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Aim To understand how beneficial bacteria assist chilli plants (Capsicum annuum) in defence against biotrophic or hemibiotrophic pathogens. Method and Results We quantified marker genes of plant defence pathways in Phytophthora capsici-infected chilli pepper treated with anti-oomycete plant growth-promoting rhizobacteria, Bacillus amyloliquefaciens, Bacillus velezensis and Acinetobacter sp. Plants displayed strong resistance, and the pathogen load in the roots was significantly lower in infected plants treated with bacterial biocontrol agents at all time points tested (1, 2 and 7 days after pathogen inoculation, p < 0.05). Gene expression profiling revealed that P. capsici infection in the absence of beneficial bacteria led to the upregulation of a wide array of defence genes. The addition of biocontrol bacteria modulated defence by further enhancing genes involved in programmed cell death, such as CaLOX1, CaPAL1, CaChitIV and CaPTI1, while suppressing others CaLRR1, a negative regulator of cell death. Conclusions Our results suggest that the bacteria exerted a combined effect by directly antagonizing the pathogen and enhancing the expression of key plant defence genes, including those involved in cell death, causing resistance at early stages of infection by this hemibiotrophic pathogen.

Some bacteria can antagonize and competitively exclude pathogens by producing antimicrobial substances in their immediate environment. These antimicrobial agents can be grouped into enzymes, protein such as polypeptides and non-protein compounds. Of the various crop pests and pathogens that devastate harvests, oomycetes and fungi are the most broadly distributed groups causing global issues in food security and environmental damage in natural ecosystems. In this study, three bacterial isolates, Bacillus amyloliquefaciens (UQ154), Bacillus velezensis (UQ156) and Acinetobacter sp. (UQ202), previously shown to promote plant growth and anti-oomycete properties against Phytophthora spp., were selected for further investigation. These isolates were subjected to extractions with dichloromethane. The crude extracts were fractionated by preparative silica gel thin-layer chromatography and tested for their anti-oomycete activities against Phytophthora capsici. The results show that nine out of 16 fractions exhibited high (> 50%) P. capsici growth inhibition, comparable to the application of the live bacterial isolates. Using GCMS analyses, the compounds identified from the bacterial fractions were pyrrolopyrazine and various phenolic compounds, including the previously identified diketopiperazine compounds (DKPs) from the crude bacterial extracts. The ability of these bacterial isolates and their bioactive compounds to inhibit plant pathogen growth deserves further investigations, offering a potential roadmap for the development of bio-pesticides.

Achromobacter spanius UQ283 is a soilborne bacterium found to exhibit plant growth-promoting and disease-suppressing attributes in several plant species. Accordingly, we used long-read sequencing to determine its complete genome sequence., Achromobacter spanius UQ283 is a soilborne bacterium found to exhibit plant growth-promoting and disease-suppressing attributes in several plant species. Accordingly, we used long-read sequencing to determine its complete genome sequence. The assembled genome will aid in understanding the multifaceted interactions between plant growth-promoting rhizobacteria, pathogens, and plants.

Biotic interactions through diffusible and volatile organic compounds (VOCs) are frequent in nature. Soil bacteria are well-known producers of a wide range of volatile compounds (both organic and inorganic) with various biologically relevant activities. Since the last decade, they have been identified as natural biocontrol agents. Volatiles are airborne chemicals, which when released by bacteria, can trigger plant responses such as defence and growth promotion. In this study, we tested whether diffusible and volatile organic compounds (VOCs) produced by soil bacterial isolates exert anti-oomycete and plant growth-promoting effects. We also investigated the effects of inoculation with VOC-producing bacteria on the growth and development of Capsicum annuum and Arabidopsis thaliana seedlings. Our results demonstrate that organic VOCs emitted by bacterial antagonists negatively influence mycelial growth of the soil-borne phytopathogenic oomycete Phytophthora capsici by 35% in vitro. The bacteria showed plant growth promoting effects by stimulating biomass production, primary root growth and root hair development. Additionally, we provide evidence to suggest that these activities were deployed by the emission of either diffusible organic compounds or VOCs. Bacterial VOC profiles were obtained through solid phase microextraction (SPME) and analysis by gas chromatography coupled with mass spectrometry (GC–MS). This elucidated the main volatiles emitted by the isolates, which covered a wide range of aldehydes, alcohols, esters, carboxylic acids, and ketones. Collectively, twenty-five VOCs were identified to be produced by three bacteria; some being species-specific. Our data show that bacterial volatiles inhibits P. capsici in vitro and modulate both plant growth promotion and root system development. These results confirm the significance of soil bacteria and highlights that ways of harnessing them to improve plant growth, and as a biocontrol agent for soil-borne oomycetes through their volatile emissions deserve further investigation.