Agrobacterium rhizogenes-Mediated Transformation and Its Biotechnological Applications in Crops - Crop Improvement: New Approaches and Modern Techniques - page 15

transgenic clones show 40-60 % reduction of GA content compared to wild-type
leaves. The reduction of GA content is indirectly cause stem elongation and planar
leaf blade growth (Dehio et al. 1993 ). When the wild-types of N. tabacum treated
by gibberellin biosynthesis inhibitors, rol A expressing plants and wild types show
similar phenotypes. On the other hand, when rol A transgenic plants treated with GA,
the phenotype of transgenic plant not completely restored (Dehio and Schell 1993 ;
Dehio et al. 1993 ). All these shows that the rol A gene has been considered in playing
an important role in modulating hormone physiology of GA and polyamine metabo-
lism (Sun et al. 1991 ; Dehio and Schell 1993 ; Dehio et al. 1993 ; Prinsen et al. 1994 ;
Martin-Tanguy et al. 1996 ; Veena and Taylor 2007 ). It was thought that the sensitiv-
ity of auxin response might correlate with plasma membrane H + ATPase activity ob-
served in rol A expressing transgenic plants (Maurel et al. 1991 ; Vansuyt et al. 1992 ).
There is data suggesting that there is an antagonism between rol A and rol B genes
in general. An observation of additional transcripts ranging from 2.1 to 2.8 kb in
size explains this antagonism (Durand-Tardif et al. 1985 ). Size of transcription of
rol A would be more than 2 kb. This would span the whole rol B sequence, leading to
the generation of an antisense message for rol B. Its occurrence could be the major
cause of antagonism between rol A and rol B in the transformed plant cells. Probably,
existence of a mechanism prevents co-expression of rol A and rol B (Capone et al.
1989 ; van Altvorst et al. 1992 ; Veena and Taylor 2007 ).
rol B
The rol B gene size ranging 765 (strain 8196) to 840 bp (strain 2659) length depend-
ing on the strain and encodes 254-279 amino acid protein which has molecular
weight of 30 kDa localized in the plasma membrane (Filippini et al. 1996 ; Meyer
et al. 2000 ; Veena and Taylor 2007 ). rol B gene is present in all Ri plasmids with
approximately 60 % identity between strains (Meyer et al. 2000 ). RolB proteins
encoded by pRi1724 and pRi2659 have a 17 amino acid longer N-terminal stretch
than the RolB proteins encoded by pRi1855 (pRiA4) (Meyer et al. 2000 ). The phys-
ical presence of the rol B gene in T L -DNA segment of Ri plasmid of the infecting
Agrobacterium in leaf tissues of plants regenerated from selected rhizoclones was
demonstrated by a positive PCR amplification (Pal et al. 2012 ).
The reports have shown that the RolB may have a critical role in early steps of
hairy-root induction (Bellincampi et al. 1996 ). The root induction is totally allevi-
ated when rol B gene is inactivated in the pRiA4 on kalanchoe leaves (White et al.
1985 ). rol B also has capacity nearly as much as the wild type A. rhizogenes T-
DNA for enhancing rooting and hairy root formation on wounded N. tabacum stems
(Cardarelli et al. 1987b ; Bellincampi et al. 1996 ; Altamura and Tomassi 1998 ; Binns
and Costantino 1998 ) and leaves (Spena et al. 1987 ).
Phenotypical abnormalities such as root meristem neoformation on leaf discs and
fast growth of rol B-transgenic plants and growth pattern of rol B-induced roots are
characterized by fast growth, high branching, and plagiotropism. As a result of these