Agrobacterium rhizogenes-Mediated Transformation and Its Biotechnological Applications in Crops - Crop Improvement: New Approaches and Modern Techniques - page 9

type strains are non-polar whereas all other strains are polar. Agropine type strains
give rise to the formation of the hairy roots regardless of the orientation of the disc
and the strains other than agropine type form hairy roots when the disc is placed
inverted orientation. The presence of second T-DNA encoding genes responsible
for auxin production possibly causes observed variation in the polarity of infection
in the plant cells transformed by the agropine-type Ri plasmid (Meyer et al. 2000 ;
Veena and Taylor 2007 ) (Fig. 1.5 ).
RiT-DNAGenes
Independent transformations of both left T-DNA (T L -DNA) (about 15-20 kb) and
right T-DNA (T R -DNA) (about 8-20 kb) to the plant genome termed as “split”
T-DNA are carried out by Agropine strains pRi, whereas mannopine strains only
transfer a single T-DNA (T L -DNA). T L -DNA of pRi contains the four rol genes,
designated as rol A, rol B, rol C and rol D (Schmulling et al. 1988 ; Petersen et al.
1989 ; Gelvin 2003 ; Bensaddek et al. 2008 ). In Ri plasmid, T L -DNA and T R -DNA
are separated from each other by at least 15 kb of non-integrated DNA, which is
represented by T-Central DNA (TC-DNA) as seen in Fig. 1.5 .
The phenotype of hairy root is related with the genes whose products act as the
determinants located on T L -DNA (Tepfer 1984 ; Taylor et al. 1985 ; Jouanin et al.
1987b ; Nakamura et al. 1988 ; Schmulling et al. 1988 ; Sinkar et al. 1988 ) whereas
the genes on the T R -DNA would only play a role in root induction (Cardarelli et al.
1985 ; Ryder et al. 1985 ; Cardarelli et al. 1987a ; Smulders et al. 1991 ). Two frag-
ments, defined as T L -DNA and T R -DNA, can be transferred and integrated indepen-
dently into the plant genome during the infection process. However, the integration
capacity of T L -DNA was much higher than T R -DNA (Chilton et al. 1982 ; Costan-
tion et al. 1984 ; David et al. 1984 ; Grant et al. 1991 ; Phelep et al. 1991 ; Nilsson
and Olsson 1997 ; Holefors et al. 1998 ; Sevon and Oksman-Caldentey 2002 ; Kumar
et al. 2006 ; Navarrete et al. 2006 ; Bensaddek et al. 2008 ). Furthermore, the present
findings imply that a higher number of Ri-T-DNA copies integrated into the plant
genome increase the phenotypic effect in the Ri-line (Christensen et al 2008 ).
T R -DNA
It was found that the right T-DNA (T R -DNA) contains genes homologous to T-DNA
of A. tumefaciens Ti plasmid (Huffman et al. 1984 ; Jouanin 1984 ; Vilaine and Casse-
Delbart 1987 ; Hansen et al. 1991 ; Chandra 2012 ). Among them, the most impor-
tant genes are those homologous to the tms 1 and tms 2 of the Ti-plasmid. tms 1 and
tms 2 genes play important roles in auxin biosynthesis in A. tumefaciens (Inze et al.
1984 ; Schröder et al. 1984 ; Thomashow et al. 1984 , 1986; Vilaine and Casse-Delbart
1987 ). Homology, mutagenesis and complementation experiments show that the two