The tomato MADS-box gene SlMBP9 negatively regulates lateral root formation and apical dominance by reducing auxin biosynthesis and transport.

KEY MESSAGE: Overexpression of SlMBP9 reduced auxin biosynthesis and transport, and negatively regulated lateral root formation and apical dominance. MADS-box transcription factors play a critical role in plant development. In this study, we describe SlMBP9, a novel MADS-box gene that is expressed in the roots of tomato plants. Tomato lines that over- or under-expressed SlMBP9 were generated using a transgenic approach. The number of lateral roots (LRs) were reduced in SlMBP9-overexpressing lines but slightly increased in SlMBP9-silenced lines. A physiological index revealed that the auxin content significantly decreased in the root maturation zone of the overexpression lines. In addition, gene expression analysis revealed that the expression of the polar auxin transporter genes PIN1 and ABCB19/MDR1 and genes involved in auxin biosynthesis was downregulated in the stems of overexpression lines, which is consistent with the reduced accumulation of auxin in the root maturation zone. Exogenous indole-3-acetic acid (auximone) rescued the lateral root phenotypes of the SlMBP9-overexpressing lines. Overexpression of SlMBP9 resulted in dwarf plants, enhanced lateral buds and reduced the gibberellin content in the stems. Together, these results suggest that SlMBP9 plays a negative role in the process of auxin biosynthesis and transport.