Effects of Brassinosteroid Associated with Auxin and Gibberellin on Apple Tree Growth and Gene Expression Patterns

Brassinosteroid is an important hormone that interacts with auxin and gibberellin to regulate plant growth and development. However, there is currently a relative lack of information regarding brassinosteroid in apple. Previous study confirmed that exogenous brassinosteroid treatments increased growth and endogenous brassinosteroid, auxin, as well as gibberellin levels of apple nursery trees. Here we succeeded to find that exogenous brassinosteroid treatments upregulated the transcript expression of auxin biosynthesis, transport, and positive signal transduction genes as well as gibberellin biosynthesis genes. In contrast, the application of exogenous brassinosteroid downregulated the expression of genes encoding negative regulators of auxin and gibberellin signal transductions. Rapid responses of several brassinosteroid-, auxin-, and gibberellin-related genes to the brassinosteroid and brassinazole treatments inferred the crosstalk of BR and IAA or GA. Furthermore, the expression levels of cell growth-related genes were also enhanced by exogenous brassinosteroid. Auxin and gibberellin treatments also influenced growth of apple tree and enhanced the expression of brassinosteroid- and cell growth-related genes. These results indicate that the growth of apple tree is regulated by the interaction between brassinosteroid, auxin, and gibberellin. Our work opened new avenues for deep portfolio of apple tree growth and laid the foundation for future studies aimed at elucidating the mechanisms regulating hormone interactions-mediated growth in apple trees.