Mechanism Of Dwarfing Effect Of M9Used As Rootstock Or Interstem For Apple

Grafting makes the horticultural plants more torlerant to environmental and biotic stress, but also alters the growth and development of the scion. Apple trees grafted onto dwarfing rootstock M9produce dwarfing tree architecture with high yield and were widely used in production. However, there are different hypotheses to explain how to dwarf. After the intensive study, the changes of hormone between scion and rootstock would be the major reason in those increasing studies. The mechanisms whereby rootstocks induce dwarfing on their scion remain unclear.The experimental materials were3-year-old M9, Baleng Crab, Fuji/M9, Fuji/M9/Baleng Crab and Fuji/Baleng Crab. The expressions of gene families encoding auxin polar transporters, PIN and LAX, in the phloem of those rootstocks were detected by using qRT-PCR.100mg/L1-naphthaleneacetic acid (NAA) and6-benzylaminopurine (BA) were foliar or root applied to Fuji/M9and Fuji/M9/Baleng Crab or N-1-naphthylphthalamic acid acid (NPA) was applied in the stem of Fuji/Baleng Crab after gridling. Then the IAA, NAA, Z, BA contents, relative expressions of MdPINs and shoot growth were measured.There are13members in PIN gene familiy and8members in LAX gene familiy, and they were nominated as MdPIN1-13and LAX1-8respectively based on phylogenetic relationship with Arabidopsis. MdPIN8, MdPIN10and MdPIN13were preferentially and differently expressed in the phloem of M9, Baleng, Fuji/M9, Fuji/M9/Baleng and Fuji/Baleng Crab when compared with other MdPINs. MdPIN8showed a significantly lower expression in interstem than in scion and rootstock of Fuji/M9/Baleng Crab. However, the expression of MdPLN8, MdPIN10and MdPIN13did not change significantly in the scion and the rootstock of Fuji/Baleng Crab. These indicated that lower expression of MdPIN8in the M9interstem phloem limited IAA basipetal transport. Likewise, MdLAXl and MdLAX8were preferentially expressed in the phloem of scion.when compared whit those other MdLAXs. The expressions of MdLAXl and MdLAX8showed no significant change in phloem scion, interstem and rootstock, indicating the expression of MdLAXs was not a key factor that limited IAA basipetal transport in M9.In Fuji/M9, foliar or root application of NAA increased root auxin (NAA+IAA) levels on day1, maintained a high levels7days and then decreased in the following days, which have a consistent with the expression of MdPIN8and MdPIN10. But there were no significantly increase in root zeatin contents and shoot growth after NAA treatments compared to that in controls. These indicated the poor root IAA supply was not the reason of the lower Z content in root of M9. After foliar and root application of BA, leaf cytokinin (zeatin+BA) levels increased significantly, root IAA content increased to2.5-fold and2.4-fold respectively when compared to the original value and shoot growth has a significantly increasing to3.1-fold compared to the controls in both foliar and root BA treatments. The results indicated that exogenous BA compensated the lower zeatin contents and relieved the dwarfing effect of M9rootstock. When NAA was root-applied in Fuji/M9/Baleng Crab, the IAA+NAA contents in root and leaf increased on day1, maintained7days and decreased on the following days. the zeatin contents in root and leaf increased significantly up to106.31and141.04ng/g FW respectively on week2. The shoot growth increased significantly to2.9-fold when compared to the controls indicating the lower zeatin contents was closely related to the poor IAA supply in the root. When NAA was foliar sprayed in scion of Fuji/M9/Baleng Crab, IAA+NAA contents in the leaf, scion-bark and interstem-bark increased and maintained high levels, IAA+NAA contents in the rootstock-bark and root didn’t change significantly. While IAA+NAA contents in interstem were higher than in scion and rootstock of M9. The zeatin content in the root and leaf did not change significanty3weeks after treatment, but no recovering of shoot growth occurred than NAA root application. The expression of MdPIN8and MdPIN10was consistent with the changes of IAA+NAA content. But the expression of MdPIN8and MdPIN10in the phloem of scion was significantly higher than that in interstem and rootstock. After day1of NPA treatment the stem-bark of Fuji/Baleng Crab, the root IAA contents decreased. The zeatin contents in the root and leaf continuously decreased after day3of treatment and the shoot growth decreased there after. These data indicated that the poor IAA supply in the root was closely related with the limited IAA basipetal transport in M9.Taken together, the dwarfing mechanism differed between M9rootstock and M9interstock. The dwarfing of M9rootstocks was due to the weak zeatin synthesis in roots. The dwarfing effect was initiated by inherently lower expression of MdPIN8in the M9interstem phloem, which limited IAA basipetal transport and caused poor root IAA supply.