Exogenous 6-benzyladenine application affects root morphology by altering hormone status and gene expression of developing lateral roots inMalus hupehensis

Malus hupehensisis an extensively used apple rootstock in China. In the current study,M. hupehensisseedlings were treated with exogenous 2.2 mu m6-benzyladenine (6-BA) so as to investigate the mechanism by which 6-BA affects lateral root development. The results indicate that 6-BA treatment promotes elongation and thickening of both root and shoot inM. hupehensis, but reduces the number of lateral roots, as well as reducing the auxin level after 6-BA treatment. Moreover,MhAHK4,MhRR1andMhRR2were also significantly up-regulated in response to 6-BA treatment. Expression levels of auxin synthesis- and transport-related genes, such asMhYUCCA6,MhYUCCA10,MhPIN1andMhPIN2, were down-regulated, which corresponds with lower auxin levels in the 6-BA-treated seedlings. A negative regulator of auxin,MhIAA3, was induced by 6-BA treatment, leading to reduced expression ofMhARF7andMhARF19in 6-BA-treated seedlings. As a result, expression ofMhWOX11,MhWOX5,MhLBD16andMhLBD29was blocked, which in turn inhibited lateral root initiation. In addition, a lower auxin level decreased expression ofMhRR7andMhRR15, which repressed expression of key transcription factors associated with root development, thus inhibiting lateral root development. In contrast, 6-BA treatment promoted secondary growth (thickening) of the root by inducing expression ofMhCYCD3;1andMhCYCD3;2. Collectively, the changes in hormone levels and gene expression resulted in a reduced number of lateral roots and thicker roots in 6-BA-treated plants.