Agrobacterium rhizogenes-Mediated Transformation and Its Biotechnological Applications in Crops - Crop Improvement: New Approaches and Modern Techniques - page 17

gene encodes 178-180 amino acid protein (approximately 20 kDa) that share more
than 65 % identity with each other (Meyer et al. 2000 ).
rol C transformed plants exhibited reduced apical dominance leading to increased
branching, dwarfed plants with short internodes, lanceolate leaves, early flowering,
reduced flower size and reduced pollen production (Schmulling et al. 1988 ). Dwarf-
ing was caused by reduced epidermal cell size in internodes (Oono et al. 1990 ).
Regulation of expression of rol C is complex, and varies depending upon the exis-
tence of the complete T-DNA sequences. In addition, root production was increased
compared to untransformed plants, but decreased compared to plants transformed
with the complete set of rol genes (Palazòn et al. 1998 ). Expressing rol C shows
phloem-specific expression in the root, low expression in the leaf, and no expres-
sion in the shoot tip (Schmulling et al. 1988 ; Estruch et al. 1991 ). However, rol C is
highly expressed in leaves when the whole T-DNA is present (Durand-Tardif et al.
1985 ; Leach and Aoyagi 1991 ). More recently, rol C gene has been shown to play
a role in formation of shoot meristems, hence suggesting its important role in the
formation of pluripotent stem cells (Gorpenchenko et al. 2006 ).
The rol C promoter is utilized extensively for phloem-specific gene expression
making it a useful tool in some biotechnological programs on pathogen resistance.
Replication of many plant viruses, including luteoviruses, reoviruses and most
geminiviruses transmitted by hemipteran vectors occur exclusively in phloem-
associated tissues. Therefore, by introducing an insecticidal gene that is toxic to
hemipteran vectors under the control of phloem-specific rol C is a promising way
for the control of such viruses through its expression in transgenic plants (Graham
et al. 1997 ). Similarly, a plant lectin with insecticidal activity is encoded by ASAL
( Allium sativum leaf agglutinin) gene and under control of the rol C promoter, it
confers resistance against various hemipteran pests in transgenic rice, tobacco and
chickpea plants (Saha et al. 2007 ).
rol C is known to stimulate rooting by an auxin-like effect of the gene (Schmull-
ing et al. 1988 ; Zuker et al. 2001 ; Casanova et al. 2003 ). An increase in auxin
sensitivity may lead to occurrence of the auxin-like effect. In fact, in comparison
between rol C transgenic N. tabacum protoplasts and their wild-type counterparts
showed that more sensitivity was recorded in transgenic N. tabacum in the measure-
ment of transmembrane hyperpolarization in response to auxin (Maurel et al. 1991 ;
Shoja 2010 ).
Also, abscisic acid (ABA), polyamine, and ethylene levels are extensively re-
duced due to rol C expression. The promoter of rol C activated by sucrose was found
to be very high (Yokoyama et al. 1994 ; Faiss et al. 1996 ), implying that rol C may be
influencing the source-sink relationship of a plant by regulating sugar metabolism
and transport (Nilsson et al. 1996a , b ; Martin-Tanguy 2001 ).
Alike rol B, the rol C gene is able to stimulate the production of high levels of sec-
ondary metabolites such as tropane alkaloids (Bonhomme et al. 2000 ), pyridine al-
kaloids, indole alkaloids (Palazon et al. 1998 ), ginsenosides (Bulgakov et al. 1998 )
and anthraquinone phytoalexins (Bulgakov et al. 2002b ; Shkryl et al. 2008 ; Shoja
2010 ) in transgenic plants.