Molecular characterization of the SPL gene family in Populus trichocarpa

SPLs, a family of transcription factors specific to plants, play vital roles in plant growth and development through regulation of various physiological and biochemical processes. Although Populus trichocarpa is a model forest tree, the PtSPL gene family has not been systematically studied.

Here we report the identification of 28 full-length PtSPLs, which distribute on 14 P. trichocarpa chromosomes. Based on the phylogenetic relationships of SPLs in P. trichocarpa and Arabidopsis, plant SPLs can be classified into 6 groups. Each group contains at least a PtSPL and an AtSPL. The N-terminal zinc finger 1 (Zn1) of SBP domain in group 6 SPLs has four cysteine residues (CCCC-type), while Zn1 of SPLs in the other groups mainly contains three cysteine and one histidine residues (C2HC-type). Comparative analyses of gene structures, conserved motifs and expression patterns of PtSPLs and AtSPLs revealed the conservation of plant SPLs within a group, whereas among groups, the P. trichocarpa and Arabidopsis SPLs were significantly different. Various conserved motifs were identified in PtSPLs but not found in AtSPLs, suggesting the diversity of plant SPLs. A total of 11 pairs of intrachromosome-duplicated PtSPLs were identified, suggesting the importance of gene duplication in SPL gene expansion in P. trichocarpa. In addition, 18 of the 28 PtSPLs, belonging to G1, G2 and G5, were found to be targets of miR156. Consistently, all of the AtSPLs in these groups are regulated by miR156. It suggests the conservation of miR156-mediated posttranscriptional regulation in plants.

A total of 28 full-length SPLs were identified from the whole genome sequence of P. trichocarpa. Through comprehensive analyses of gene structures, phylogenetic relationships, chromosomal locations, conserved motifs, expression patterns and miR156-mediated posttranscriptional regulation, the PtSPL gene family was characterized. Our results provide useful information for evolution and biological function of plant SPLs.