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Exogenous 6-benzyladenine application affects the root morphology, by altering hormonal status, and gene expression of developing lateral root in Malus hupehensis.

Malus hupehensis is extensively used apple rootstock in China(Mao et al., 2017). In the current study, Malus hupehensis seedlings were treated with exogenous 2.2µM 6-benzyladenine (6-BA) so as to investigate the mechanism by which 6-BAaffects lateral root (LR) development. Results indicated that 6-BA treatment promoted the elongation and thickening of both root and shoot in Malus hupehensis, while reduced the number of LRs, as well as auxin level declined by 6-BA treatment. Moreover, MhAHK4, MhRR1 and MhRR2 were also significantly upregulated in response to 6-BA treatment. Expression levels of auxin synthesis and transport-related genes, such as MhYUCCA6, MhYUCCA10, MhPIN1, and MhPIN2 were downregulated, which corresponded with the lower auxin levels in the 6-BA-treated seedlings. A negative regulator of auxin, MhIAA3 was induced by 6-BA, which leads to declined expression of MhARF7 and MhARF19 in 6-BA-treated seedlings. Resulting, blocked expression of MhWOX11, MhWOX5, MhLBD16 and MhLBD29 were observed, which in turn inhibited LR initiation. In addition, lower auxin level decreased the expression of MhRR7 andMhRR15, which repressed the expression of key transcription factors associated with root development, thus inhibiting LR development. In contrast, 6-BA treatment promoted secondary growth (thickening) of the root by inducing the expression of MhCYCD3;1 and MhCYCD3;2. Collectively, the changes in hormone levels and gene expression resulted in a decreased number of LRs and thicker roots in 6-BA-treated plants.