Transformation of the apple rootstock M.9/29 with the rolB gene and its influence on rooting and growth.

To improve the rooting ability, the dwarfing apple rootstock M.9/29 was transformed with the rolB gene by Agrobacterium-mediated gene transfer. The use of sorbitol in the induction medium resulted in a successful transformation, while the use of sucrose failed to give any transformants. Totally 14 putative clones, named ARB1-14, were obtained from ten different leaves. Polymerase chain reaction (PCR) and Southern analyses confirmed that all the clones contained the nptII and rolB genes, while only four of them contained the intact gus gene. The in vitro rooting test showed that all the tested clones rooted to 83-100% on the hormone free rooting medium, while only 1% for the control plants. The root number of the transgenic clones ranged from 3.5 to 9, while the control plants produced only one root. Growth analysis showed that the clone ARB9 and ARB10 had a significant reduced node number and stem length compared with the control plants. However, the relative growth rate (RGR) of the tested clones was similar to that of the control plants, indicating that RGR is not directly related to dwarfism of a plant. The clone ARB10 also showed a significant reduced internode length compared with the control plants. The root length and root morphology did not differ between the transgenic clones and the untransformed control plants.