Spatial and temporal expression of a rice prolamin gene RP5 promoter intransgenic tobacco and rice

Alcohol-soluble prolamin is one of the major storage proteins in the rice seed. To study the spatial and temporal expression of a prolamin gene RP5, we introduced the RP5/β-D-glucuronidase (GUS) chimeric gene into tobacco (Nicotiana tabacum) and rice (Oryza sativa) via Agrobacterium tumefaciens-mediated transformation. Expression of the GUS reporter gene in transgenic tobacco was not restricted to the endosperm of seeds, but was also seen in the vascular tissue of stems. In transgenic rice, a high level of GUS activity was detected exclusively in the endosperm cells, with stronger expression in the aleurone layer and the subaleurone cells of the endosperm. Temporal expression patterns of the transgene in the two host plants were quite similar, the levels of GUS activity in transgenic seeds reaching the maximum 12-16 days after flowering (DAF) (mid-maturation). Northern-blot analysis showed that the GUS and endogenous RP5 transcripts accumulated nearly in parallel during rice seed development. These results suggest that the RP5 promoter is sufficient to confer the endosperm-specific expression and the temporal control in the homologous rice host, but not in the heterologous host, tobacco. Furthermore, the transgenic rice lines could transmit the RP5/GUS chimeric gene to R2 seeds without gene silencing, and their expression levels remained as high as the primary transformants, indicating that the RP5 promoter is potentially useful in molecular breeding to express a gene of interest at high levels in rice endosperm.