rol-Transgenic Populus tremula: root development, root-borne bud regeneration and in vitro propagation efficiency

 The potential use of the rol genes from Agrobacterium rhizogenes to improve the root system horticultural characteristics was evaluated in transgenic aspen (Populus tremula) plants, harboring the rol genes under their native promoters. Southern blot and RT-PCR analyses confirmed the presence and expression of A. rhizogenes rolC and rolB genes in four different phenotypically selected transgenic clones. Several of the observed phenotypic modifications were related to rol-gene expression and included, in particular, modified root systems. All in vitro-cultured rol-transgenic plants exhibited extensive root formation in a hormone-free medium, as well as a larger root surface area and mass, as compared to a uidA (β-glucuronidase-encoding)-transgenic aspen line and control (non-transformed) plants. Adventitious root formation in stem segments of rol-transgenic plants exhibited very rapid kinetics, resulting in a much shorter rooting time for rol-transgenic stem segments (e.g. 10 days for 80% rooting in rol-transgenic lines T-26 and T-27, as compared to more than 18 days for control non-transformed or uidA-transgenic aspen plants). rol-Transgenic plants maintained the capacity for 100% rooting throughout the year, versus 70–80% rooting in non-transformed plants during the winter. The four rol-transgenic lines exhibited differences in root development; in two of them enhanced root development was accompanied by increased shoot fresh weight. The root:shoot fresh weight ratio was always higher in rol-transgenic lines than in non-transformed plants. In the T-27 rol-transgenic line, the propagation coefficient of shoot-bud regeneration in liquid root culture was almost three times higher than in non-transformed plants. To the best of our knowledge this is the first report on quantitative phenotypic alterations in rol-transgenic woody plants. Received: 1 December 1997 / Accepted: 8 March 1998