Structure and function of root‐inducing (Ri) plasmids and their relation to tumor‐inducing (Ti) plasmids

Abstract

The abilities of Agrobacterium tumefaciens and A. rhizogenes to transform dicotyle‐dons and cause crown gall and hairy root disease are caused by the presence of tumor inducing (Ti) and root inducing (Ri) plasmids. During transformation plasmid T‐DNA (transferred DNA) is inserted into the plant genome. The T‐region is flanked by 25 bp direct repeats, which are essential for transfer. The T‐regions contain oncogenes that are expressed in the plants. Some of these code for enzymes that synthesize auxin or cytokinin. Another type, present in Ri plasmids only, appears to impose a high hormone sensitivity on the infected tissue. The T‐DNA also contains genes for enzymes synthesizing opines, which the bacteria catabolize. The T‐DNA transfer is initiated by the induction of genes in the virulence (vir) region of the plasmid by phenolic compounds secreted by wounded tissue. The products of the vir‐genes and of chromosomal genes mediate transfer of T‐DNA to the plant cells. Crown gall disease is caused by production of auxin and cytokinin by the transferred T‐DNA. The T‐DNA of Ri plasmids codes for at least three genes that each can induce root formation, and that together cause hairy root formation from plant tissue. Current results indicate that the products of these genes induce a potential for increased auxin sensitivity that is expressed when the transformed cells are subjected to a certain level of auxin. After this stage the transformed roots can be grown in culture without exogenous supply of hormones.