Homology studies demonstrate colinear organization of the transferred regions of plasmids pRi 1855 and pRi 8196 from Agrobacterium rhizogenes

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.

A common organization of the T-DNA genes expressed in plant hairy roots induced by different plasmids of Agrobacterium rhizogenes

The recently described pathogenic plasmid from Agrobacterium rhizogenes 2659 induces "hairy root" proliferation in infected plants which synthesize a new opine type, provisionally called cucumopine. The T-DNA restriction site map of 2659 differs from other pRi T-DNAs, i.e., mannopine and agropine. However all these three different T-DNAs have a similar organization as detected through hybridization. This, previously shown by our cross-hybridization studies between mannopine and agropine pRi T-DNAs, is here extended to the cucumopine pRi T-DNA; each of the three pRi T-DNA is composed of successive DNA regions that would find actual counterparts in the two other T-DNAs. Equivalence of DNA regions is suggested by their similar length, location and organization in the three T-DNAs and by their sequence homology substantial enough to be reproducibly cross-hybridized in all our experiments. This suggests that the three pRi T-DNAs share most of their functions. This was sought by analysing the pRi 2659 T-DNA gene expression in transformed plant tissues. The mRNA species observed in pRi 2659 carrot hairy roots of in vitro cultures give a pattern that is rather unvariable and resembles those described for some agropine type pRi transformant tissues. This transcript pattern is consistent with the preservation in the pRi 2659 T-DNA of counterparts of most of the open reading frames detected in the TL-DNA of agropine type pRis.

Homology mapping of T-DNA regions on three Agrobacterium rhizogenes Ri plasmids by electron microscope heteroduplex studies

Recombinant plasmids carrying segments of the Agrobacterium rhizogenes T-DNA regions of the three Ri plasmids 1855 (TL-DNA only), 8196, and 2659 were used for establishing homology maps by electron microscope examination of heteroduplexes. Plasmid DNA was linearized by digestion with suitable restriction endonucleases in order to generate large T-DNA segments. Heteroduplexes were prepared in 50% formamide and spread under standard conditions. Measurements of double and single strands allowed the drawing of homology maps. The three T-DNAs share mainly two homologous sequences of respectively about 2.5 and 1.5 kb, bracketing a largely nonhomologous central part which is about 5.5 kb long. The T-DNAs from pRi1855 and pRi2659 appear to be more related to each other than to that of pRi8196. With reference to the published nucleotide sequence of the TL-DNA of pRiA4 (probably identical to that of pRi1855), ORFs 8 and 14 seem to be the most conserved sequences of the three T-DNAs. The significance of these conserved sequences is unclear since the genetic loci involved in rhizogenicity of agropine strains identified previously are located in nonhomologous regions.

Physical map of the T-DNA region of Agrobacterium rhizogenes strain NCPPB2659

The T-DNA region of the plasmid responsible for hairy root in the Agrobacterium rhizogenes strain NCPPB2659 is identified and characterized by its physical map.