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Molecular Plant Breeding 2011, Vol.2, No.8, 48
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Northern blotting analysis showed the inactivation of
cecropin B
gene occurred at transcriptional level.
Transcriptional gene silencing in plant is often
associated with DNA methylation in the promoter
region and the 5’ un-translated region. DNA
methylatin-correlated gene inactivation can be
released after treatment with methylation inhibitors
such as 5
-
azacytidine (Kohli et al., 1999; Kumpatla et
al., 1997). Further experiments need to be conducted
for investigating whether DNA methylation is
involved in the
cecropin B
gene silence.
An interesting result of the present study is that we
found Jingyin 119 is a special transgenic line for
transgenes to be expressed stably. Both
cecropin B
and
bar gene were stably expressed in Jingyin 119 donor
and all its resulting hybrids, ignoring the complex
crossbreeding transmission and across generations.
Previously, the stable inheritance and expression of
exotic
bar
and
cecropin B
gene in this Jingyin 119
transgenic line was confirmed over six successive
generations (Kumpatla et al., 2001). Here we
presented the foreign
bar
and
cecropin B
gene in this
transgenic line kept expressing across long-term
generations of self-pollination (T12) and after multiple
crossing transmission. It is interesting to inquire into
the following questions: is the integration structure of
exotic
bar
and
cecropin B
in Jingyin 119 donor very
suitable for their expression? Are there safe
chromosomal locations in rice genome for transgenes
to reside and escape from the rice genome supervision
and modification system, just as the locations in
Jingyin 119 genome where exotic bar and
cecropin B
integrated? Meyer (1995) proposed that endogenous,
transcriptional active sequences contain cis-acting
flanking regions are necessary for adequate
functioning of genetic machinery, for these sequences
can facilitate DNA bending or elicit different
trans-acting factors. Such ‘isochore’ sequences are
special sequences. Any deviation from isochore
structure is recognized as a foreign element, which
may lead to elimination or silencing of transgene.
Thus, how and why foreign bar and
cecropin B
in
Jingyin 119 could escape from the recognition and
attack of the rice genome defending system?
Information regarding the chromosomal locations
favouring transgene expression is very limited so far.
The excellent performance of transgenes in Jingyin
119 transgenic line provides a promising hint to seek
for these suitable chromosomal locations for transgene
integration. Further experiments are necessary to
discover the transgene structure and integration
positions on rice chromosome of
bar
and
cecropin B
gene in Jingyin 119 transgenic.
Another phenomenon we found in present study is
that conventional crossing transmission can affect the
expression status of non-selected
cecropin B
gene. The
inactivation of
cecropin B
gene in crosses of TR 6 and
the maintaining successive expression activity of
cecropin B
gene in some crosses of Ming B implied
that effect from host genotypic constitution on
transgene expression can not be ignored. Under these
conditions, the effects from integration position, copy
number and rearrangement of transgenic loci on
cecropin B
expression could be ruled out, because the
integration patterns of
cecropin B
gene in hybrids are
exactly the same to their corresponding transgenic
donors. The mechanism to explain how host genotypic
constitution influenced transgene expression is
possible at epigenetic level, such as change of
methylation status of
cecropin B
gene expression
cassette. The alteration in genotype backgrounds by
conventional crosses might prevent, or at least delay,
the epigenetic modification process of transgenes in
some circumstances, which were illustrated by the
expression activity of
cecropin B
in some hybrids of
Ming B.
On the other hand, we used the progenies of
transgenic donors and hybrids as materials to
investigate the transgene expression behaviour in
crossing transmission. It remains unclear whether
transgene inactivation in hybrids occurred imme-
diately after cross (in F
1
generation) or through
generations. The results of F1 hybrid of TR 5/TR 6
may throw some light on this question. Cecropin B
gene silenced in TR 5 donor but expressed normally in
TR 6 donor. The subsequent F1 hybrid of TR 5/TR 6
lost its express activity of
cecropin B
(Figure 1D).
This implied the inactivation of
cecropin B
gene
occurred immediately after cross. Fojtová et al (2006)
revealed in tobacco that meiosis could not alter the