To expand on nickmo's answer, this from a general knowledge site:
In early Roman times, wheat was the grain of choice for the wedding ceremony, as wheat, not rice, symbolized fertility. The virginal bride carried a sheaf of wheat in her hand throughout the ceremony, or wore a garland of wheat in her hair. Instead of the bride tossing a bouquet, as is traditionally done today, wedding guests tossed grains of wheat at her, and young, single girls clambered for the grains that bounced off of the young bride, believing that these grains could ensure them a trip down the bridal path soon thereafter.
The wheat tossing custom fell by the wayside under the reign of Queen Elizabeth I of England, when the once airborne wheat instead was baked into small cakes, which the guests then crumbled and tossed over the bride's head. Even this tradition gave way to another, in which a large wheat cake was baked, then eaten, not tossed. Wedding guests, literally left empty-handed, had no recourse but to find a suitable substitute for the costly wheat cakes. They needed something to toss at the bride to reinstate themselves as active participants in the ceremony. The natural choice was none other than cheap, clean, white rice, and the tradition then born has stuck to this day