Abstract
Problem statement: Chitin, a common constituent of insect exoskeleton, could be hydrolyzed by chitinase. The research was conducted to screen chitinolytic rhizobacteria isolated from rhizosphere of chilli pepper and to determine their chitinase activity in degrading chitin of whitefly, Bemisia tabaci Genn. (Hemiptera: Aleyrodidae). Whitefly is recognized as an important pest on many crops including chilli pepper. Approach: Screening and molecular identification based on 16S rRNA sequence of chitinolytic isolates, chitinase productions, measurement of chitinase activity, characterization of chitinase and effect of the chitinase treatment on whitefly were studied. Results: A total of 25 isolates of rhizobacteria formed a clear zone on solid chitin media. Two isolates, i.e., I.5 and I.21 isolates had the highest chitinolytic index. Based on sequence of 16S rRNA gene, the isolates of I.5 and I.21 were identified as Bacillus sp. and Bacillus cereus, respectively. The highest chitinolytic index and specific activity of I.5 isolate was 0.94 and 0.11 U mg-1 proteins, respectively. Maximum production of I.5 chitinase was occured after 36 h cultivation at 30°C and pH 7.0. The highest chitinolytic index and specific activity of I.21isolate was 0.75 and 0.114 U mg-1 proteins, respectively. Maximum production of I.21 chitinase was occured after 36 h cultivation at 55°C and pH 7.0. Cell culture and crude enzyme of the isolates were tested on chitin of B. tabaci and the effect was observed using a microscope and sterile water was used as a negative control. Hydrolytic observation showed that crude enzyme of I.21 isolate could degrade chitin of B. tabaci exoskeleton and the activity was better than that of I.5 isolate. Conclusion: Chitinase produced by Bacillus cereus I.21 strain has potential application as biocontrol agents for B. tabaci.
Highlights
Root colonizing bacteria that exert beneficial effects on plant development via direct or indirect mechanisms have been defined as Plant Growth Promoting Rhizobacteria (PGPR)
The screening was conducted based on chitinolytic index that was defined as a ratio of a clear zone and colony diameter
Properties of chitinolytic isolates: There were 25 PGPR isolates performing a clear zone around their colony on chitin agar (Table 1)
Summary
Root colonizing bacteria (rhizobacteria) that exert beneficial effects on plant development via direct or indirect mechanisms have been defined as Plant Growth Promoting Rhizobacteria (PGPR). Application of PGPR may become a promising biocontrol agent for plan diseases. Large scale application of PGPR to crop as inoculants will substantially reduce the use of chemical fertilizers and pesticides, which often pollute the environment (Bloemberg and Lugtenberg, 2001). The whitefly, Bemisia tabaci Genn., (Hemiptera: Aleyrodidae) is an important pest of many crops. It attacks more than 500 plant species belong to 63 plant families. The insect is a vector of plant viruses member of Geminivirus group (Damayanti et al, 2007). Whitefly-transmitted plant viruses have increased in prevalence and distribution (Damayanti et al, 2007). The impact has been devastating with yield losses ranging from 20-10%, depending upon the crop, season and prevalence of the whitefly. Some diseases associated with the whitefly are lettuce necrotic yellows, irregular ripening of tomato, silver leaf of squash, cotton leaf curl, tobacco leaf curl and cassava mosaic
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