Basic HTML Version
Deng et al., 2011, Ectopic expression of an
AGAMOUS
homolog
NTAG1
from
Chinese narcissus accelerated earlier flowering and senescence in Arabidopsis, Molecular Plant Breeding Vol.2 No.3 (doi: 10.5376/mpb.2011.02.0003)
18
the
35S::NTAG1
transgenic plants were often yellow
which is characteristic of senescence. Most leaves of
the transgenic plants began to lose green when Wt
plant bolted (Figure 1L). To analyze the effect of the
ectopic expression of
NTAG1
on senescence, the
MDA content, a stress or senescence indicator, was
assayed in the 14-d-old plants. It showed that the
transgenic plants had much higher MDA level than
Wild type plants (Figure 2E). In addition, the
expression of
SENESCENCE ASSOCIATE GENE 12
(
SAG 12
), a senescence marker (Gan and Amasino,
1995) was advanced by ectopic expression of
NTAG1
(Figure 2D)
2 Discussion
In Arabidopsis,
AG
is required for both floral
meristem determinacy and reproductive organ identity
(Bowman et al., 1989; Lohmann and Weigel, 2002).
In Arabidopsis,
AG
mutation results in the expansion
region of the
A
gene class into the center of the flower,
which makes stamens change into petals and carpels
into sepals. In addition, there are additional abnormal
flower produced in the center of
ag
flowers suggesting
that the floral meristem of this mutant is indeterminate.
To explore the possible molecular mechanisms
involved in double Narcissus flower, we isolated and
characterized
NTAG1
gene, an
AG
homolog from both
Chinese narcissus varieties. Both the results that the
sequence and expression pattern exhibited same in
both varieties, and evidence of functional
NTAG1
gene
existing in double flowers showed by ectopic tests in
Arabidopsis, indicate that the increase of petal number
in narcissus flower is not caused by the sequence
mutation or a deregulation of expression of
NTAG1
gene. Gao etc. al (Gao et al., 2008) isolated a
MADS-box gene
NTMADS1
, an
AG
ortholog, from
Chinese narcissus double flower plants and speculated
that the sequence variation from that of
NTAG
is the
reason of double flower formation in narcissus.
According to our data, such sequence difference may
be caused by PCR or resulted from the plant materials
got from different places. Based on the flowering
ABCE model, the double flower might be caused by
the restriction of the
AG
ortholog expression, or by the
expansion of A-function genes expression domains
(Dubois et al., 2010). So further analysis of
NTAG1
expression domains, the isolation and characterization
of other C-function genes and A-function genes in
narcissus, will be necessary to draw a clear
conclusion.
Not similar to the most
AG
homolog transgenic plants
(Mizukami and Ma, 1992; Mizukami and Ma, 1997;
Tzeng et al., 2002), ectopic expression of
NTAG1
in
Arabidopsis is not often accompanying with the
homeotic transformation flower (Less than 10% lines
in T
1
generation). Especially with the generation
increased, the transgenic plants developed normal
flowers. There are three explanations for such data.
Firstly, the expression of
NTAG1
gene in most
transgenic plants, especially in higher generations,
was not enough for the flower transformation.
Secondly,
NTAG1
gene might not be the only one
C-function genes in narcissus. Indeed, the C-function
is shared by two partially redundant genes in many
plant species (Dubois et al., 2010). For instance,
AG
performs the sexual organ identity and floral meristem
termination, whereas,
SHATTERPROOF
(
SHP
) is
involved in later carpel development stages in
Arabidopsis (Causier et al., 2005). Conversely, in
Antirrhinum majus
L., the ortholog of
SHP
,
PLENA
is
essential for sexual organ identity (Davies et al., 1999).
It is therefore of interest to identify
SHP
lineage in
narcissus. Additionally, in line with the quartet model
(Theissen, 2001), another reason might be the
different
NTAG1
interaction factors existed in
narcissus from that in Arabidopsis.
The novel phenotypes of the
NTAG1
transgenic plants
include early flowering, losing inflorescence
indeterminacy and increasing branch number, which
are usually candidate characters of ornamental plants
selected by people. So it will be an ideal candidate
gene for the genetic modification in narcissus or other
plants. However, ectopic expression of
NTAG1
accelerated earlier senescence. This might be a
consequence of the competition between sink and
source, the poor source caused by fewer and smaller
leaves, the competitive sink characterized by early
reproductive development. So in order to obtain more
novel phenotypes, adjust the
NTAG1
expression
driven by certain promoter such as inducible promoter
await for future performance.