Genetic manipulation of the genes for clonal seeds results in sterility in cotton

Heterosis is a common phenomenon in plants and has been extensively applied in crop breeding. However, the superior traits in the hybrids can only be maintained in the first generation but segregate in the following generations. Maintaining heterosis in generations has been challenging but highly desirable in crop breeding. Recent study showed that maternally produced diploid seeds could be achieved in rice by knocking out three meiosis related genes, namely REC8, PAIR1, OSD1 to create MiMe in combination with egg cell specific expression of BBM transcription factor, a technology called clonal seeds. Interestingly, there has been very limited reports indicating the feasibility of this approach in other crops.

In this study, we aimed to test whether clonal seeds could be created in cotton. We identified the homologs of the three meiosis related genes in cotton and used the multiplex CRISPR/Cas9 gene editing system to simultaneously knock out these three genes in both A and D sub-genomes. More than 50 transgenic cotton plants were generated, and fragment analysis indicated that multiple gene knockouts occurred in the transgenic plants. However, all the transgenic plants were sterile apparently due to the lack of pollen. Pollination of the flowers of the transgenic plants using the wild type pollens could not generate seeds, an indication of defects in the formation of female sexual cells in the transgenic plants. In addition, we generated transgenic cotton plants expressing the cotton BBM gene driven by the Arabidopsis egg cell specific promoter pDD45. Two transgenic plants were obtained, and both showed severely reduced fertility.

Overall, our results indicate that knockout of the clonal seeds related genes in cotton causes sterility and how to manipulate genes to create clonal seeds in cotton requires further research.