Transcription Factor SmSPL2 Inhibits the Accumulation of Salvianolic Acid B and Influences Root Architecture

The SQUAMOSA PROMOTER-BINDING PROTEIN-LIKE (SPL) transcription factor play vital roles in plant growth and development. Although 15 SPL family genes have been recognized in the model medical plant Salvia miltiorrhiza Bunge, most of them have not been functionally characterized to date. Here, we performed a careful characterization of SmSPL2, which was expressed in almost all tissues of S. miltiorrhiza and had the highest transcriptional level in the calyx. Meanwhile, SmSPL2 has strong transcriptional activation activity and resides in the nucleus. We obtained overexpression lines of SmSPL2 and rSmSPL2 (miR156-resistant SmSPL2). Morphological changes in roots, including longer length, fewer adventitious roots, decreased lateral root density, and increased fresh weight, were observed in all of these transgenic lines. Two rSmSPL2-overexpressed lines were subjected to transcriptome analysis. Overexpression of rSmSPL2 changed root architectures by inhibiting biosynthesis and signal transduction of auxin, while triggering that of cytokinin. The salvianolic acid B (SalB) concentration was significantly decreased in rSmSPL2-overexpressed lines. Further analysis revealed that SmSPL2 binds directly to the promoters of Sm4CL9, SmTAT1, and SmPAL1 and inhibits their expression. In conclusion, SmSPL2 is a potential gene that efficiently manipulate both root architecture and SalB concentration in S. miltiorrhiza.