Application of Agrobacterium Rol Genes in Plant Biotechnology: A Natural Phenomenon of Secondary Metabolism Regulation

Abstract

The rolA, rolB and rolC genes are plant oncogenes that are carried in plasmids of the plant pathogen Agrobacterium rhizogenes. Following agrobacterial infection, these genes are transferred into the plant genome and cause tumor formation and hairy root disease. The rolB and rolC genes of Agrobacterium rhizogenes were studied extensively for the past two decades as regulators of cell growth and differentiation. A new function for the rol genes in plant-Agrobacterium interactions became apparent with the discovery that these genes are also potential activators of secondary metabolism in transformed cells in different plant families (reviewed by Bulgakov, 2008). Classically, rolB and rolC have been considered closely related genes, possessing similar biological functions. However, they demonstrated different, or even opposite, effects on cell death processes (Schmulling et al., 1988), calcium balance in transformed cells (Bulgakov et al., 2003), sensitivity to auxin (Maurel et al., 1991), growth of transformed tissues (Capone et al., 1989) and secondary metabolism (Shkryl et al., 2008). Plant-microbe interactions often lead to the development of defense mechanisms in plant cells. Since reactive oxygen species (ROS) play a pivotal role in the regulation of plant defense mechanisms, extensive experiments were performed to study the relationship between secondary metabolism (phytoalexin production) and the production of ROS in cells transformed with rol genes. Here, we summarize these results. Surprisingly, the rolB and rolC genes not only activated phytoalexin production but also suppressed intracellular ROS levels. This combination of defense responses, coupled with the effect of ROS suppression, represents a unique case in plant-microbe interactions. These findings suggest that bypassing upstream cell control mechanisms may be useful in the construction of plant cells possessing stable production of secondary metabolites. This chapter describes the new findings relating to secondary metabolism and ROS production under the individual and combined expression of the rol genes in plant cells.