Rubber tree [Hevea brasiliensis (Willd. ex A. Juss.) Müll. Arg.] is the exclusive commercial source of natural rubber. Male-sterile (MS) germplasm has proven to be valuable for rubber tree hybrid breeding, but its male sterility mechanism remains unclear. Here, morphological observations of a new MS clone Reyan93-114 showed shriveled anthers, short filaments, and withered male flowers. Transmission electron microscopy revealed the tapetal cells of the anthers were hypertrophic and vacuolated. Microspore protoplasts reduced in size, and the microspore wall had obvious defects: the incomplete intine and the insufficiently deposited exine. Organelles had disappeared in both tapetal cells and microspores. Comparative mitochondrial transcriptome analysis identified three differentially expressed genes in the MS clones: orf126, orf51, and atp8. In particular, orf126 and orf51 were unique insertions in the MS mitochondrial genomes. The former was an unknown open reading frame, and the latter was a fusion gene that contained a portion of atp9. These two genes should be studied as candidate male sterility genes in rubber tree. These results clarified the cytological causes of male sterility and expanded to understand the relationship between mitochondrial genes and male sterility in rubber tree.

中文翻译：

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橡胶树不育雄花形态学、细胞学和线粒体转录组比较分析

橡胶树 [巴西橡胶树（Willd。ex A. Juss。）Müll。Arg.] 是天然橡胶的独家商业来源。雄性不育 (MS) 种质已被证明对橡胶树杂交育种有价值，但其雄性不育机制仍不清楚。在这里，新 MS 克隆 Reyan93-114 的形态观察显示花药枯萎、花丝短和雄花枯萎。透射电子显微镜显示花药的绒毡层细胞肥大和空泡。小孢子原生质体变小，小孢子壁存在明显缺陷：内壁不完整，外壁沉积不足。绒毡层细胞和小孢子中的细胞器都消失了。比较线粒体转录组分析确定了 MS 克隆中的三个差异表达基因：orf126、orf51，和atp8。特别是，orf126和orf51是 MS 线粒体基因组中的独特插入。前者是一个未知的开放阅读框，后者是一个融合基因，含有一部分atp9。这两个基因应作为橡胶树雄性不育候选基因进行研究。这些结果阐明了雄性不育的细胞学原因，并扩展了了解线粒体基因与橡胶树雄性不育之间的关系。