Genetic information on the Ri plasmid of Agrobacterium rhizogenes determines host specificity

A critical review on use of Agrobacterium rhizogenes and their associated binary vectors for plant transformation

Agrobacterium rhizogenes, along with A. tumefaciens, has been used to affect genetic transformation in plants for many years. Detailed studies conducted in the past have uncovered the basic mechanism of foreign gene transfer and the implication of Ri/Ti plasmids in this process. A number of reviews exist describing the usage of binary vectors with A. tumefaciens, but no comprehensive account of the numerous binary vectors employed with A. rhizogenes and their successful applications has been published till date. In this review, we recollect a brief history of development of Ri-plasmid/Ri-T-DNA based binary vectors systems and their successful implementation with A. rhizogenes for different applications. The modification of native Ri plasmid to introduce foreign genes followed by development of binary vector using Ri plasmid and how it facilitated rapid and feasible genetic manipulation, earlier impossible with native Ri plasmid, have been discussed. An important milestone was the development of inducible plant expressing promoter systems which made expression of toxic genes in plant systems possible. The successful application of binary vectors in conjunction with A. rhizogenes in gene silencing and genome editing studies which are relatively newer developments, demonstrating the amenability and adaptability of hairy roots systems to make possible studying previously intractable research areas have been summarized in the present review.

Agrobacterium-mediated plant transformation: the biology behind the "gene-jockeying" tool

Agrobacterium tumefaciens and related Agrobacterium species have been known as plant pathogens since the beginning of the 20th century. However, only in the past two decades has the ability of Agrobacterium to transfer DNA to plant cells been harnessed for the purposes of plant genetic engineering. Since the initial reports in the early 1980s using Agrobacterium to generate transgenic plants, scientists have attempted to improve this "natural genetic engineer" for biotechnology purposes. Some of these modifications have resulted in extending the host range of the bacterium to economically important crop species. However, in most instances, major improvements involved alterations in plant tissue culture transformation and regeneration conditions rather than manipulation of bacterial or host genes. Agrobacterium-mediated plant transformation is a highly complex and evolved process involving genetic determinants of both the bacterium and the host plant cell. In this article, I review some of the basic biology concerned with Agrobacterium-mediated genetic transformation. Knowledge of fundamental biological principles embracing both the host and the pathogen have been and will continue to be key to extending the utility of Agrobacterium for genetic engineering purposes.

Map location on Agrobacterium root-inducing plasmids of homologies with the virulence region of tumor-inducing plasmids

Southern-type hybridizations were carried out in order to identify sequence homologies with the pTi vir loci, on an agropine-type plasmid (pRiHRI) and a mannopine-type plasmid (pRi8196) of Agrobacterium rhizogenes. The localization of the sequences hybridizing with subcloned fragments containing vir A, B, G, C, and D from pTiAch5 indicated a similar linear organization of the pTi vir loci and their homologies on pRiHRI and pRi8196, though no homology was detected on both pRi with a 1.1-kb internal fragment of virD. No homology was detected either with the vir E locus on pRiHRI vir region, nor with the virF locus on both pRi vir regions. As on nopaline pTiC58, fragments bearing the homologies with virC and virG are closer together on both pRi than on octopine pTiAch5. A preliminary functional map of the pRiHRI vir region is deduced from this study.

Developmental regulation of RI TL-DNA gene expression in roots, shoots and tubers of transformed potato (Solanum tuberosum cv. Desiree)

Expression of TL-DNA from Agrobacterium rhizogenes plasmid pRi 1855 was examined in a transformed derivative of Solanum tuberosum cv. Desiree, D9X8a. Northern blot analysis identified at least nine TL-DNA coded transcripts in roots, shoots and tubers but their relative abundance differed within and between organs. This revealed a distinctive pattern of organ specified differential expression. Grafting experiments showed that the abnormal shape of tubers of transformed potato was probably determined by TL-DNA products synthesised within the tuber and not by diffusable products synthesised in other parts of the plant. The abundance of at least one transcript, tr5, was probably determined by culture conditions. Implications for functions and control of expression of Ri TL-DNA genes are discussed. It is suggested that Ri TL-DNA provides a convenient and extensive set of model genes to study variation and stability of expression of linked foreign genes introduced into plants.