Antioxidant-mediated defense response of Carica papaya L. var. eksotika against a compatible Erwinia mallotivora strain BT-MARDI
Papaya dieback disease, caused by the bacterium Erwinia mallotivora, is the most devastating papaya disease in Malaysia. Most papaya cultivars in Malaysia are susceptible to this disease which suggests that the molecular basis of the interaction between these cultivars and the pathogen is based o...
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| Main Author: | |
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| Format: | Thesis |
| Language: | English |
| Published: |
2015
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| Subjects: | |
| Online Access: | http://psasir.upm.edu.my/id/eprint/68042/1/FBSB%202015%2015%20IR.pdf |
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| Summary: | Papaya dieback disease, caused by the bacterium Erwinia mallotivora, is the
most devastating papaya disease in Malaysia. Most papaya cultivars in
Malaysia are susceptible to this disease which suggests that the molecular
basis of the interaction between these cultivars and the pathogen is based on a
compatible interaction, leading to disease development. However, in certain
pathosystems, such interaction could also develop moderate defense response
mediated by antioxidant system. Hence, it was hypothesized that early defense
response in compatible interaction of papaya and E. mallotivora is mediated by
antioxidant system. Therefore, this study was aimed to determine the
expression of antioxidant-related genes and proteins and activity of antioxidantrelated
enzymes in the early response of Eksotika papaya to E. mallotivora
infection. In addition, this study also optimized the inoculation procedures for
artificial infection of Eksotika papaya with this pathogen. Two important
parameters in the artificial infection of E. mallotivora, inoculum concentration
and leaf position, affected significantly the disease development in papaya. The
probability of the infection depends on the number of bacteria inoculated where
greater number of bacteria is required to defeat host defenses as well as the
position of leaf where lower or older leaf is less metabolically active thus, lead
to higher susceptibility to this infection. Therefore, for efficient artificial infection
of papaya with this pathogen, the use of second leaf from the top and inoculum
concentration of 107 CFU/inoculation site was recommended. Using these
optimized parameters, the inoculation of papaya with this pathogen was carried
out and the inoculated leaf samples were collected at 0, 2, 4, 8, 12 and 24 h of
post-infection (hpi). Following the bacterial challenge, through semi-quantitative
reverse-transcription-PCR (RT-PCR) method, the expression of two selected
antioxidant marker genes [superoxide dismutase (SOD) and peroxidase] was
analyzed. SOD gene was found to be transiently up-regulated at 2 hpi
whereas, peroxidase gene was up-regulated throughout the experimental period. Through enzyme assays, enzymatic activity of SOD and peroxidase
also showed a significant progressive increase occurred after 8 hpi (pvalue<
0.05). These observations were further confirmed through protein
profiling of the infected leaf proteome at 4 and 8 hpi using two dimensionalpolyacrylamide
gel electrophoresis (2D-PAGE) and mass spectrometry
analysis where a significant up-regulation of SOD protein at 4 hpi was
observed (p-value<0.10). Overall, the findings in this study proved that
antioxidant system is involved in early defense response of papaya to E.
mallotivora infection which may suggest that this defense pathway is also
activated in this compatible interaction. This study provides new insights into
molecular mechanisms underlying this interaction. |
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