The regulatory roles of SmTIR1 gene in root development of Salvia miltiorrhiza

The roots and rhizomes of Salvia miltiorrhiza are widely used as medicine for the treatments of cardiovascular and cerebrovascular diseases in China. Root morphology and development are important factors that affect the quality and yield of this herb, but the molecular mechanism of root development of S. miltiorrhiza is still unclear. Auxin is one of the main plant hormones that regulate root development in plants, and auxin receptor TIR1 plays pivotal roles in this biological process. In this study, SmTIR1 gene was cloned and transgenic S. miltiorrhiza plantlets were obtained to investigate the function of SmTIR1 in root development of S. miltiorrhiza. The results showed that SmTIR1 was a positive regulator of growth and root development in S. miltiorrhiza. The SmTIR1 overexpression lines showed increased leaf number, root biomass, lateral root number, and lateral root diameter. The SmTIR1 RNAi lines showed the opposite morphological indices compared with the SmTIR1 overexpression lines. Besides, the contents of phenolic acids in leaves of SmTIR1 overexpression lines were remarkedly increased, but contents of bioactive compounds in roots of SmTIR1 transgenic S. miltiorrhiza showed no obvious changes. The results of RNA-seq analysis indicated that many of auxin signaling pathway genes were differentially expressed in SmTIR1 RNAi lines, SmTIR1 may affect the growth and root development of S. miltiorrhiza via influencing auxin signal transduction.