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Genomics and Applied Biology, 2011, Vol.2 No.2
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involved in systemic movement in some hosts but is
dispensable in others (Scholthof et al., 1995). It was
concluded that P19 plays an important role in plant
defense suppression through RNA silencing process
although it is not known how P19 comes into contact
with siRNAs and whether P19 is recycled during this
process (Canto et al., 2006). Moreover, Lakatos et al
(2004) assumed that RNA silencing spreads through
the vascular tissue from the place of initiation to distant
parts of the plant, and sequestration of siRNAs by P19
may block the spread of a systemic silencing signal.
Park et al (2004) and Uhrig et al (2004) found that
P19 also interacts with members of the ALY-family
genes of plant RNA binding proteins. But Zhou et al
(2000) revealed that in mouse, Drosophila, human,
yeast, ALY proteins are known to be involved in the
export of mRNAs from the nucleus prior to translation
and are incorporated into the exon junction complex
of proteins that mark mRNAs during splicing. In addition,
animal ALY is also known to be a transcriptional
coactivator that possibly functions as a chaperone to
promote the interaction of DNA-binding proteins
(Virbasius et al., 1999).
Saiardi & Quagliariello (1992) revealed that direct
sequencing of cytochrome oxidase subunit
IlI (coxIll)
mRNA with a specific primer confirms RNA editing
in sunflower (
Helianthus annus
) mitochondria. The
activity of this enzyme (cytochrome oxidase) is greatly
decreased or absent in patients with chronic granulo-
matous disease, an inherited disorder characterized by
a severe defect in host defense against bacteria and
fungi (Maly et al., 1993).
It was generally thought that RNA editing which is
carried out in the plant mitochondrial cytochrome
oxidase gene and phagocytosis in animals evolved to
control the diseases, especially against bacteria and
fungi. RNA editing of the cytochrome oxidase gene
and the ALY-family genes play an important defense
role against the plant pathogen, especially viral infection.
Moreover, we suggest that the
P19
gene plays an
additional role in controlling the viral infection as well.
These enzymes work by blocking the viral silencing
process during the infection of plant cells. The main
objective of this study is to demonstrate the regulation
of defensin genes in one susceptible and two resistant
tomato cultivars inoculated with the TBSV. Our
second objective is to report the expression of the
P19
(viral defensin gene) against the plant defense system
in one susceptible and two resistant tomato cultivars.
1 Results and Disscussion
1.1 TBSV inoculation and the expression of viral
P19
gene
Severe viral symptoms were observed in cultivar
TY20 two weeks post inoculation. Mild symptoms
appeared on the other two cultivars during the same
period. Isolation of
P19
gene through the RT-PCR
technique yielded that the expression of this gene was
high in both cultivars TY70/70 and TY70/84, while
considerably low in cultivar TY20, based on amplified
band intensity. The control samples did not give any
PCR products (Figure 1). Molnar et al (2005) revealed
that TBSV P19 is a member of a class of plant and
animal virus proteins that are known to interfere with
host defense mechanism. Our results are in agreement
with Canto et al (2006), who reported that P19 protein
of TBSV is a potent suppressor of RNA silencing and
depending on the host species; it is also required for
short- and long-distance virus movement and
symptom production. In addition, the appearance of
symptoms was initially observed severe in sensitive
which supports our finding concerning the low tomato
cultivars but it was mild in resistant cultivars,
expression of
P19
gene in sensitive tomato cultivars.
These results agree with Dangl et al (1996), who
suggested that plants also developed the ability to
Figure 1 RT-PCR for the
P19
gene in the inoculated tomato
plants using the purified virus after two weeks post inoculation
Note: M: 1 kb Ladder DNA Marker; Lane pool: Genetic pool
for healthy plant of the three examined cultivars (TY20, TY70/84
and TY70/70)