Genome-wide analysis of poplar HD-Zip family and over-expression of PsnHDZ63 confers salt tolerance in transgenic Populus simonii × P.nigra

HD-Zip is a plant-specific HB transcription factor, which participates in plant development and stress response. In this study, we identified 63 poplar HD-Zip transcription factors, which were randomly distributed on 19 chromosomes of poplar. Based on the gene structure and phylogenetic relationship, these members are divided into four groups, which have a variety of collinear evolutionary relationships. They also have rich segmental replication events and experienced strong purification selection. Based on RNA-seq analysis, we profiled the expression pattern of the 63 HD-Zip members under salt stress. Subsequently, we carried out in-depth study on the significantly up-regulated PsnHDZ63 in the stems and leaves. The transgenic Populus simonii × P.nigra plants over-expressing PsnHDZ63 displayed better morphological and physiological indexes than WT under salt stress. In addition, PsnHDZ63 enhanced salt stress tolerance of transgenic lines by combining effective stress-resistant elements to improve reactive oxygen species scavenging ability. These studies laid a foundation for a comprehensive understanding of poplar HD-Zip family members, and revealed the important role of PsnHDZ63 in plant salt tolerance.