Spermidine application affects the adventitious root formation and root morphology of apple rootstock by altering the hormonal profile and regulating the gene expression pattern

•Exogenous Spd application promoted AR formation.•Spd improved the concentration of endogenous IAA and GA3 as well as decreased the ABA and JA contents.•Spd application up-regulated the expression of Spd related genes and auxin biosynthesis related genes. Adventitious root (AR) formation is a critical process for the clonal propagation of horticultural crops. Polyamines are involved in several processes associated with root growth such as cell division and cell development. In this study, Malus prunifolia var. ringo stem cuttings were treated with exogenous spermidine (Spd) 20 μmol/L in order to investigate the physiological and molecular mechanism of Spd to promote ARs formation. The results of the present study indicated an involvement of Spd in AR formation in apple rootstock by interacting with IAA. Morphological and anatomical observation revealed that the Spd application promotes AR formation. According to our results exogenous Spd application promoted AR formation by improving the concentration of endogenous IAA and GA3 at 1, 3 and 1, 8 days respectively as well as by decreasing the ABA and JA contents at all time points except JA at 16 day in response to Spd treatment. Furthermore no significant difference was observed in BR and ZR levels at all time points except at 8 day in both groups. Spd application obviously up-regulated the expression of Spd related genes (MdSAMDC1, MdPAO and MdSPDS6) and auxin biosynthesis related genes (MdIAA7, MdIAA14 and MdIAA23) in the basal parts of apple rootstock. On the other hand MdWOX11 up-regulated the expression of MdLBD16 and MdLBD29, which induced the expression of cell cycle related genes MdCYCD1;1 and MdCYCP4;1. Collectively, our work laid the foundation for applied research and could be useful in future crop improvement programs.