Study On The Inhibition Mechanism And Fresh-keeping Effects Of Bacillus Subtilis On Kiwifruit Soft Rot

Kiwifruit soft rot is an improtant disease which often occurs in the post-ripening period.The disease is extremely harmful and can make the inside of the fruit deteriorate,lose its edible value so that causing major economic damage.Although chemical control is main measure to control the kiwifruit rot,people are looking for alternative control methods because of the harm of chemical agents.However,biological control has been widely studied and applied in the prevention and control of fruit and vegetable diseases due to its high control efficiency and low environmental pollution.We explored the inhibitory effects on kiwifruit soft rot pathogens of the biocontrol bacterium Bacillus subtilis strain BS-1.Then,we further explored the mechanism of strain BS-1's inhibition of pathogenic fungi and analysised the possible antifungi components of strain BS-1.Finally,we evaluated the preservation effects on the shelf life of kiwifruit.This topic provides a theoretical basis for biological control of kiwifruit soft rot,effectively improving kiwifruit postharvest storage quality,exploring the prospects of BS-1 strain for biological control and the development and application of BS-1 strain in the future.The specific research content and conclusions are as follows:(1)We tested the antifungi activity of BS-1 strain in vitro,the results showed that the strain had a strong antagonistic effect on the 12 tested kiwifruit soft rot pathogenic fungi.After data analysis,the size of the inhibition zone was between 4.01 mm and 13.66 mm after 7 days of experimental treatment.The inhibition zones of Botryosphaeria dothidea and Diaporthe nobilis,the main pathogens of kiwifruit soft rot,were 5.04 mm and13.66 mm,respectively.(2)Comparing the antifungal effects of the volatile components and the fermentation broth of the BS-1 strain in vitro,it was found that the growth of B.dothidea and D.nobilis in the volatile component treatment group was consistent with that of the control group.While the colony diameters of the two pathogenic fungi in the fermentation broth treatment group were significant smaller than the control group,the growth is significantly inhibited.The antifungal stability of the fermentation supernatant was further explored,and it was found that the fermentation supernatant has good antibacterial ability in the temperature range of 37 ?-100 ?,the ultraviolet irradiation time range of 0.5 h-2 h,and the pH range of 3-5 and 8-10.These results indicated that the main antibacterial active substances produced by this strain may exist in the fermentation supernatant and have antifungal stability,rather than volatile components.(3)We found that the fermentation broth can reduce the area of lesions caused by B.dothidea and D.nobilis in kiwifruit,indicating that the fermentation broth can inhibit the pathogenicity of the pathogens on the fruit,and further indicate that the antifungal substances of the BS-1 strain are present in the fermentation broth.(4)Preliminary exploration of the antifungal mechanisms of the fermentation broth showed that it could inhibit the germination of pathogenic fungal spores,destroy the mycelial cell membrane and cause the death of the hyphae,and enhance the disease resistance of the kiwifruit.The fermentation broth could promote the expression of kiwifruit respiratory burst oxidase homolog(RBOH)gene family,and stimulate stronger and more lasting reactive oxygen species(ROS)fluorescence and increase superoxide dismutase(Superoxide dismutase).Dismutase(SOD),peroxidase(Peroxidase,POD)and polyphenol oxidase(Polyphenol oxidase PPO)activities.(5)The antifungal active ingredients of the fermentation broth were identified,and the results showed that the crude protein and crude lipids extracted from the fermentation broth have antifungal effects in vitro and in vivo.Further analysis of the two components showed that the molecular weight of the antifungal protein component ranged from 34 kDa to 75 kDa,and the lipid antifungal component was probably iturin.(6)Finally,it was also found that BS-1 and fermentation supernatant significantly delayed fruit softening,improved fruit fragility,firmness and other qualities,reduced fruit weight loss rate and rot rate,and thus improved the postharvest quality and prolonged shelf life of kiwifruit.The effects of fermentation supernatant were more significant than that of BS-1.