Comprehensive analysis of Ogura cytoplasmic male sterility-related genes in turnip (Brassica rapa ssp. rapifera) using RNA sequencing analysis and bioinformatics

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.

Ogura型细胞质雄性不育（Ogura-CMS）已广泛应用于十字花科蔬菜的杂交育种产业。芜菁（Brassica rapa ssp. rapifera）是中国重要的地方十字花科蔬菜之一，以扁盘状肉质根为食用器官进行栽培。本研究对芜菁的Ogura-CMS系‘BY10-2A’及其保持系可育株（MF）‘BY10-2B’的形态学特征开展了系统分析。结果显示，Ogura-CMS芜菁的肉质根尺寸偏小，且在小孢子母细胞向四分体的发育过渡阶段，其小孢子发育与绒毡层降解过程均存在显著缺陷。雄配子发生过程中出现的异常小孢子生成与绒毡层提前降解，导致花丝缩短、花药发白萎蔫，最终使该Ogura-CMS系完全丧失雄性育性。此外，本研究通过对Ogura-CMS系与MF系的花序进行转录组测序分析，探究了芜菁中Ogura-CMS的调控机制。通过从头转录组组装共得到84132个单基因簇（unigenes），其中相较于MF系，Ogura-CMS系中共鉴定出5117个差异表达基因（DEGs），包括1339个上调基因与3778个下调基因。在该差异表达基因集合中，我们发现了多个已知功能的花药发育与小孢子形成相关基因，特别是调控绒毡层程序性细胞死亡（PCD）以及参与花粉壁形成的相关基因。此外，通过基因本体（GO）分析，我们预测有185个新基因参与雄性器官发育，其中26个差异表达基因呈现出基因型特异性表达模式。本研究为解析芜菁的花药发育机制与细胞质雄性不育调控机制提供了全面的理论基础。