MdWRKY9 overexpression confers intensive dwarfing in the M26 rootstock of apple by directly inhibiting brassinosteroid synthetase MdDWF4 expression.

Dwarfing rootstocks enable high-density planting and are therefore highly desirable in modern apple (Malus domestica) production. M26 is a semi-dwarfing rootstock that is used worldwide, but identifying intensive dwarfing rootstock is a major goal of apple breeding programs. Herein, we show that MdWRKY9 mediates dwarfing by directly inhibiting the transcription of the brassinosteroid (BR) rate-limiting synthetase MdDWF4 and reducing BR production. We found that the transcriptional factor MdWRKY9 is highly expressed in all tested dwarfing rootstocks. Transgenic lines of M26 rootstock overexpressing MdWRKY9 exhibit further dwarfing, which resulted from the reduced BR levels and was reversed via exogenous brassinolide treatment. Both an in vivo chromatin immunoprecipitation (ChIP) analysis and an in vitro electrophoretic mobility shift assay (EMSA) indicated that MdWRKY9 binds to the promoter of MdDWF4. Furthermore, MdWRKY9 repressed MdDWF4 expression in stable transgenic apple plants as determined by quantitative PCR. In addition, RNA-interfered expression of MdWRKY9 in transiently transformed apple calli led to a significant increase of MdDWF4, suggesting MdWRKY9 plays a critical role in regulating the expression of MdDWF4. We report a novel dwarfing mechanism in perennial woody plants that involves WRKY-controlled BR production, and present a new dwarfing M26 rootstock for potential applications in apple production.