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Fig. 1.1
Scanning electron
micrograph of attachment of
Agrobacterium rhizogenes
strain R1000 to sunflower
(
Helianthus annuus
L.) coty-
ledonary node cell
Citovsky
2000
; Giri and Giri
2007
; Murugesan et al.
2010
) (Fig.
1.1
). It is a close
relative of the better known
A. tumefaciens
, which is the best-characterized species
among the genus
Agrobacterium
(Rao
2009
; Ozyigit
2012
) (Fig.
1.1
).
All
A. rhizogenes
strains are characterized by the presence of a large root induc-
ing (Ri) plasmid containing a highly conserved “core” DNA region required for
hairy root formation (Filetici et al.
1987
; Gelvin
2003
; Veena and Taylor
2007
).
Like the crown gall disease, which is caused by
A. tumefaciens
(Ream
2002
; Mc-
Cullen and Binns
2006
; Ozyigit
2012
)
A. rhizogenes
causes hairy root (root-mat)
disease in infected plants through genetic transformation (Weller and Stead
2002
;
Weller et al.
2005
).
Hairy Root Disease
The “hairy root” is the term first used in 1900 by Stewart et al. (as quoted by Hil-
debrandt 1934). The distinctive symptom of hairy root disease is the formation of
a mass of roots. Following the
A. rhizogenes
infection, hairy root formation occurs
as a result of protruding large numbers of small roots as fine hairs directly from
the infection site (Chandra
2012
) (Fig.
1.2
). Besides the plagiotropic root growth,
hairy-root disease is characterized as short internodes, a high degree of lateral
branching, wrinkled leaves, reduced apical dominance, reduced fertility, profusion
of root hairs, abnormal flower production, advanced flowering, increased number
of flowers, enhanced growth rates and changed secondary metabolite accumulation
(Ackermann
1977
; Tepfer
1983
; Balandrin et al.
1985
; Charlwood and Charlwood
1991
; Pellegrineschi et al.
1994
; Flores et al.
1999
; Lee et al.
2001
; Keil
2002
; Ca-
sanova et al.
2004
; Veena and Taylor
2007
) (Fig.
1.2
).
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