Abstract：Abstract Expansin (EXP) is a class of proteins that causes cell wall relaxation in plants, which play an important role in petal cell expansion during flower opening. In order to explore the expression characteristics of EXP genes in Osmanthus fragrans, 12 unigene with high homology to plant EXP genes were identified from the transcriptome database of floral buds of O. fragrans in this study, and their cDNA sequences were cloned from the petals of the full flowering stage of O. fragrans. According to the sequence alignment in the NCBI database, 12 unigene sequences were named OfEXPA1~OfEXPA8, OfEXPB1~OfEXPB2, OfEXLA1, and OfEXLB1(GenBank No.: KY968700~KY968711), respectively. BLASTX search and comparison with ORF Finder tool analysis revealed that except OfEXPA1, OfEXPA5 and OfEXLB1 genes, whose 5' cDNA sequences were incomplete, the other 9 EXP genes had the full-length cDNA sequences. Sequence analysis showed that the deduced amino acid residues of 9 genes with full-length cDNA sequences were ranging from 240~274, the predicted molecular weights were ranging from 26.06 to 29.7 kD, and pI were ranging from 4.77 to 9.74. The deduced amino acid sequences of the OfEXPA1~OfEXPA8 and OfEXPB1~OfEXPB2 included the typical structural characteristics of all expansins, with eight typical Cys residues in the N-termini of the proteins, four Trp residues near the C-termini and most of the EXPA and EXPB proteins contained histidine domain motif sequence HFD. And OfEXLA1 had the characteristic motif of CDQC, and the OfEXLB1 motif had not been reported. The results of qRT-PCR analysis found that all EXP genes were expressed in different flower bud developmental stages of O. fragrans ‘Yanhong Gui’. The overall expression pattern of OfEXPA1, OfEXPA3, OfEXPA5, OfEXPA6, OfEXPA7, OfEXPA8, OfEXPB1, OfEXPB2 and OfEXLB1 was similar, the expression in the former stage of flower bud development (B1-B6) was significantly higher than that of petal expansion stages (B7-FF), and the expression decreased gradually in these stages; While OfEXPA2, OfEXPA4 and OfEXLA1 were lowly expressed in the former stages, but rapidly increased in the obvious expansion period of petals (B6), and presented high expression in petal expansion stages. The phylogenetic tree analysis showed that OfEXPA4 and OfEXPA2 had a close relationship with GgEXPA1 in gladiolus (Gladiolus grandiflorus) and RhEXPA1 in rose (Rosa chinensis), respectively, and the expression patterns of OfEXPA2 and OfEXPA4 were similar to those of GgEXPA1 and RhEXPA1 genes, suggesting that these two genes were the key members of expansin gene family related to the process of flower opening of O. fragrans; According to the characteristics of OfEXLA1 expression, it was speculated that it was also involved in the regulation of flower opening. The results provide a theoretical basis for the comprehensive understanding of the mechanism of flower opening and the function of EXP genes.
