CHAPTER 26 - Agrobacterium tumefaciens Ti Plasmid-Derived Plant Vectors for Dicotyledonous and Monocotyledonous Plants

Abstract

This chapter reviews the bacterium Agrobacterium tumefaciens which induces tumors called crown galls on dicotyledonous plants but not on monocotyledonous plants. A segment of bacterial DNA called T-(transferred) DNA is introduced into plant cells during tumor induction and becomes integrated into the genome of these plant cells. T-DNA contains a number of genes that are expressed in the transformed plant cells. DNA sequence analysis has shown that these genes have eukaryotic regulatory sequences. The expression of the aux and cyt genes from the T-DNA in transformed plant cells leads to the synthesis of enzymes that are involved in the production of the auxin indole acetic acid and the cytokinin isopentenyl-AMP. Auxins and cytokinins are phytohormones that are essential for the growth of plant cells. The overproduction of these compounds in crown gall cells forms the basis of their unlimited growth as tumor cells. Infection of plants with Agrobacterium, therefore, leads to the formation of large tumors. Agrobacteria carry large plasmids that contain genes involved in tumor induction. These plasmids, which are called Ti (tumor-inducing) plasmids, not only contain the T-region but also a series of other genes that are involved in tumor induction and that are called the vir (virulence) genes.