Characterization and genetic mapping of a classic-abortive-type recessive genic-male-sterile mutant ap90 in rice (Oryza sativa L.).

In this study, we obtained a stable male sterile mutant abortive pollen 90 (ap90) from the mutant library of Zhonghui 8015 (wild-type, WT), an indica restorer line, induced by Ethyl Methy Sulfone (EMS). Compared to the WT, the ap90 mutant displayed no significant differences in plant height, plant type, tiller number, heading date and other agronomic traits, but the anthers were thinner, light creamy yellow and pollen grains were completely abortive. Semi-thin sections observation of anther development at different stages showed that the ap90 mutant carried an abnormal development process of anther wall cells. Namely, the tapetum cell degradation was obviously abnormal, the microspore cells could not form a normal pollen wall structure during mitosis and the starch filling process was blocked which eventually resulted in the degradation of microspores into threadlets and failure of anther dehiscence. Scanning electron microscopic (SEM) observations of the anther surface and pollen exine suggested that the anther epidermis of mutant ap90 were shrunk and covered by more compactly arranged cuticles. The shape of Ubisch distributed on the inner surface of anther locule were irregular, closely arranged and disordered. The pollen grains were shriveled and the sporopollenin on the pollen exine were abnormally arranged. Genetic analysis showed that the ap90 phenotype was controlled by a pair of recessive nuclear genes. Gene preliminary mapping located the mutation site into a 491.73 kb interval between RM21421 and RM21435 on the long arm of rice chromosome 7. Further Mut-Map sequencing analysis confirmed that there was a 37 bp deletion and a following single base substitution in the second exon region of LOC_Os07g22850 in the ap90 mutant, which resulted in the shifted coding sequence, prematurely terminated transcription and translation, leading to the entire abortive pollen and sterile spikelet in the ap90 mutant. Expression pattern analysis results demonstrated that the OSAP90 gene was specifically expressed in anthers and the OsAP90 protein was mainly located in the endoplasmic reticulum (ER). The qPCR results suggested that the relative expression level of many male sterility-related genes in the ap90 mutant was affected by the mutation site, which further proved that 0sAP90 played an important role during the formation of Ubisch and pollen wall in rice anther development. [ABSTRACT FROM AUTHOR]