Discovery of Male Sterility and Molecular Characterization in Yellow Mustard (Sinapis alba)

A male sterile (ms) plant Y145-13-11 was discovered in the inbred progeny produced via bud-pollination of open-pollinated plants of the accession SRS 2765 in yellow mustard. The male sterility of Y145-13-11 is stable in the field and greenhouse. Y145-13-11 was crossed with 10 different inbred lines to produce F1 seeds with the aim to identify the restorer (R) and maintainer (B) lines. The male fertility of at least 10 F1 hybrid plants was studied for each cross. All of the tested lines functioned as a restorer line for Y145-13-11.
Semi-thin section analysis indicated that the microspore of Y145-13-11 had vacuolated tapetal cells and was completely lack of exine, which is in contrast to that of the fertile plants with normal tapetum and exine. Transmission electron microscopy studies revealed that the tetrads of Y145-13-11 had abnormal primexine deposition, suggesting the occurrence of abortion at the tetrads stage. Scanning electron microscopy indicated that Y145-13-11 exhibited severe pollen wall defection due to the lack of sporopollenin.
To reveal the molecular mechanism underlying the male sterility of Y145-13-11, RNA-Seq analysis was performed to profile the transcriptome of the flower buds of the two male sterile plants Y145-13-11-1 and Y145-13-11-2, and the two male fertile plants (Y145-13-11 × Y3709)-1 and (Y145-13-11 × Y3709)-2. The flower buds were classified into two different stages: 1) Stage 1 (before/at meiosis): bud length<1mm, and 2) Stage 2 (tetrad stage): 1mm<bud length<2mm. cDNA libraries at each stage for each plant were generated. In total, 385 and 352 up-regulated genes at stage 1 and stage 2, respectively, were found in the two fertile plants (Y145-13-11 × Y3709)-1 and (Y145-13-11 × Y3709)-2. Interestingly, of the 352 up-regulated genes at stage 2, four pollen sporopollenin synthesis genes, CYP704B1, ACOS2, PKSA and TKPR2, exhibited down-regulated expression in the male sterile plants Y145-13-11-1 and Y145-13-11-2, which were further confirmed by Real-time PCR. These findings suggested that the genes involved in the biosynthesis of sporopollenin was blocked, thus leading to the formation of defect pollen exine in Y145-13-11. Further cloning and validation of the genes controlling male sterility of Y145-13-11 is underway.