SPECIFICITY OF A PROMOTER FROM THE RICE TUNGRO BACILLIFORM VIRUS FOR EXPRESSION IN PHLOEM TISSUES

The major promoter region for the transcription of the genome of rice tungro bacilliform virus (RTBV), a newly described badnavirus, has bee n identified. Fragments of the RTBV genome upstream of the site of tra nscription initiation were isolated and tested for promoter activity u sing a beta-glucuronidase reporter gene (gusA). Assays of transient gu sA expression were performed following introduction of the chimeric ge ne into protoplasts via electroporation. The chimeric RTBV-promoter. g usA gene was more active in rice protoplasts than in maize or tobacco protoplasts, but was weaker than gusA controlled by an enhanced 35S pr omoter from cauliflower mosaic virus. Analysis of gusA gene expression following introduction of chimeric reporter genes into intact leaves via micro-projectile bombardment indicated that the GUS activity is pr esent primarily in vascular tissues. Transgenic rice plants carrying t he chimeric gusA gene had GUS activity only in the phloem of the vascu lar bundles in the leaf. Tissue printing studies demonstrated that RTB V accumulates in the vascular bundles of infected rice leaves. The res ults of our study indicate that phloem-specific expression from the RT BV promoter is an intrinsic property of the viral promoter.