Overexpression of miR390b promotes stem elongation and height growth in <i>Populus</i>

Abstract

MicroRNA390 (miR390) is involved in plant growth and development by down-regulating the expression of the downstream genes <i>trans-acting short interfering RNA3</i> (<i>TAS3</i>) and <i>AUXIN RESPONSE FACTORs</i> (<i>ARFs</i>). There is a scarcity of research on the involvement of the miR390-<i>TAS3-ARFs</i> pathway in the stem development of <i>Populus</i>. 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, <i>TAS3.1</i> and <i>TAS3.2</i> were identified as the target genes of miR390 in poplar and were negatively regulated by miR390 in the apex. The transcription levels of <i>ARF3.2</i> and <i>ARF4</i> 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-<i>TAS3</i>-<i>ARFs</i> pathway in poplar is involved in stem elongation and plant height growth.