A promoter derived from taro bacilliform badnavirus drives strong expression in transgenic banana and tobacco plants

Taro bacilliform virus (TaBV) is a pararetrovirus of the genus Badnavirus which infects the monocotyledonous plant, taro (Colocasia esculenta). A region of the TaBV genome spanning nucleotides 6281 to 12 (T1200), including the 3' end of open reading frame 3 (ORF 3) and the intergenic region to the end of the tRNAmet-binding site, was tested for promoter activity along with four different 5' deletion fragments (T600, T500, T250 and T100). In transient assays, only the T1200, T600, T500 fragments were shown to have promoter activity in taro leaf, banana suspension cells and tobacco callus. When these three promoters were evaluated in stably transformed, in vitro-grown transgenic banana and tobacco plants, all were found to drive near constitutive expression of either the green fluorescent protein (GFP) or β-glucuronidase (GUS) reporter gene (uidA) in the stem (or pseudostem), leaves and roots, with strongest expression observed in the vascular tissue. In transgenic banana leaves, the T600 promoter directed four-fold greater GUS activity than that of the T1200, T500 and the maize polyubiquitin-1 promoters (Ubi-1). In transgenic tobacco leaves, the levels of GUS expression directed by the three promoters was between four and ten-fold lower than that of the double Cauliflower mosaic virus (CaMV) 35S (D35S) promoter. These results indicate that the TaBV-derived promoters may be useful for the high-level constitutive expression of transgenes in either monocotyledonous or dicotyledonous species.