Overexpression of miR390b promotes stem elongation and height growth in .

MicroRNA390 (miR390) is involved in plant growth and development by down-regulating the expression of the downstream genes () and (). There is a scarcity of research on the involvement of the miR390- pathway in the stem development of . Here, differentially expressed miRNAs during poplar stem development were screened by small RNA sequencing analysis, and a novel function of miR390b in stem development was revealed. Overexpression of miR390b (OE-miR390b) resulted in a large increase in the number of xylem fiber cells and a slight decrease in the cell length at the longitudinal axis. Overall increases in stem elongation and plant height were observed in the OE-miR390b plants. According to transcriptome sequencing results and transient co-expression analysis, and were identified as the target genes of miR390 in poplar and were negatively regulated by miR390 in the apex. The transcription levels of and were significantly repressed in OE-miR390b plants and strongly negatively correlated with the number of xylem fiber cells along the longitudinal axis. These findings indicate that the conserved miR390-- pathway in poplar is involved in stem elongation and plant height growth.