Genome-wide investigation of SQUAMOSA PROMOTER BINDING PROTEIN-LIKE genes in Liriodendron and functional characterization of LcSPL2

Premise The plant-specific SQUAMOSA promoter-binding protein-like (SPL) transcription factors play a pivotal role in various developmental processes, including leaf morphogenesis and vegetative-to-reproductive phase transition. Liriodendron chinense and Liriodendron tulipifera are widely used in landscaping due to their tulip-like flowers and peculiar leaves. However, the SPL gene family in Liriodendron has not been identified and systematically characterized. Methods We systematically identified and characterized the SPLs family members in Liriodendron, including phylogeny, gene structure, syntenic analyses. Subsequently, we quantified the expression patterns of LcSPLs across various tissue sites through RT-qPCR assays, hence identified the target gene, LcSPL2. Finally, we characterized the functions of LcSPL2 via ectopic transformation. Results Altogether, 17 LcSPL and 18 LtSPL genes were genome-widely identified in L. chinense and L. tulipifera respectively. All the 35 SPLs were grouped into nine clades. Both species had three SPL gene pairs arising from segmental duplication events, and the LcSPLs displayed high collinearity with the L. tulipifera genome. RT-qPCR assays showed that SPL genes were differentially expressed in different tissues, especially. Because LcSPL2 is highly expressed in pistils and leaves, it was selected to describe the SPL gene family of L. chinense by ectopic expression. We showed that overexpression of LcSPL2 in Arabidopsis thaliana resulted in earlier flowering and fewer rosette leaves. Moreover, we observed that overexpression of LcSPL2 in A. thaliana up-regulated the expression levels of four genes related to flower development. Conclusions This study identified SPL genes in Liriodendron and characterized the function of LcSPL2 in advancing flower development.