Abstract
Longan (Dimocarpus longan Lour.) is an important commercial fruit tree in southern China. The embryogenesis of longan affects the quality and yield of fruit. A large number of alternative splicing events occurs during somatic embryogenesis (SE), which is regulated by serine/arginine-rich (SR) proteins. However, the functions of SR proteins in longan are poorly understood. In this study, 21 Dlo-SR gene family members belonging to six subfamilies were identified, among which Dlo-RSZ20a, Dlo-SR30, Dlo-SR17, Dlo-SR53 and Dlo-SR32 were localized in the nucleus, Dlo-RSZ20b, Dlo-RSZ20c, Dlo-RSZ20d, Dlo-SC18, Dlo-RS2Z29, Dlo-SCL41, and Dlo-SR33 were localized in chloroplasts, and Dlo-RS43, Dlo-SC33, Dlo-SC37, Dlo-RS2Z33, Dlo-RS2Z16, Dlo-RS2Z24, Dlo-SCL43, Dlo-SR112, and Dlo-SR59 were localized in the nucleus and chloroplasts. The Dlo-SR genes exhibited differential expression patterns in different tissues of longan. The transcript levels of Dlo-RSZ20a, Dlo-SC18, Dlo-RS2Z29, DLo-SR59, Dlo-SR53, and Dlo-SR17 were low in all analyzed tissues, whereas Dlo-RS43, Dlo-RS2Z16, Dlo-RS2Z24, and Dlo-SR30 were highly expressed in all tissues. To clarify their function during SE, the transcript levels of Dlo-SR genes were analyzed at different four stages of SE, comprising non-embryonic callus (NEC), friable-embryogenic callus (EC), incomplete compact pro-embryogenic culture (ICpEC) and globular embryo (GE). Interestingly, the transcript levels of Dlo-RS2Z29 and Dlo-SR112 were increased in embryogenic cells compared with the NEC stage, whereas transcript levels of Dlo-RSZ20a, Dlo-RS43, Dlo-SC37, and Dlo-RS2Z16 were especially increased at the GE stage compared with the other stages. Alternative splicing events of Dlo-SR mRNA precursors (pre-mRNAs) was detected during SE, with totals of 41, 29, 35, and 44 events detected during NEC, EC, ICpEC, and GE respectively. Protein-protein interaction analysis showed that SR proteins were capable of interaction with each other. The results indicate that the alternative splicing of Dlo-SR pre-mRNAs occurs during SE and that Dlo-SR proteins may interact to regulate embryogenesis of longan.