Agrobacterium rhizogenes-Mediated Transformation and Its Biotechnological Applications in Crops - Crop Improvement: New Approaches and Modern Techniques - page 6

kov et al. 2004 ). Hairy roots are capable of growing in the absence of exogenous
plant hormones on the plant cells due to the presence of T-DNA. Agrobacterium
species are highly adapted for sophisticated parasitic relationship with host plants
and thus found to establish a unique ecological niche by genetically engineering
(Vilkar et al. 1987 ).
GallProteins
One of the similarities of Ri and Ti plasmid is that bearing nearly identical or-
ganization of the vir operons (Zhu et al. 2000 ). Only noticeable difference can
be seen is neither genomes nor Ri plasmids of A. rhizogenes contains vir E1 and
vir E2 genes (Moriguchi et al. 2001 ; Hodges et al. 2004 ). As known from studies
about A. tumafaciens VirE2 is a single-stranded DNA binding protein and VirE1
acts as a chaperone of VirE2. The VirE2 covers single-stranded T-DNA (T-strands)
from nuclease attack (Rossi et al. 1996 ; Ozyigit 2012 ) and involves nuclear im-
port of T-DNA to the plant cells (Yusibov et al. 1994 ; Rossi et al. 1996 ; Zupan
et al. 1996 ; Gelvin 1998 ). vir E genes play critical roles in pathogenesis of A. tu-
mefaciens (Christie et al. 1988 ; Citovsky et al. 1992 ; Ward and Zambryski 2001 ;
Duckely and Hohn 2003 ; Ozyigit 2012 ). However, the absence of vir E genes or no
other homolog genes in the A. rhizogenes genome clearly shows that vir E genes
are not necessary in the mechanism of hairy root induction (Moriguchi et al. 2001 ).
Recent studies imply that despite sharing no homology, the GALLS gene located
on the Ri plasmid can substitute VirE2 function in A. tumefaciens (Hodges et al.
2004 ). GALLS protein differs from VirE2 with ATP-binding and helicase motifs
resembling to those in TraA protein involved in conjugation. Both GALLS and
VirE2 contain nuclear localization sequences and a C-terminal type IV secretion
signal. Mutations in these domains lead to loss of GALLS ability to substitute for
VirE2 (Sinkar et al. 1988 ; Hodges et al. 2006 ). However, mechanism of GALLS
protein in A. rhizogenes is still not fully known. All these facts reveal that in spite
of differences in their virulence systems, the Ti and Ri plasmids are share a com-
mon ancestor. However, the way of T-DNA transfer and those other variations in
T-DNA processing also show signs of independent evolution from each other. Cur-
rent understanding of the molecular bases of the differences between hairy root and
gall formation will be accelerated by further studies on genome sequencing and
comparison of various Agrobacterium strains (Hodges et al. 2006 ).
Ri Plasmid
Ri plasmid in all A. rhizogenes strains has a region known as T-DNA which carries
genes ( rol -genes) involved in root initiation and development and genes essential
for opine biosynthesis (Slightom et al. 1986 ; Hansen et al. 1994a ). Agrobacterium