Comparison of promoters in transgenic rice

Reports of the use of rice storage protein gene promoters to express transgenes in rice grain have demonstrated that rice grain can be used as a production platform for end-use quality or seed-based edible vaccines. The generation of transgenic rice with multitraits (gene stacking), which requires the use of multiple transgenes under the control of different promoters, necessitates the use of promoters from rice and other cereals, as this is highly advantageous in reducing homology-based transcriptional gene silencing. Using the green fluorescent protein gene (gfp) as a reporter gene and a transgenic rice platform, promoters of storage protein and non-storage protein genes from barley, wheat and rice were compared with regard to their spatial and temporal control of expression. Storage protein promoters from barley (549-bp B-hordein and 433-bp D-hordein) and wheat (425-bp high-molecular-weight glutenin) directed the expression of green fluorescent protein (GFP) in endosperm but not embryo; however, expression was leaky, as it was also observed in seed maternal tissues, leaf and root tissues. As expected, the rice promoters (1350-bp alpha-glutelin B-1 and 1007-bp alpha-globulin) directed the endosperm-specific expression of GFP in transgenic rice. Our results indicate that seed-specific promoters from barley and wheat, although containing endosperm and GCN4 motifs, which are important for endosperm-specific expression in rice, may not be spatially regulated in the same manner as they are in their native species. The analysis of GFP expression under the control of various promoters in rice grain indicates that promoters from other cereals can drive high levels of endosperm-specific expression in rice, but their utility for seed-specific expression may depend on their tissue specificity.