The ASYMMETRIC LEAVES1 ortholog PagAS1a promotes xylem development and plant growth in <i>Populus</i>

ASYMMETRIC LEAVES1 (AS1) is a central regulator of leaf polarity establishment in <i>Arabidopsis</i>. However, the functions of its ortholog genes in <i>Populus</i> are unclear. In this study, we performed gene expression analysis and found that the <i>AS1</i> ortholog gene <i>PagAS1a</i> exhibited significantly higher expression levels in the secondary phloem than in the secondary xylem within the wood formation zone. Subcellular localization analysis showed that <i>PagAS1a</i> localizes in the nuclei. Compared to the wild type, both <i>PagAS1a</i> overexpression (<i>PagAS1a</i>-OE), and dominant repression (<i>PagAS1a</i>-SRDX) plants exhibited enhanced height and stem diameter growth. Analysis of stem cross-sections revealed that both overexpression and dominant repression of <i>PagAS1</i> promoted secondary xylem development. We performed transcriptome sequencing (RNA-seq) for gene expression analysis, and identified 2,736 significantly differentially expressed genes (DEGs) in <i>PagAS1a</i>-OE plants. GO analysis revealed that genes associated with photosynthesis, lignin biosynthetic, and cellulose biosynthetic pathways were significantly enriched in up-regulated DEGs in <i>PagAS1a</i>-OE, while genes associated with lignin catabolic, and phenylpropanoid catabolic pathways were significantly enriched in down-regulated DEGs. Together, our results suggest that the <i>AS1</i> ortholog <i>PagAS1a</i> is an important regulator of secondary xylem development and plant growth in <i>Populus</i>.