Effect of exogenous Brassinolide (BR) application on the morphology, hormone status, and gene expression of developing lateral roots in Malus hupehensis

Abstract

Malus hupehensis seedlings were treated with exogenous applications of 0.5 mg L−1 brassinolide (BR) in order to investigate the mechanism by which BR promotes and effects lateral root development. Root morphology, plant height, plant crown diameter, and endogenous levels of Brassinolide (BR), Auxin (IAA), Abscisic acid (ABA), and Gibberellin (GA3) in lateral roots were evaluated. Additionally, differential gene expression of genes related to root development were examined in treated and untreated roots by RT-qPCR during lateral root development. Results indicated that BR treatment promoted both root and shoot growth and auxin levels increased, while ABA and GA3 declined. Additionally, there was also a significant upregulation in the expression of MdBAK1, MdBRI1, and MdBZR1 in response to the BR treatment. Transcript levels of key auxin synthesis and transport genes, such as MdYUCCA6, MdYUCCA10, MdPIN1, MdPIN2, MdPIN3, which corresponded with higher auxin levels in the BR-treated samples. MdIAA23 were induced, but the expression of MdIAA5, a negative regulator of MdARF7 and MdARF19, was downregulated in BR-treated apple seedlings, leading to an elevated expression of MdARRO1; which in turn increased lateral root development. On the other hand, MdBRI1 induced MdWOX5 expression, resulting in enhanced expression of the cell cycle related genes: MdCYCD1;1, MdCYCD3;1, and MdCYCD3;2; leading to upregulated expression of MdLBD29. Collectively, the changes in gene expression and hormone levels resulted in an increased number of lateral roots, and other growth characteristics in BR-treated plants.