Mitotic pollen abnormalities are linked to Ogura cytoplasmic male sterility in Chinese cabbage(Brassica rapa L. ssp. pekinensis)

Ogura cytoplasmic male sterility(Ogura CMS) was first identified in wild radish(Raphanus sativus) and resulted in complete pollen abortion. However, the molecular mechanism of Ogura CMS in Chinese cabbage remains unclear.A cytological analysis confirmed nuclear degradation during the late uninucleate stage of pollen development, which diminished by the tricellular stage. Concurrently, tapetal cells exhibited abnormal enlargement and vacuolation starting from the tetrad stage. Serious developmental defects were observed in the pollen wall. During early pollen development, genes associated with cytochrome c and programmed cell death(PCD) were upregulated in the Ogura CMS line, while genes involved in pollen wall mitosis were downregulated. Conversely, at the late stage of pollen development, peroxisome and autophagy-related genes in the Ogura CMS line were upregulated. The mitochondrial orf138 gene mutation triggered the PCD process in tapetal cells, leading to their abnormal enlargement and the degradation of their contents, eventually resulting in vacuolation at the tricellular stage. These tapetal defects hindered the provision of adequate sporopollenin and nutrients to the microspores, consequently leading to abnormal pollen wall development and abnormal mitosis in the microspores. Ultimately, nuclear dispersion commenced during the late uninucleate stage, and autophagy occurred in the late stage of pollen development. Consequently, the plant could not produce functional pollen, resulting in male sterility in Chinese cabbage. Studies of Ogura CMS can promote the production and application of male sterile materials and enrich male sterile resources, which is of great significance for hybrid breeding.