437 Transformation of Dwarfing Apple and Pear Rootstocks with the rolB Gene and Its Influence on Rooting and Growth M. Welander, L.H. Zhu and X.Y. Li Swedish University of Agricultural sciences Department of Crop Science P.O. Box 44 SE-230 53 Alnarp Sweden Keywords: field trial, Malus, Pyrus communis, in vitro rooting, ex vitro rooting Abstract Regeneration and transformation protocols have been elaborated for the dwarfing apple rootstocks M26 and M9/29 and the dwarfing pear rootstock BP10030. Transformed plants have been verified by PCR and Southern blot analyses. The in vitro rooting ability of transformed shoots without auxin treatment varied between 83-100% whereas untransformed shoots did not root at all. For nodal cuttings of transformed clones the rooting ability varied between 91-95% and 20-46% for untransformed plants. Not only the rooting percentage but also the root number increased for the transgenic clones. The root number per cutting varied between 14 and 31 for the transgenic clones and between 1 and 5 for the untransformed plants. Growth analysis revealed that some transformed clones showed significantly shorter internode length and stem length compared to the untransformed controls. This means that the rolB gene not only improved the rooting ability, but also introduced some dwarf characteristics into the transformed plants. The permission for field trial on the transformed rootstocks has been obtained from the Board of Agriculture in Sweden. Plants of different transformed clones and the untransformed controls of M26 and M9/29 have been produced and grown in the field for two years. Five apple cultivars commonly used in Sweden and Europe have been budded onto the rootstocks to evaluate the influence of transgenes on growth and development of the cultivars. INTRODUCTION Dwarfing rootstocks are used worldwide for reducing the plant size and increasing yield efficiency. Although many rootstock options are available for fruit trees, there are no ideal rootstocks in commercial production. Among the dwarfing apple (Malus) rootstocks, M9 is the most commonly used one in Europe, but has a low rooting ability and weak tree anchorage in soil. Increasing the rooting ability would add a great value to this rootstock. Today there are no satisfactory dwarfing pear (Pyrus) rootstocks. Pyrus itself is difficult to propagate and ‘Quince’, which is used as an alternative, is incompatible with some important commercial scion cultivars. The pear rootstock BP10030 (Pyrus), selected in Balsgård, Sweden, is a promising dwarfing rootstock with good yield efficiency. However, the low rooting ability limits its nursery production. Improving the rooting ability of this rootstock should greatly increase its commercial value and also enable growers to produce new material with cuttings instead of layering and stool-bedding. The objectives of our research have been to evaluate the rooting ability of the apple rootstocks M26 and M9 (M9/29) and the pear rootstock BP10030 transformed with the rolB gene and to perform growth analysis on these materials in the greenhouse. Furthermore we intend to identify the effects of the rolB gene on growth, flowering and fruit set by grafting several scion cultivars onto the rolB transformed rootstocks in the field. XI th Eucarpia Symp. on Fruit Breed. & Genetics Eds. F. Laurens and K. Evans Acta Hort. 663, ISHS 2004