Agrobacterium rhizogenes-Mediated Transformation and Its Biotechnological Applications in Crops - Crop Improvement: New Approaches and Modern Techniques - page 12

Petersen et al. 1989 ; Lee et al. 2001 ; Bensaddek et al. 2008 ). It has been reported
that the T L -DNA of the agropine-type Ri plasmid consists of at least 18 open read-
ing frames (ORF). ORF 10, 11, 12 and 15 coincided with rol A, rol B, rol C and rol D,
respectively (Slightom et al. 1986 ; Scorza et al. 1994 ).
Rol Genes
The T-DNAs have many other genes other than those opine and hormone synthesis
genes. Although their functions are not well characterized, they are known to have
very strong effects on growth. At least four genetic loci (rolA, B, C and D) were
identified in the T-DNA regions of pRiA4 by a series of deletions and transposon
insertions studies and shown to play important roles of root-inducing properties
of A. rhizogenes on the T L -DNA (Table 1.1 ) (White et al. 1985 ). The rol genes lo-
cated on the T L -DNA of Ri plasmid modify auxin and cytokinin biosynthesis and/
or endogenous hormone levels and their expressions stimulate the formation of
roots in transformed tissues (Nilsson et al. 1993a ; Maurel et al. 1994 ; Moritz and
Schmülling 1998 ; Shen et al. 1990 ; Bonhomme et al. 2000 ; Ishizaki et al. 2002 ;
Hong et al. 2006 ; Bensaddek et al. 2008 ). Studies have focused on characterizing
the three rol genes named as rol A, rol B, and rol C because they are considered es-
sential for the hairy root initiation based on transposon “loss-of-function” analysis
(White et al. 1985 ). Induced adventitious root formation by rol A, rol B and rol C
genes is shown on tobacco, kalanchoe and tomato leaves (Cardarelli et al. 1987a ;
Spena et al. 1987 ; Vilaine et al. 1987 ; Spano et al. 1988 ; van Altvorst et al. 1992 ; Ki-
yokawa et al. 1994 ) and plants carrying these genes are morphologically equivalent
to those carrying the whole T L -DNA (Spano et al. 1988 ). Inactivation or overexpres-
sion of various rol genes in stable transgenic lines or hairy-root cultures exhibits dif-
ferent variations in plant phenotypes and root morphology (Schmulling et al. 1988 ;
Martin-Tanguy et al. 1996 ; Casanova et al. 2004 ).
rol A
The rol A gene is found on all Ri plasmids and encodes a small protein with a
molecular mass of approximately 11 kDa (Nilsson and Olsson 1997 ). The rol A
gene sequence length differs in various A. rhizogenes strains ranges from 279 to
423 bp (Meyer et al. 2000 ). Analysis of amino acid sequences showed that rol A
encodes a protein with basic isoelectric point (PI 11.2). It also contains a frequent
sequence motif common in DNA-binding proteins (Suzuki 1989 ) and proposed
to function as a regulatory transcription factor (Levesque et al. 1988 ; Veena and
Taylor 2007 ).
A dramatic reduction in several classes of hormones, including auxin, cytokinin,
gibberellic acid (GA) and abscisic acid triggered by the expression of rol A gene is