rol-Gene expression in transgenic aspen (Populus tremula) plants results in accelerated growth and improved stem production index

Stem and trunk growth, axillary bud break and branching habits are extremely important parameters of wood production in forest trees. The possibility of altering tree form by transformation with genes responsible for hormone biosynthesis and/or activity is most attractive. We examined four different phenotypically selected transgenic clones of a model tree - Populus tremula - expressing rol genes from Agrobacterium rhizogenes under their native promoters. Several of the observed phenotypic modifications were correlated with rol-gene expression, including breaking of stem apical dominance which resulted in the development and branching of up to four axillary buds per explant, as compared to a lack of axillary bud break in a uidA (beta-glucuronidase-encoding)-transgenic aspen line and control (non-transformed) plants. rol-Transgenic plants also exhibited a higher cumulative stem length and enhanced growth rate, and hence a higher stem production index. During their first and second years in the greenhouse, rol-transgenic aspen plants exhibited enhanced growth and delayed winter dormancy relative to non-transformed plants. Although initially rol-transgenic plants had smaller, wrinkled leaves, these changes were not observed in the 2-year-old plants, which exhibited a phenotypically true-to-type leaf shape.