Transcriptome Analysis Reveals New Insights into MdBAK1-Mediated Plant Growth in Malus domestica. 
BAK1 effects on plant stress responses have been well documented, but little is known regarding its effects on plant growth.  In this study, we functionally characterized MdBAK1.  Overexpressing MdBAK1 in Arabidopsis thaliana and apple trees promoted growth.  Longitudinal stem cells were longer in transgenic plants than in wild-type plants.  The size and number of cells and the area of the transverse stem were greater in the transgenic lines than in the wild-type plants.  Moreover, transgenic A. thaliana and apple plants were more sensitive to an exogenous brassinosteroid.  A transcriptome analysis of wild-type and transgenic apple revealed that MdBAK1 overexpression activated the brassinosteroid and ethylene signals, xylem production, and stress responses.  Trend and Venn analyses indicated that carbohydrate, energy, and hormone metabolic activities were greater in transgenic plants during different periods.  Moreover, a weighted gene coexpression network analysis proved that carbohydrate, hormone, and xylem metabolism as well as cell growth may be critical for MdBAK1-mediated apple tree growth and development.  Compared with the corresponding levels in wild-type plants, the endogenous brassinosteroid, cytokinin, starch, sucrose, trehalose, glucose, fructose, and total sugar contents were considerably different in transgenic plants.  Our results imply that MdBAK1 helps to regulate the growth of apple tree through the above-mentioned pathways.  These findings provide new information regarding the effects of MdBAK1 onplant growth and development.