1. Introduction

Agrobacterium species harboring tumor-inducing (Ti) or hairy root-inducing (Ri) plasmids cause crown gall or hairy root diseases, respectively in plants. Agrobacterium tumefaciens is a plant pathogen that induces tumor on a wide variety of dicotyledonous plants and the disease is caused by tumor-inducing plasmid (pTi). Similarly Agrobacterium rhizogenes is a plant pathogen that induces hairy roots on a wide variety of dicotyledonous plants and the disease is caused by root-inducing plasmid (pRi). Virulence (vir) genes of Ri as well as of Ti plasmids are essential for the T-DNA (Figure 26.1 & 26.2) transfer into plant chromosomes . These natural plasmids provide the basis for vectors to make transgenic plants. The plasmids are approximately 200 kbp in size. Both pTi and pRi are unique in two respects: (i) they contain some genes, located within their T-DNA, which have regulatory sequences recognized by plant cells, while their remaining genes have prokaryotic regulatory sequences, (ii) both plasmids naturally transfer a part of their DNA, the T-DNA, into the host genome, which makes Agrobacterium a natural genetic engineer.

Complete sequence analysis confirms that the pathogenic plasmids contain gene clusters for DNA replication, virulence, T-DNA, opine utilization and conjugation. T-DNA genes have lower G + C content, which is presumably suitable for expression in host plant cells. Besides these genes, each plasmid has a large number of unique genes. Even plasmids of the same opine type differ considerably in gene content and are highly chimeric in structures. The plasmids seem to interact with each other and with plasmids of other members of the Rhizobiaceae and are likely to shuffle genes of infection between Ti and Ri plasmids. Plasmid stability genes are talked about, which are important for plasmid evolution and construction of useful strains.

 

1.1. The Ti plasmid

The Ti plasmid contains all the genes which required for tumor formation. Virulence genes (vir-genes) are also located on the Ti plasmid. The vir genes encode a set of proteins responsible for the excision, transfer and integration of the T-DNA into the plant nuclear genome.

The basic elements of the vectors designed for Agrobacterium-mediated transformation that were taken from the native Ti-plasmid

• The T-DNA border sequences, at least the right border, which initiates the integration of the T-DNA region into the plant genome

• The vir genes , which are required for transfer of the T-DNA region to the plant, and

• A modified T-DNA region of the Ti plasmid, in which the genes responsible for tumor formation are removed by genetic engineering and replaced by foreign genes of diverse origin, e.g., from plants, bacteria, virus. When these genes are removed, transformed plant tissues or cells regenerate into normal-appearing plants and, in most cases, fertile plants.