Gene expression profiles and metabolic pathways responsible for male sterility in cybrid pummelo

Abnormal expression of genes regulating anther and pollen development and insufficient accumulation of male sterility (MS)- related metabolites lead to MS in cybrid pummelo Male sterility (MS) is a major cause of seedlessness in citrus, which is an important trait for fresh fruit. Understanding the mechanism of MS is important for breeding seedless citrus cultivars. In this study, we dissected the transcriptional, metabolic and physiological mechanisms of MS in somatic cybrid of pummelo (G1 + HBP). G1 + HBP exhibited severe male sterility, manifesting as retarded anther differentiation, abnormal anther wall development (especially tapetum and endothecium), and deficient pollen wall formation. In the anthers of G1 + HBP, the expression of genes regulating anther differentiation and tapetum development was abnormal, and the expression of genes regulating endothecium secondary lignification thickening and pollen wall formation was down-regulated. The transcription of genes involved in MS-related biological processes, such as jasmonic acid (JA) signaling pathway, primary metabolism, flavonoid metabolism, and programmed cell death, was altered in G1 + HBP anthers, and the accumulation of MS-associated metabolites, including fatty acids, amino acids, sugars, ATP, flavonols and reactive oxygen species (ROS), was down-regulated in G1 + HBP anthers. In summary, abnormal expression of key genes regulating anther and pollen development, altered transcription of key genes involved in MS-related metabolic pathways, and insufficient accumulation of MS-related metabolites together lead to MS in G1 + HBP. The critical genes and the metabolism pathways identified herein provide new insights into the formation mechanism of MS in citrus and candidate genes for breeding seedless citrus.