Performance And Mechanism Of Bacillus Subtilis SL-44 On Disease Prevention And Growth-promotion Of Capsicum Annuum

In view of the serious problems of red pepper（Capsicum annuum）disease in seedling stage caused by Rhizoctonia solani in Xinjiang region,the biological control method was adopted to inhibit plant disease.The effect of disease prevention and growth-promoting by Bacillus subtilis SL-44 was investigated,in order to biocontrol R.solani and promote the growth of pepper plants;The defense-related resistance of plant was investigated to analyze disease control mechanism and signal pathways activated by SL-44;The antimicrobial compounds was extracted and identified from SL-44 culture filtrate to clarified the types of antimicrobial lipopeptides.The main conclusions are summarized below:1.This study was aimed at evaluating the ability of SL-44 to control R.solani and promote pepper growth.Strain SL-44 was an Bacillus subtilis isolated from plant rhizosphere.Pot experiment results indicated that the dry and fresh weights of pepper in SL-44 and Rhizoctonia solani（S-R）treatment were45.5%and 54.2%higher than those in R.solani（R）treatment,and 18.2%and 31.8%higher than those in CK（control）treatment.The plant height in S-R treatment increased by 14.2%and 9.0%compared with those in the R and CK treatments,respectively.In vitro antagonism assay showed that SL-44 exhibited strong antifungal activity against the mycelial growth of R.solani with an inhibition rate of 42.3%.The amount of phosphorus dissolved by SL-44 reached 60.58 mg?L^-1 in broth.In addition,7.5μg?mL^-1 IAA were secreted by SL-44.Strain SL-44 inhibited the growth of R.solani and improved biomass of pepper plants.SL-44 has ability in colonizing and interacting with plant root.Mass exchange and information transmission between the pepper plants and SL-44 mutually promoted their development.Bacillus subtilis SL-44 has a great potential as biocontrol agent against R.solani on pepper plants.2.The effect of SL-44 on the defense responses of pepper plants was investigated,and the signal pathways that the defense responses of pepper plants stimulated by SL-44 were analyzed.After 12 hours of mixed culture,destruction and breakage of R.solani mycelia caused by SL-44 cell-free filtrates were observed;The activity of defense-related enzymes SOD,POD,CAT,PPO and PAL in pepper leaves inoculated with SL-44 was significantly higher than that of the control;plant histochemistry and fluorescein staining experiments showed that a large amount of reactive oxygen species（ROS）and callose accumulated in the leaf of the pepper in SL-44 treatment.However,chitinase andβ-1,3-glucanase activity have no increasing in plant leaves in SL-44 treatment compared to the control;RT-PCR experiments showed that the expression level of CaPINII which related to Jasmonic acid-dependent signaling pathway was significantly increased in SL-44 treatment.Based on the above results,Bacillus subtilis SL-44activates the JA-dependent signaling pathway in pepper plants,resulting in induced systemic resistance（ISR）in pepper plant.3.The antagonistic compounds were obtained by separating,filtering and concentrating Bacillus subtilis SL-44 culture filtrate.The stability of antagonistic compounds against temperature,pH,organic solvents was examined.By examining the effects of the antagonistic compounds,it was found that when the temperature was 30°C and the pH was 5.0,the inhibition zone reached the maximum.While the organic solvent was chloroform,the best antagonistic effect was obtained.The identification of antagonistic compounds revealed that sufactin,iturin and fengycin which belong to lipopeptides were contained.The optimization of the fermentation medium for SL-44 strain revealed that the maximum OD₆₀₀ in LB medium was 13.6.Meanwhile,the pH of SL-44 broth of LB medium is also within a reasonable range.While the TY medium is more suitable for producing large amounts of spores by SL-44,it is also suitable for biological control treatment of SL-44 on large-scale application in agriculture.Based on the above analysis,Bacillus subtilis SL-44 has good control effect on Rhizoctonia solani growth and can promote the growth of pepper,it also induce systemic resistance in pepper.The key to SL-44 inhibit Rhizoctonia solani is to produce antifungal lipopeptides like iturin and fengycin.Therefore,Bacillus subtilis SL-44 has potential application value and is expected to be widely used as a biocontrol agent in modern agriculture.