الفهرس 
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Abstract
Biotic stress is considered one of the limiting factors for agricultural productivity. In the present study, Rhizoctonia solani was used as a model for biotic stress in plants. R. solani is known to be one of the most harmful pathogens that affect productivity of several crops. Many synthetic fungicides are recently used to control the disease despite their adverse consequences on human health and environment. Therefore, one of the pesticides alternatives is induction of plant resistance against a wide range of pathogens. In this study, green bean seeds were exposed to UV-C radiation for 60 min and/or treated with each of salicylic acid (SA, 5mM), glycine betaine (GB, 5mM), mannitol (5 mM) and mixed solution of all of the previous compounds for 24 hours to stimulate defensive mechanisms in seeds and grown plants against R. solani. To evaluate the effectiveness of these treatments, two experiments were designed as follows:
1- Green house experiment: in which control or treated seeds were sown in both sterilized and soil infested with R. solani. Therefore, the final design of experiment consisted of 20 treatment sets. After two weeks, root samples were collected to evaluate disease index (DI) and plant resistance machinery through determination of total phenols, antiradical activity of roots extracts, lipid peroxidation, proline concentration and some plant defensive enzymes activities.
2- In vitro experiment: which included testing the accumulation of phytoalexins in the germinated seeds after treatments through an antifungal assay using ethanolic extracts of treated germinated seeds against R. solani. Also, evaluation of the inhibitory effect of these extracts against cell wall degrading enzymes secreted by R. solani.
The results can be summarized as follows:
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SUMMARY AND CONCLUSION
M.M. ABOUL FOTOUH (2020), Ph.D., Fac. Agr., Ain Shams Univ.
A- Green house experiment
1. Disease index (DI)
All seed treatments significantly decreased the disease index (DI) in seedlings grown from treated seeds as compared with control. Seed treatment with SA (5 mM), mannitol (5 mM) and GB (5 mM) individually had the most reductive impact on DI with decrements of 58, 53 and 54.6%, respectively as compared with control. Also, it was noticed that infection with R. solani negatively affected the root system of control group, while seed treatments enhanced plant rooting in infected seedlings as compared with control.
2. Lipid peroxidation
Malondialdehyde levels were measured as an indication for lipid peroxidation. Results showed that infection with R.solani led to a significant increase in the levels of MDA as compared to non-infected groups. However, all seed treatments reduced the impact of infection on lipid peroxidation levels as compared with infected control and the most effective treatment was mix + UV which alleviated the MDA concentration by 60% as compared with control. It could be observed that the results of MDA concentrations were consistent with the severity of disease symptoms on plants roots.
3. Proline concentration
Results showed that infected roots exhibited higher proline concentrations compared with non-infected groups. Among all treatments, only UV attained a significant increase over the infected control in proline concentration with an increase of 34.4 %. In contrast, GB, GB + UV, mix and mix + UV treatments significantly reduced proline concentration in roots of plants grown from treated seeds as compared with infected control.
4. Total phenols concentration
In general, seed treatments led to the accumulation of soluble phenols in roots of non-infected green bean seedlings as compared with control. Infection with R.solani led to significant increments in the
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SUMMARY AND CONCLUSION
M.M. ABOUL FOTOUH (2020), Ph.D., Fac. Agr., Ain Shams Univ.
concentration of total phenols in all treatments except for mannitol, SA and GB as compared with their corresponding control. The highest total phenols were attained in roots of plants grown from seeds treated with mix + UV. Also, UV and GB + UV treatments led to significant increases over the infected control.
5- Free radical scavenging activity of root extracts
All extracts showed modest results in DPPH inhibition despite the significant variations. The highest antiradical activity was observed in the case of mix + UV treatment with an increment of 8% in DPPH inhibition as compared with non-infected control. Also, seed treatment with each of mannitol, SA, GB and GB + UV significantly enhanced the anti-radical activity in comparison with control. Extracts of infected roots exhibited lower antiradical activity than non-infected groups except for UV which led to the maximum antiradical activity (35.93%) among all treatments with a significant increase over the control. Mix + UV elevated the percentage of DPPH inhibition by 4% over that of the infected control, while the anti-radical activity of extracts of other treatments were lower than the control.
6- Phenylalanine ammonia lyase (PAL)
The activity of PAL was reduced in roots of plants grown from primed seeds in comparison with control except for mannitol + UV, mix and mix + UV. Seed treatment with mix solution attained the highest activity with almost 5-fold the control value. Moreover, infection with R.solani led to a considerable increase in PAL activity. Such response was more pronounced with UV (5-fold increase) and SA + UV (2-fold increase) compared to their respective control. UV and SA + UV were the most efficient treatments in stimulation of PAL activity in roots of infected plants.
7- Superoxide dismutase (SOD)
Seed treatment with mix + UV led to the greatest activity (105.7 Unit.mg-1 protein) followed by GB and UV treatments with increments of 48.5 and 12.8% as compared with non-infected control. SOD activity was
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SUMMARY AND CONCLUSION
M.M. ABOUL FOTOUH (2020), Ph.D., Fac. Agr., Ain Shams Univ.
elevated because of infection in all treatments. The highest activity was obtained with both SA + UV and GB treatments with increments of 54.6 and 38.8%, respectively as compared with infected control.
8- Guaiacol peroxidase (G-POD)
Seed priming with each of UV-C, SA + UV, GB and mix + UV led to significant increases in guaiacol peroxidase (G-POD) activity as compared to control. Infection with R.solani significantly raised G-POD activity in roots in all treatments. The response to infection was most pronounced with SA treatment. Combined treatment of mix and UV-C achieved the highest activity among all treatments with an increment of 95.4% compared with infected control. Other treatments had increases varying from 9.8% to 59.9% over the infected control.
9- Polyphenol oxidase (PPO)
Results showed that PPO activity was positively affected by all treatments in both infected and non-infected groups. The most efficient treatment was mix + UV which caused a significant increase by 118.8% in comparison with infected control.
10- Chitinase
UV-C irradiation alone or with SA attained the maximum activity with significant increases by 53.5 and 53.9%, respectively over non-infected control. Also, mannitol and mannitol + UV elevated chitinase activity in roots of non-infected plants as compared with control. Infection with R.solani led to significant increments in chitinase activity in control group and plants grown from seeds treated with UV, mannitol, GB, GB + UV, mix and mix + UV. UV was the most effective treatment in stimulation of chitinase activity.
B- In vitro experiment
1- Antifungal assay
Seed treatment with mix solution and UV-C together gave the maximum results in the inhibition of radial growth with an increment of 30.12% over the control. Also, UV, SA, SA + UV, GB + UV and mix solution treatments had a pronounced effect on the accumulation of
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SUMMARY AND CONCLUSION
M.M. ABOUL FOTOUH (2020), Ph.D., Fac. Agr., Ain Shams Univ.
antifungal compounds in germinated seeds but lower than that of mix +UV.
2- Effect of seeds ethanolic extracts on CWDE activities
i. Pectin methyl esterase (PME)
The activity of pectin methyl esterase (PME) was negatively affected by the germinated seeds ethanol extracts of all treatments. However, PME activity was mostly inhibited by the ethanolic extracts of germinated seeds treated with SA + UV, mannitol and mannitol + UV with inhibition percentages of 79, 75 and 75%, respectively. Also, extracts of germinated seeds treated with mix solution + UV or UV-C alone exhibited the same impact on PME activity as they lowered the activity by 67 and 68%, respectively.
ii. Polygalacturonase (PG)
Extracts of all treatments negatively acted on PG with inhibition percentages ranged from 6 to 27%. Such effect was maximally observed in the case of mix + UV treatment (27%) followed by mix treatment with an inhibition percentage reached to 24% as compared with the solvent control.
iii. Pectate lyase (PeL)
Treatment of green bean seeds with SA, GB + UV or mix solution attained the highest inhibitory effect for germinated seeds extracts against PeL produced by R. solani. The extent of inhibition reached to 38, 42 and 42%, respectively. Such effect was also observed with lesser extent with mix + UV treatment (32%).
iv. Cellulase
All extracts led to a significant inhibition for the enzyme activity. Extracts of germinated seeds treated with mannitol, glycine betaine and mix solution showed the most inhibitory activity to cellulase with percentages of 16, 17 and 16%, respectively. Also, extract of germinated seeds treated with glycine betaine + UV significantly lowered cellulase activity as compared with control.
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SUMMARY AND CONCLUSION
M.M. ABOUL FOTOUH (2020), Ph.D., Fac. Agr., Ain Shams Univ.
Conclusion
Results of the present study apparently declared that seed treatment with UV-C, SA, mannitol and glycine betaine enhanced plant resistance against R. solani by activation of defense mechanisms which included: (1) induction of phytolaexins biosynthesis, (2) Up regulation of pathogenesis-related proteins such as chitinase, (3) activation of the antioxidant system and (4) accumulation of CWDEs inhibitors. Such responses acted against the pathogenicity factors of R. solani and led to lower disease index values in plants grown from treated seeds.
However, it can be recommended to study seed treatments with mannitol and glycine betaine at higher concentrations in order to increase their effectiveness. Also, the chemical nature of CWDEs inhibitors in the extracts of germinated seeds in the present study needs to be further investigated to elucidate their structure and their mechanism of inhibition