Exogenous IAA improves the seedling growth of Syringa villosa via regulating the endogenous hormones and enhancing the photosynthesis

Auxin is a core regulator of plant growth and an effective promoter of cell division and cell expansion. Effects of auxin on improving plant growth of Syringa plants remain poorly elucidated. The present study was carried out to investigate the effects of exogenous indole-3-acetic acid (IAA) on morphogenesis, endogenous hormone content, photosynthesis capacity and find the basis of IAA-induced physiological effects of Syringa villosa by transcriptome sequencing technology. The results indicated that exogenous IAA significantly contributed to the plant height, stem diameter, leaf area and leaf number, which improved the accumulation of whole plant quality. Contents of endogenous IAA and gibberellic acid (GA3) were markedly higher than control after IAA treatment, while abscisic acid (ABA) content was significantly declined. For photosynthetic parameters, photosynthesis capacity was promoted by the application of IAA, as evidenced by a significant increase in net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (Tr) and a lower intercellular CO2 concentration (Ci). In addition, the transcriptome of S. villosa was first sequenced which revealed that genes related to the synthesis and signal transduction of growth-promoting hormones such as IAA, GA3, cytokinin (CTK), and brassinolide (BR) were significantly up-regulated within 2 h of IAA treatment. The expression levels of genes related to photosynthesis, antenna proteins, and carbon fixation of photosynthetic organisms were also enhanced by exogenous IAA. Additionally, the application of exogenous IAA had a positive effect on growth and development by rapidly inducing genes related to cell division and cell expansion. By profiling the transcriptome of IAA response, potential target genes were identified based on the hormone levels and photosynthesis. These results provide insights for further functional genomic studies on the regulatory mechanisms of IAA underlying Syringa plants architecture.