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 been identified. Fragments of the RTBV genome upstream of the site of transcription initiation were isolated and tested for promoter activity using a beta-glucuronidase receptor gene (gusA). Assays of transient gusA expression were performed following introduction of the chimeric gene into protoplasts via electroporation. The chimeric RTBV-promoter: gusA gene was more active in rice protoplasts than in maize or tobacco protoplasts, but was weaker than gusA controlled by an enhanced 35S promoter 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 present primarily in vascular tissues. Transgenic rice plants carrying the chimeric gusA gene had GUS activity only in the phloem of the vascular bundles in the leaf. Tissue printing studies demonstrated that RTBV accumulates in the vascular bundles of infected rice leaves. The results of our study indicate that phloem-specific expression from the RTBV promoter is an intrinsic property of the viral promoter.