Agrobacterium rhizogenes-Mediated Transformation and Its Biotechnological Applications in Crops - Crop Improvement: New Approaches and Modern Techniques - page 21

et al. 1989 ). In N. tabacum leaf discs harboring rol B and ORF13 genes had capac-
ity to induce rooting almost as well as the full length of T L -DNA (Aoki and Syono
1999 ). The results obtained via co-inoculation of leaf discs achieved using the rol A,
rol B and rol C with either ORF13 or ORF14 showed a limited root induction on car-
rot disks (Capone et al. 1989 ). A comparison from the studies showed that there is
no homology between ORF13/ORF14 and auxin biosynthetic genes. Furthermore,
unlike the genes controlling biosynthesis of auxin (Camilleri and Jouanin 1991 ),
ORF13 and ORF14 have no activity for the induction of roots on N. tabacum leaf
discs (Cardarelli et al. 1987b ). A highly divergent gene family known as plast gene
family is constituted by rol B, rol C, ORF13 and ORF14. They have similar func-
tions and are thought to be evolutionary related (Levesque et al. 1988 ).
The ORF13 gene is approximately 600 bp in size, encoding a 197-200 amino
acid protein, whose expression leads to higher levels in leaves and roots (Durand-
Tardif et al. 1985 ; Veena and Taylor 2007 ). ORF13 gene leads to the formation of
induce cell proliferation such as dense green and rapidly proliferating callus on
transformed carrot root and tobacco leaf discs (Capone et al. 1989 ; Frundt et al.
1998 ; Dodueva 2007 ). Wound-inducible and organ-specific expression of ORF13 in
transgenic plants lead to a variety of characteristic modifications including irregular
formation of leaves, severe leaf nervure, shortened and variable internode length,
abnormal and asymmetric flowers, agravitropic root growth and a reduced cell
number and cell size in the root (Hansen et al. 1993 , 1997 ; Lemcke and Schmulling
1998 ; Veena and Taylor 2007 ). Accelerated expression level in ORF13 gene trig-
gered a more severe reduction of growth in stem and roots through TC-dependent
overproduction of the ORF13 gene product, affecting both cell number and cell size
in the root. Interestingly, growth and gravitropism was normal in the ORF13 high
expressers (Lemcke and Schmulling 1998 ).
Expression of ORF13 provokes specific phenotype similar to cytokinin-treated
plants however free or bound cytokinin content of the transformed tissues shows
no difference from wild-type (Medford et al. 1989 ; Hansen et al. 1993 ; Lemcke
and Schmulling 1998 ). Furthermore, the shoot part of the ORF13 transformed plant
does not resemble cytokinin-overproducing plants, indeed the growth reduction re-
sults from the inhibition of cell division in the apical meristems and development
of leaves (Lemcke and Schmülling 1998 ). Some of the phenotypic alterations in
transgenic plants are thought to arise from interaction of ORF13 with hormone
signaling pathways. ORF13 may play roles in hormone homeostasis and regulation
of the cell cycle in infected cells (Veena and Taylor 2007 ). The observations and
grafting of transgenic shoots onto wild type plants revealed that ORF13 may cause
the production of a diffusible factor with cytokinin-like activity (Hansen et al. 1993 ;
Dodueva 2007 ).
Since the only T-DNA gene that induces cell proliferation is ORF13, when in-
oculated with both carrot discs and tobacco leaf discs produce green callus (Hansen
et al. 1993 ; Frundt et al. 1998 ). Application of exogenous cytokinin increases the
number of roots produced from ORF13 tobacco leaf discs, but does not change root
induction on untransformed, even though there was no difference in endogenous cy-
tokinin levels (Specq et al. 1994 ; Lemcke and Schmulling 1998 , Britton et al. 2008 ).