Agrobacterium rhizogenes-Mediated Transformation and Its Biotechnological Applications in Crops - Crop Improvement: New Approaches and Modern Techniques - page 11

Fig. 1.7 Schematic representation of gene locations on T L -DNA
Narasu 2000 ; Christey 2001 ). Deletion of the right border of nopaline-type or oc-
topine-type T-DNA in Ri plasmids appears to affect virulence. Also, mutations cre-
ated within this region have the same effect as removing the tms loci of Ti plasmid
resulted with being avirulent on plants. The deletion of T L -DNA in Ri plasmids
is being less susceptible to oncogenic transformation than the T R -DNA deletion
(Vilaine and Casse-Delbart 1987 ). Expression of the T R -DNA alone can induce root
formation in some plants, but the resulting phenotype is not as strong as when both
T L - and T R -DNA are introduced together (Vilaine and Casse-Delbart 1987 ).
T L -DNA
The size of T L -DNA of agropine type Ri-plasmid is about 19-20 kb in length but,
unlike the T R -DNA, it does not appear to be closely related to any other characterized
loci of Ti-plasmids (Huffman et al. 1984 ; Vilaine and Casse-Delbart 1987 ; Aoki and
Syono 1999 ; Chandra 2012 ). In many species, T L -DNA size seems almost constant,
except in Nicotiana tabacum consisting shorter T L -DNA (Jouanin et al. 1987b ). The
mannopine/cucumopine type T-DNAs and the agropine type T L -DNA contain two
strongly conserved regions which flank an only partially homologous central region
(Filetici et al. 1987 ; Brevet and Tempe 1988 ; Aoki and Syono 1999 ; Chandra 2012 ).
A substance carrying out stimulation of hairy root differentiation under the influence
of endogenous auxin is synthesized by genes of T L -DNA (Ooms et al. 1986 ; Shen
et al. 1988 ; Giri and Narasu 2000 ; Mishra and Ranjan 2008 ).
As a result of mutagenesis in T L -DNA of Ri plasmid, the loss or attenuation of
virulence is shown (White et al. 1985 ). The T L -DNA of Ri plasmids carrying several
loci is identified to be essential for hairy root induction (so-called rol genes for root
oncogenic loci) (Fig. 1.7 ). Transposon mutagenesis in the T L -DNA has identified
at least four genes ( rol A, rol B, rol C and rol D) involved in tumorigenesis as affect-
ing some plants (White et al. 1985 ; Estramareix et al. 1986 ; Slightom et al. 1986 ;
Vilaine and Casse-Delbart 1987 ; Meyer et al. 2000 ; Christensen et al. 2008 ). All rol
genes have been shown to carry out formation of hairy root phenotype (White et al.
1985 ; Cardarelli 1987a ; Jouanin 1987a ; Vilaine et al. 1987a ; Schmulling et al. 1988 ;