Genomic analysis of PIN-FORMED genes reveals the roles of SmPIN3 in root architecture development in Salvia miltiorrhiza

Salvia miltiorrhiza is a widely utilized medicinal herb in China. Its roots serve as crucial raw materials for multiple drugs. The root morphology is essential for the quality of this herb, but little is known about the molecular mechanism underlying the root development in S. miltiorrhiza. Previous study reveals that the polar auxin transport is critical for lateral root development in S. miltiorrhiza. Whether the auxin efflux carriers PIN-FORMEDs (PINs) are involved in this process is worthy investigation. In this study, we identified nine SmPIN genes in S. miltiorrhiza, and their chromosome localization, physico-chemical properties, and phylogenetic relationship were analyzed. SmPINs were unevenly distributed across four chromosomes, and a variety of hormone responsive elements were detected in their promoter regions. The SmPIN proteins were divided into three branches according to the phylogenetic relationship. SmPINs with close evolutionary distance showed similar conserved motif features. The nine SmPINs showed distinct tissue-specific expression patterns and most of them were auxin-inducible genes. We generated SmPIN3 overexpression S. miltiorrhiza seedlings to investigate the function of SmPIN3 in the root development in this species. The results demonstrated that SmPIN3 regulated the root morphogenesis of S. miltiorrhiza by simultaneously affecting the lateral root development and the root anatomical structure. The root morphology, patterns of root xylem and phloem as well as the expressions of genes in the auxin signaling pathway all altered in the SmPIN3 overexpression lines. Our findings provide new insights for elucidating the regulatory roles of SmPINs in the auxin-mediated root development in S. miltiorrhiza.