Agrobacterium rhizogenes-Mediated Transformation and Its Biotechnological Applications in Crops - Crop Improvement: New Approaches and Modern Techniques - page 18

rol D
The rol D gene is found only in T L -DNA of agropine type Ri plasmids. It is also
the only rol gene that is incapable of inducing root formation on its own (Mauro
et al. 1996 ). The rol D gene size 1,032 bp and encodes a protein of 344 amino acids
(Meyer et al. 2000 ; Christey 2001 ). This is a cytosolic protein with a sequence
similar to ornithine cyclodeaminase (OCD) that catalyzes the conversion of orni-
thine to proline. Proline is an osmoprotectant and its accumulation is considered
to be a defense response as a result of environmental stress in many plant species
(Mauro et al. 1996 ; Trovato et al. 2001 ; Bettini et al. 2003 ). High levels of proline
accumulation are in flowers suggesting a role in flowering (Trovato et al. 2001 ).
The pleiotropic effects induced by expression of rol D gene in transgenic plants are
increased flowering and reduced rooting (Mauro et al. 1996 ; Trovato et al. 2001 ).
Although flower yield is accelerated, the flowers show heteromorphic incompa-
bility, which prevents self-fertilization. Production of viable seeds is achieved
through manually-selfed plants (Mauro et al. 1996 ). However, it should be noted
that these experiments were conducted using the rol D sequence from pRi1855. It
has been reported that the induction of flowering is not performed by rol D from
pRiHRI (Lemcke and Schmulling 1998 ). rol D exhibits poor tissue- or organ-spe-
cific expression in comparison with other rol genes but is shown to have a pre-
dominantly developmental expression pattern (Vilaine and Casse-Delbart 1987 ).
Activity is seen in the elongating and expanding tissues of each organ in adult
plants, but never in apical meristems. As the plants age, expression decreases and
ceases at senescence. The mutations in rol D appear to accentuate callus growth
giving rise to initiation of tumor formation resembling the Ti-plasmid infection
(Trovato et al. 1997 ).
rol B TR (rol B HomologueinT R -DNA)
A rol B homolog on T R -DNA in the agropine type Ri plasmid was discovered and
named as rol B TR . Excluding the 5′ or 3′ flanking sequences, there is a 53 % nucleo-
tide similarity between rol B TR and rol B in their sequences (Bouchez and Camilleri
1990 ). The expression of rol B TR in N. tabacum is shown to cause phenotypical
alterations such as wrinkled leaves bent strongly downward, formed shoots at the
base of the stem and retarded growth is observed which are different than rol B
phenotype. Two big differences were noted by the alignment of protein sequences
of rol B and rol B TR . First, a CX5R motif is absent in rol B TR and second, N-terminal
part of RolB TR contains 14 amino acids and mutations in the corresponding se-
quence in rol B TR gene cause abolishment of the altered phenotype (Lemcke and
Schmulling 1998 ).