Rol genes and root initiation and development

Due to their extensive growth potential, transgenic root systems arising from inoculation with Agrobacterium rhizogenes became popular in the last decade as model systems in domains as diverse as production of secondary metabolites, interactions with pathogens and symbionts, examination of gene importance in control of root development or in regulation of gene expression in roots. Wild-type bacterial strains have also been considered as useful tools to stimulate rooting on recalcitrant cuttings or microcuttings as they cause abundant root initiation at the site of inoculation. Root initiation and the in vitro growth characteristics of transformed roots result from the transfer of genes located on the root-inducing plasmid (Ri) to plant cells and their expression therein. Two sets of pRi genes are involved in tlhe root induction process: the rol (root loci) genes located in the TL region and the aux genes of the TR region. Some of these genes being able to interact, the system appears also as a new tool to study the role of auxin in the process of root initiation. The distinctive phenotype of the transformed roots which are capable of hormone autonomous growth seems to be controlled mainly by the rol genes. These rol genes, i.e. the genetic loci rol A, rol B, rol C and rol D correspond to open reading frames ORFs 10, 11, 12 and 15. In vitro experiments determined the functions of the Rol B and Rol C proteins but the functions of Rol A and Rol D are still unknown. Altered metabolism of developmental regulators or modified sensitivity to auxin have been suspected to mediate root induction and morphological abnormalities of transformed roots and plants. The target cells for transformation and the cells which are competent for root initiation will be characterized as well as the subsequent development of transgenic roots provided with various constructs from the whole T-DNA to single rol genes. Results dealing with auxin contents in relation with root growth kinetics, phenotype and structure, will also be presented and discussed with the potential use of the rol genes to control root biomass.