Agriculture Reference
In-Depth Information
Fig. 1.6
Schematic represen-
tation of gene locations on
T
R
-DNA
morphogenic loci located on the T
R
-DNA are counterpart of the tms loci located
on the Ti plasmids and involve in hairy root tumorigenesis (White et al.
1985
). In
A. rhizogenes
infected
Nicotiana glauca
tissue, the transcripts of the tms loci of Ri
plasmids are found to be similar in size to those transcripts found in the tms region
of Ti-plasmids (Willmitzer et al.
1983
; Taylor et al.
1985
; Vilaine and Casse-Delbart
1987
). Similar transcripts were also found in carrot plants regenerated from tissues
infected with
A. rhizogenes
(De Paolis et al.
1985
; Vilaine and Casse-Delbart
1987
).
The root induction is probably due to auxin biosynthesis carried out by the aux loci
located on T
R
-DNA. The aux loci are found to be homologous to the tms loci of
A.
tumefaciens
T-DNA (Vilaine and Casse-Delbart
1987
).
aux
1,
aux
2,
rol
B
TR
,
mas
1,
mas
2, and
ags
genes located on the T
R
-DNA are
responsible for the biosynthesis of agropine and auxin, which cause differences
in hairy root growth and morphology when compared to non-transformed roots
(Fig.
1.6
). It was also reported that the presence of these genes on transformed plant
cells caused increase auxin sensitivity (Grant et al.
1991
; Lambert and Tepfer
1992
;
van der Salm et al.
1997
; Hansen et al.
1997
; Meyer et al.
2000
; Alpizar et al.
2006;
Nemoto et al.
2009
).
Sequence analysis revealed two open reading frames corresponding to proteins
of 749 amino acids as
aux
1 gene protein and 466 amino acids
aux
2 gene protein
(De Paolis et al.
1985
; Camilleri and Jouanin
1991
; Gaudin and Jouanin
1995
;
Christensen et al.
2008
; Chandra
2012
). Auxin biosynthetic pathway comprises two
steps. The
t2m
(tryptophan 2- monooxygenase) gene product encoded by the
aux
1
catalyzes the conversion of tryptophan to indole-3-acetamide (IAM) (Comai and
Kosuge
1982
; Van Onckelen et al.
1986
; Camilleri and Jouanin
1991
). Then, IAM
is converted to indole-3-acetic acid (IAA) by IAM hydrolase, the product of the
aux
2 (Jouanin
1984
; Schröder et al.
1984
; Thomashow et al.
1984
). The T-DNA of
mannopine, cucumopine and mikimopine type strains in Ri plasmids do not carry
aux
genes. Since these strains are still capable to induce a “hairy-root” phenotype, it
can be said that the presence of the
aux
genes on T
R
-DNA is not necessary to gener-
ate hairy root phenotype. It has been demonstrated that the
aux
genes are required
to support the “hairy root” phenotype and to extend the host range of the bacterium
(White et al.
1985
; Cardarelli et al.
1987b
; Hansen et al.
1991
; Sevon and Oksman-
Caldentey
2002
).
Hybridization experiments also revealed that the genes encoding agropine bio-
synthesis (
ags
) are also located on the T
R
-DNA region (Willmitzer et al.
1982
;
Huffman et al.
1984
; Lahners et al.
1984
; Vilaine and Casse-Delbart
1987
; Giri and
Search WWH ::
Custom Search