Genome-wide identification and characterization of the SBP-box gene family in Petunia.

BACKGROUND: SQUAMOSA PROMOTER BINDING PROTEIN (SBP)-box genes encode a family of plant-specific transcription factors (TFs) that play important roles in many growth and development processes including phase transition, leaf initiation, shoot and inflorescence branching, fruit development and ripening etc. The SBP-box gene family has been identified and characterized in many species, but has not been well studied in Petunia, an important ornamental genus.

RESULTS: We identified 21 putative SPL genes of Petunia axillaris and P. inflata from the reference genome of P. axillaris N and P. inflata S6, respectively, which were supported by the transcriptome data. For further confirmation, all the 21 genes were also cloned from P. hybrida line W115 (Mitchel diploid). Phylogenetic analysis based on the highly conserved SBP domains arranged PhSPLs in eight groups, analogous to those from Arabidopsis and tomato. Furthermore, the Petunia SPL genes had similar exon-intron structure and the deduced proteins contained very similar conserved motifs within the same subgroup. Out of 21 PhSPL genes, fourteen were predicted to be potential targets of PhmiR156/157, and the putative miR156/157 response elements (MREs) were located in the coding region of group IV, V, VII and VIII genes, but in the 3'-UTR regions of group VI genes. SPL genes were also identified from another two wild Petunia species, P. integrifolia and P. exserta, based on their transcriptome databases to investigate the origin of PhSPLs. Phylogenetic analysis and multiple alignments of the coding sequences of PhSPLs and their orthologs from wild species indicated that PhSPLs were originated mainly from P. axillaris. qRT-PCR analysis demonstrated differential spatiotemperal expression patterns of PhSPL genes in petunia and many were expressed predominantly in the axillary buds and/or inflorescences. In addition, overexpression of PhSPL9a and PhSPL9b in Arabidopsis suggested that these genes play a conserved role in promoting the vegetative-to-reproductive phase transition.

CONCLUSION: Petunia genome contains at least 21 SPL genes, and most of the genes are expressed in different tissues. The PhSPL genes may play conserved and diverse roles in plant growth and development, including flowering regulation, leaf initiation, axillary bud and inflorescence development. This work provides a comprehensive understanding of the SBP-box gene family in Petunia and lays a significant foundation for future studies on the function and evolution of SPL genes in petunia.