Genome-wide identification, classification and expression analysis of the PHD-finger protein family in Populus trichocarpa

The plant homeobox domain (PHD) proteins are widespread in eukaryotes, and play important roles in regulating chromatin and transcription. Comprehensive analyses of PHD-finger proteins have been performed in animals, but few plant PHD-finger proteins involved in growth and development have been characterized functionally. In this study, we conducted a genome-wide survey of PHD-finger proteins in Populus trichocarpa by describing the phylogenetic relationship, gene structure, and chromosomal location and microarray analyses of each predicted PHD-finger family member. We identified 73 PHD-finger genes (PtPHD1-73) and classified them into eleven subfamilies (A-K) by phylogenetic analysis. Seventy-two of the 73 genes were unevenly distributed on all 19 chromosomes, with seven segmental duplication events. Analysis of the Ka (non-synonymous substitution rate)/Ks (synonymous substitution rate) ratios suggested that the duplicated genes of the PHD-finger family mainly underwent purifying selection with restrictive functional divergence after the duplication events. Expression profiles analysis indicated that 67 PHD-finger genes were differentially expressed in various tissues. Quantitative real-time RT-PCR (qRT-PCR) analyses of nine selected PtPHD genes under high salinity, drought and cold stresses were also performed to explore their stress-related expression patterns. The results of this study provide a thorough overview of poplar PHD-finger proteins and will be valuable for further functional research of poplar PHD-finger genes to unravel their biological roles.