Phenotypic characterization of a female sterile mutant in rice

A female sterile mutant, derived from a spontaneous mutation, was first discovered in rice (Oryza sativa L. ssp. indica) restorer line 202R. With normal flowering, the mutant exhibits an extremely low seed-setting rate. When the mutant is crossed as a pollen donor, the seeds set normally; whereas when it is used as a pollen receiver, no seeds are obtained even with mixed pollen grains of different varieties sprinkled over the stigmas. The floret of the mutant, consisting of six stamens and one pistil, looks the same as that of the wild type in the male-female organs, except that less than 10% of the mutant florets have three stigmas on the ovary. Although the mutant has a low seed-setting rate, its pollen fertility is approximately 87.1%, which is equal to that of the wild type. In addition, more than 90% of the mature embryo sacs of the mutant have complete inner structures. At every stage after pollination, the sperm, embryo, and endosperm are not found in the mutant embryo sac, whereas the disintegration of the egg cell that does not accomplish fertilization is visible. Through observations with a fluorescence microscope, we have found that the pollen grains germinate normally, whereas the pollen tube abnormally elongates in the style-transmitting tissue. The mutant pollen tubes display various defects in the style, such as slower elongation, conversed elongation, distorted elongation, swollen tips, or branched tips. As a result, the growth of the pollen tubes ceases in the style, and, therefore, the pollen tubes cannot reach the embryo sac and the process of double fertilization is blocked. Based on these observations, we conclude that this mutant, designated as fs-202R, is a novel type of female sterile mutation in rice, which causes the arrest of the elongation of the pollen tube.