Stringently and developmentally regulated levels of a cytoplasmic double-stranded RNA and its high-efficiency transmission via egg and pollen in rice.

A very restricted amount of high-molecular-weight double-stranded RNA (dsRNA) has been found in healthy japonica rice plants. We discriminated dsRNA-carrying rice plants from noncarriers. The endogenous dsRNA was localized in the cytoplasm (about 100 copies per cell) and was transmissible to progeny plants by mating. In crosses between carriers and noncarriers, the RNA was transmitted efficiently to F1 plants via both egg and pollen. The rice dsRNA was maintained at an almost constant level by host plant cells from generation to generation. The high-efficiency transmission of the endogenous dsRNA to progeny plants appears to depend on the autonomously controlled replication of the dsRNA localized in cytoplasmic vesicles. However, an increase in copy number (about 10-fold) of the dsRNA was observed during the suspension culture of host cells. The number of copies of dsRNA returned to the original low value in regenerated plants, suggesting that the copy number is stringently and developmentally regulated in rice cells.