An AUX/IAA gene PlIAA9 regulates lignin deposition during secondary growth of herbaceous peony (Paeonia lactiflora Pall.) stems

• PlIAA9 , a canonical Aux/IAA gene, was highly expressed in the vascular bundles of stems. • Silencing of PlIAA9 weakened stem strength accounted for undeveloped xylem and less lignin deposition. • Overexpression of PlIAA9 increased stem strength by improving xylem development and lignin accumulation. • Silencing of PlIAA9 decreased the expression levels of transcriptional regulators of lignin biosynthesis. Secondary growth of stems significantly affects lodging resistance of plants, which is a core factor in plant agronomic traits. Aux/IAA genes have been shown to be involved in the secondary growth of stems. However, as we know, there has been little research conducted on the involvement of herbaceous peony ( Paeonia lactiflora Pall.) Aux/IAA gene in the secondary growth of stems. In this study, a canonical Aux/IAA gene PlIAA9 was isolated from herbaceous peony. Expression patterns analysis demonstrated that PlIAA9 is highly expressed in the vascular bundles of stems. Silencing of PlIAA9 in herbaceous peony resulted in significant reductions in stem diameter and strength. The microstructure observations of stems showed that the cell layers of vascular cambium and xylem, as well as the xylem radial width were remarkablely decreased in silenced herbaceous peony. The histochemical staining and physiological analysis revealed that silencing of PlIAA9 in herbaceous peony signitificantly reduced the lignin content. On the contrary, overexpression of PlIAA9 in tobacco ( Nicotiana tabacum L.) exhibited opposite phenotypes, stem characteristics and lignin content. Furtthermore, expression analysis showed that expression levels of transcriptional regulators of lignin biosynthesis were significantly decreased in PlIAA9- silenced herbaceous peony. Collectively, PlIAA9 played a positive role in the regulation of secondary growth in stems by affecting lignin deposition. The findings of this study would offer insightful understanding of the pivotal role played by AUX/IAA genes in the secondary growth process of herbaceous peony stems.