.

Key words: petunia, gene silencing, amiRNA, chalcone synthase gene