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KEY MESSAGE: oxa CMS is a new cytoplasmic male sterility type in Brassica juncea. oxa CMS is a cytoplasmic male sterility (CMS) line that has been widely used in the production and cultivation of stem mustard in the southwestern China. In this study, different CMS-type specific mitochondrial markers were used to confirm that oxa CMS is distinct from the pol CMS, ogu CMS, nap CMS, hau CMS, tour CMS, Moricandia arvensis CMS, orf220-type CMS, etc., that have been previously reported in Brassica crops. Pollen grains of the oxa CMS line are sterile with a self-fertility rate of almost 0% and the sterility strain rate and sterility degree of oxa CMS is 100% due to a specific flower structure and flowering habit. Scanning electron microscopy revealed that most pollen grains in mature anthers of the oxa CMS line are empty, flat and deflated. Semi-thin section further showed that the abortive stage of anther development in oxa CMS is initiated at the late uninucleate stage. Abnormally vacuolated microspores caused male sterility in the oxa CMS line. This cytological study combined with marker-assisted selection showed that oxa CMS is a novel CMS type in stem mustard (Brassica juncea). Interestingly, the abortive stage of oxa CMS is later than those in other CMS types reported in Brassica crops, and there is no negative effect on the oxa CMS line growth period. This study demonstrated that this novel oxa CMS has a unique flower structure with sterile pollen grains at the late uninucleate stage. Our results may help to uncover the mechanism of oxa CMS in Brassica juncea.

A new non-heading Chinese cabbage CMS line M119A was characterized and specific molecular markers were developed to classify different CMS types. One new non-heading Chinese cabbage (Brassica rapa L.) cytoplasmic male sterile (CMS) line M119A was obtained by interspecific crosses between the recently discovered hau CMS line of Brassica juncea and B. rapa. Furthermore, the line was characterized and compared with other five isonuclear-alloplasmic CMS lines. The M119A line produced six stamens without pollen and only two stamen fused together in fewer flowers. Tissue section indicated that anther abortion in M119A may have occurred during differentiation of the archesporial cells without pollen sac. All the six CMS lines were grouped into three types based on the presence of three PCR fragments of 825, 465 and 772 bp amplified with different mitochondrial genes specific primers. The 825-bp fragment was amplified both in 09-10A and H201A using the specific primer pair P-orf224-atp6, and showed 100 % identity with the mitochondrial gene of pol CMS. The 465-bp fragment was amplified in 30A and 105A using the primer pair P-orf138 and shared 100 % identity with the mitochondrial gene of ogu CMS. The 772-bp fragment was amplified in M119A and H203A using the primer pair P-orf288 and showed 100 % identity with the mitochondrial gene of hau CMS. Therefore, these markers could efficiently distinguish different types of isonuclear-alloplasmic CMS lines of non-heading Chinese cabbage, which were useful for improving the efficiency of cross-breeding and heterosis utilization in cruciferous vegetables.