Enhancement of resistance against Rhizoctonia solani by glycine betaine and UV-C radiation in green bean (Phaseolus vulgaris L.)

Rhizoctonia solani is considered one of the most harmful pathogens affecting green bean productivity. Phaseolus vulgaris, L. cv. paulista seeds were subjected to UV-C for 60 min. and/or immersed in glycine betaine (GB, 5 mM) for 24 hours to stimulate plant biochemical defenses against R. solani. After treatments, seeds were sown in soil infested with R. solani, then seedling samples were collected after two weeks to evaluate plant resistance and some antioxidant enzymes activities. The results clearly demonstrated that disease index (DI) was highly reduced by GB (54.7%) followed by UV+GB (38.3%) and UV (35.6%). Seed treatment with UV-C for 60 min or GB (5 mM) for 24 h alone or together enhanced plant rooting in infected seedlings comparing with control. Also, there was a high correlation between the levels of lipid peroxidation and Rhizoctonia root rot disease index. Seed treated with UV-C, GB or both mitigated the effect of infection on the lipid peroxidation (Malondialdehyde levels) with percentages of 26.8, 40.2 and 29.3% respectively in comparison with the infected control. Regarding to the plant defensive enzymes, GB was the only treatment that triggered a significant increment in the superoxide dismutase (SOD) activity in roots of both infected and non-infected seedlings as compared to the control. On the other hand, guaiacol peroxidase (G-POD) and polyphenol oxidase (PPO) activities were elevated by all treatments in infected plant roots in comparison with control. The maximum activity for G-POD was attained with UV treatment (9042.5 U.mg-1 protein) followed by GB+UV, GB and infected control with values of 7776.5, 7152.4 and 6508.8 U.mg-1 protein, respectively. Similarly, PPO exhibited higher activities with significant differences in all treatments over the infected control with percentages of 54, 59.4 and 48.7% for UV, GB and UV+GB respectively. Finally, it can be concluded that treatment of green bean seeds with UV-C and GB enhanced green bean seedlings tolerance to R. solani by activation of the antioxidant enzymes and activation of defense-related enzymes like polyphenol oxidase. The results of combination of UV-C and GB treatments indicated the absence of synergetic effects in induction of plant tolerance.