Sequence characterization and expression analysis of NAC genes from Betula platyphylla

The expression analysis of 21 BpNAC genes in stem, xylem from different growth season, and tension wood indicated that they might participate in xylem and tension wood development in birch. NAC transcription factors play important roles in the growth and development of plants. The NAC family has many members; however, the functions of the majority of them are unknown, especially in woody plants. In this study, to characterize the NAC members and explore NAC genes' association with xylem development, 21 full-length NAC genes were identified in Betula platyphylla, a deciduous woody plant. Most of the 21 NAC proteins contain a complete NAC DNA-binding domain and a variable transcriptional regulation domain. Motif analysis showed that proteins within the same group mostly have similar motifs; however, there were no obvious similarities in gene structures among the identified BpNACs, based on exon/intron analysis. Cis-elements analysis indicated that promoters of the 21 BpNAC genes were enriched in ABRE and AC-box elements, suggesting that BpNAC genes might be related to ABA signal transduction pathways and xylem development. Quantitative real-time PCR analysis indicated that the 21 BpNACs were differentially expressed in the stem of B. platyphylla. In different stages of the growth season, BpNACs showed expression differences in the developing xylem. These genes also responded to artificial bending in tension wood and opposite wood, to varying degrees, and exhibited distinct expression patterns at different treatment times. The results indicated that these BpNAC genes might participate in tension wood development. Our study provided the basis for further functional investigations of BpNAC genes in birch.