Abstract
Ogura-type cytoplasmic male sterility (Ogura-CMS) has been widely used in the hybrid breeding industry for cruciferous vegetables. Turnip (Brassica rapa ssp. rapifera) is one of the most important local cruciferous vegetables in China, cultivated for its fleshy root as a flat disc. Here, morphological characteristics of an Ogura-CMS line ‘BY10-2A’ and its maintainer fertile (MF) line ‘BY10-2B’ of turnip were investigated. Ogura-CMS turnip showed a reduction in the size of the fleshy root, and had distinct defects in microspore development and tapetum degeneration during the transition from microspore mother cells to tetrads. Defective microspore production and premature tapetum degeneration during microgametogenesis resulted in short filaments and withered white anthers, leading to complete male sterility of the Ogura-CMS line. Additionally, the mechanism regulating Ogura-CMS in turnip was investigated using inflorescence transcriptome analyses of the Ogura-CMS and MF lines. The de novo assembly resulted in a total of 84,132 unigenes. Among them, 5,117 differentially expressed genes (DEGs) were identified, including 1,339 up- and 3,778 down-regulated genes in the Ogura-CMS line compared to the MF line. A number of functionally known members involved in anther development and microspore formation were addressed in our DEG pool, particularly genes regulating tapetum programmed cell death (PCD), and associated with pollen wall formation. Additionally, 185 novel genes were proposed to function in male organ development based on GO analyses, of which 26 DEGs were genotype-specifically expressed. Our research provides a comprehensive foundation for understanding anther development and the CMS mechanism in turnip.
Highlights
As an important and valuable resource, male-sterile varieties are extensively exploited in crop hybrid breeding
Despite 10 generations of back-crossings, Ogura-Cytoplasmic male-sterility (CMS) plants showed a reduction in fleshy root size compared with its maintainer fertile (MF) line (Fig 1A–1C)
During the reproductive growth phase, the Ogura-CMS line was distinguishable from the MF line by its short filaments and withered white anthers (Fig 1D and 1E)
Summary
Plant materials and growth conditionsPreviously, the Ogura-CMS line ‘BY10-2A’ of B. rapa ssp. rapifera was developed by inter-specific hybridization between B. rapa ssp. chinensis as the Ogura-CMS cytoplasm donor and fertile B. rapa L. ssp. rapifera, followed by 10 recurrent generations of back-crossing. The Ogura-CMS line ‘BY10-2A’ of B. rapa ssp. Rapifera was developed by inter-specific hybridization between B. rapa ssp. Chinensis as the Ogura-CMS cytoplasm donor and fertile B. rapa L. ssp. Rapifera, followed by 10 recurrent generations of back-crossing. CMS line and its maintainer fertile (MF) line ‘BY10-2B’ were cultivated in the experimental farm of Wenzhou Vocational College of Science and Technology, Wenzhou, Zhejiang, China. Plant morphological analysis and floret structure observation. Plants were observed and photographed at 32, 48, 110, and 180 days after germination. The length and diameter of fleshy roots were measured at 110 and 180 days after germination. A week after the first anthesis, florets of both the Ogura-CMS and MF lines were collected. The floret structures were observed under a Leica MZ16FA stereoscopic microscope (Leica Microsystems, Wetzlar, Germany)
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