ROL GENES AND ROOT INITIATION AND DEVELOPMENT

Due to their extensive growth potential, transgenic root systems arisi ng from inoculation with Agrobacterium rhizogenes became popular in th e last decade as model systems in domains as diverse as production of secondary metabolites, interactions with pathogens and symbionts, exam ination of gene importance in control of root development or in regula tion of gene expression in roots. Wild-type bacterial strains have als o been considered as useful tools to stimulate rooting on recalcitrant cuttings or microcuttings as they cause abundant root initiation at t he site of inoculation. Root initiation and the in vitro growth charac teristics of transformed roots result from the transfer of genes locat ed on the root-inducing plasmid (Ri) to plant cells and their expressi on 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 au x 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 th e process of root initiation. The distinctive phenotype of the transfo rmed 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 O RFs 10, 11, 12 and 15. In vitro experiments determined the functions o f the Rol B and Rol C proteins but the functions of Rol A and Rol D ar e still unknown. Altered metabolism of developmental regulators or mod ified sensitivity to auxin have been suspected to mediate root inducti on and morphological abnormalities of transformed roots and plants. Th e target cells for transformation and the cells which are competent fo r root initiation will be characterized as well as the subsequent deve lopment of transgenic roots provided with various constructs from the whole T-DNA to single rol genes. Results dealing with auxin contents i n relation with root growth kinetics, phenotype and structure, will al so be presented and discussed with the potential use of the rol genes to control root biomass.