Rooting and Other Characteristics of a Transgenic Walnut Hybrid (Juglans hindsii x J. regia) Rootstock Expressing rolABC

Walnuts ( Juglans spp.) are difficult-to-root woody plants. The rolABC genes ( rolA + rolB + rolC ), derived from the bacteria Agrobacterium rhizogenes , have been shown to increase the rooting potential of other difficult-to-root woody plants. We inserted the rolABC genes into somatic embryos of a `Paradox' hybrid ( J. hindsii × J. regia ) clone PX1 using the A. tumefaciens gene transfer system. A transgenic sub-clone, designated PX1 rolABC 2-2 was selected and compared to the untransformed clone for a variety of phenotypic characteristics, including rooting potential. Transformed and untransformed shoots were budded onto seedling J. regia rootstock in the greenhouse and established in the field. Transformed trees displayed reduced internode length, an increase in lateral branching, and wrinkled leaves. In another test, a commercial persian walnut cultivar J. regia `Chandler' was grafted onto rooted cuttings of both the untransformed and transformed plants. The presence of the rolABC genes in the rootstock had no visible effects on the grafted scion. Several of these trees were excavated from the field and the root systems of each genotype were examined for root number, diameter, and biomass. Trees with the rolABC rootstock had significantly more small diameter roots compared to the controls and less recovered biomass. Tests of the rooting potential of leafy semi-hardwood cuttings for two years resulted in 14% to 59% rooting of the transformed cuttings compared to 51% to 81% rooting of the control. Both transformed hardwood cuttings and microshoots in tissue culture also rooted significantly less (52% and 29% respectively) than untransformed hardwood cuttings and tissue cultured shoots (82% and 54% respectively). Thus, although the rolABC genes induced a shorter internode length and a more fibrous root system (typical of rol -tranformed plants), they were not useful for increasing the rooting potential, and as rootstock they did not affect the phenotype of the scion.