Low expression of PIN gene family members is involved in triggering the dwarfing effect in M9 interstem but not in M9 rootstock apple trees

In many perennial woody crops, true-to-type scion is propagated via grafting techniques. The vigour of apple trees can be reduced by grafting on to dwarfing rootstock like M9. However, the mechanisms as to how rootstocks induce dwarfing on their scion remain unclear. Here, members of the gene families encoding auxin polar transporters, PIN and LAX, were analysed in M9, Fuji, and Baleng Crab. Of the 13 members, MdPIN8, MdPIN10 and MdPIN13 were preferentially and differently expressed in the scion, interstem or rootstock in grafting complexes. The dynamic changes in phytohormone contents, relative expressions of MdPINs, and shoot growth after 1-naphthaleneacetic acid (NAA), 6-benzylaminopurine (BA) or N-1-naphthylphthalamic acid (NPA) treatments were evaluated in 3-year-old Fuji/M9, Fuji/M9/Baleng Crab and Fuji/Baleng Crab. In Fuji/M9, foliar or root application of NAA increased root auxin (NAA + IAA) levels but did not significantly increase root zeatin content or shoot growth. Leaf cytokinin (zeatin + BA) levels and shoot growth increased significantly after foliar or root application of BA. In Fuji/M9/Baleng Crab, root application of NAA induced a significant increase in root and leaf zeatin contents and shoot growth, but NAA foliar sprays did not, because the expression of MdPIN8 in the bark of interstem M9 did not increase. When stem auxin transport was inhibited by NPA in Fuji/Baleng Crab, root zeatin level and shoot growth declined dramatically. It was proposed that the dwarfing effect was initiated by inherently lower expression of MdPIN8 in M9 interstem, or by poor root zeatin synthesis in M9.