Abstract
Bacterial isolates obtained from the rhizosphere of Arabidopsis and a plantless compost potting mix was screened for anti-oomycete activity against Phytophthora capsici, Phytophthora citricola, Phytophthora palmivora, and Phytophthora cinnamomi. Three out of 48 isolates exhibited more than 65% inhibition against all tested Phytophthora species and were selected for further studies. These strains, named UQ154, UQ156, and UQ202, are closely related to Bacillus amyloliquefaciens, Bacillus velezensis, and Acinetobacter sp., respectively, based on 16S rDNA sequence analysis. The isolates were evaluated for their ability to fix nitrogen, solubilize phosphate, as well as for siderophore, indoleacetic acid, cell wall degrading enzymes and biofilm production. Their plant growth promoting activities were evaluated by measuring their effect on the germination percentage, root and shoot length, and seedling vigor of lettuce plants. All of these traits were significantly enhanced in plants grown from seeds inoculated with the isolates compared with control plants. Moreover, bacteria-inoculated P. capsici-infected chili plants exhibited improved productivity based on CO2 assimilation rates. Both real-time quantitative PCR and disease severity index revealed significant decreases in pathogen load in infected chili root tissues when plants were previously inoculated with the isolates. Biocontrol activity may result from the secretion of diketopiperazines as identified by Gas chromatography-mass spectrometry analysis of bacterial cultures’ extracts. Collectively, this work demonstrates the potential of bacterial isolates to control Phytophthora infection and promote plant growth. They can, therefore be considered as candidate microbial biofertilizers and biopesticides.
Highlights
Food production and agricultural yields must increase worldwide to meet the demands of an increasing human population (Bommarco et al, 2013; Ibarrola-Rivas et al, 2017; Tilman et al, 2017)
Soil bacterial isolates were identified by near-full length 16S rDNA gene amplification and sequencing
The highest hits with 99% similarity obtained after blasting the sequence reads against the National Center for Biotechnology Information database1 are shown in Supplementary Table S1
Summary
Food production and agricultural yields must increase worldwide to meet the demands of an increasing human population (Bommarco et al, 2013; Ibarrola-Rivas et al, 2017; Tilman et al, 2017). Phytophthora cinnamomi, a soilborne oomycete holds a special interest due to its broad host range, mostly of woody species (Bergot et al, 2004; Cahill et al, 2008) It is considered one of the most profoundly destructive forest pathogens globally, and is responsible for severe crop diseases (Zentmyer, 1980; Shearer and Tippett, 1989). In Australian soils, P. cinnamomi is widely distributed and causes significant devastation in in horticultural crops such as macadamia, pineapple and avocado and in many endemic plant species (Cahill et al, 2008) Phytophthora capsici is another soil-borne oomycete that can survive in the soil for several years as oospores. We identified six diketopiperazines (DKPs); from extracts of each of the three bacterial isolates namely hexane, dichloromethane, and ethyl acetate
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