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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.

Plant diseases are the primary issue that reduces agricultural yield and production, causing significant economic losses and instability in the food supply. In plants, citrus is a fruit crop of great economic importance, produced and typically grown in about 140 countries. However, citrus cultivation is widely affected by various factors, including pests and diseases, resulted in significant yield and quality losses. In recent years, computer vision and machine learning have been widely used in plant disease detection and classification, which present opportunities for early disease detection and bring improvements in the field of agriculture. Early and accurate detection of plant diseases is crucial to reducing the disease’s spread and damage to the crop. Therefore, this paper employs a two-stage deep CNN model for plant disease detection and citrus diseases classification using leaf images. The proposed model consists of two main stages; (a) proposing the potential target diseased areas using a region proposal network; (b) classification of the most likely target area to the corresponding disease class using a classifier. The proposed model delivers 94.37% accuracy in detection and an average precision of 95.8%. The findings demonstrate that the proposed model identifies and distinguishes between the three different citrus diseases, namely citrus black spot, citrus bacterial canker and Huanglongbing. The proposed model serves as a useful decision support tool for growers and farmers to recognize and classify citrus diseases.

The Gram-negative bacteria Xanthomonas oryzae pv. oryzae, the causative agent of bacterial leaf blight (BLB), received attention for being an economically damaging pathogen of rice worldwide. This damage prompted efforts to better understand the molecular mechanisms governing BLB disease progression. This research revealed numerous virulence factors that are employed by this vascular pathogen to invade the host, outcompete host defence mechanisms, and cause disease. In this review, we emphasize the virulence factors and molecular mechanisms that X. oryzae pv. oryzae uses to impair host defences, recent insights into the cellular and molecular mechanisms underlying host-pathogen interactions and components of pathogenicity, methods for developing X. oryzae pv. oryzae-resistant rice cultivars, strategies to mitigate disease outbreaks, and newly discovered genes and tools for disease management. We conclude that the implementation and application of cutting-edge technologies and tools are crucial to avoid yield losses from BLB and ensure food security.

Phytophthora capsici causes root, crown and fruit rot on many plant species including tomato and other solanaceous species. Plant growth promotion and suppression of P. capsici on tomato were assessed for three soil bacterial isolates, namely Bacillus amyloliquefaciens (UQ154), Bacillus velezensis (UQ156) and Acinetobacter sp. (UQ202). Cultures were applied as seed treatments (pre and post-infection inoculation) plus a soil drench at transplanting. The bacterial isolates significantly promoted growth of seedlings, as measured by root length, total fresh weight, and seedling vigor. We observed a reduction in pathogen load in tomato roots in both treatments using quantitative Polymerase Chain Reaction (qPCR). This work confirms the broad-spectrum activity of these bacterial isolates for our previous findings of biocontrol activity on different plants.

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.

Bacterial leaf blight (BLB) and sheath brown rot (SBR), caused by Xanthomonas oryzae pv. oryzae (Xoo) and Pseudomonas fuscovaginae, respectively, are bacterial diseases that lead to substantial yield losses in rice. Natural plant-based products represent a sustainable alternative to combat bacterial diseases due to their biodegradability and overall safety. However efficient ways of delivering them are crucial to their success. In an attempt to maximize the antibacterial properties of botanical bactericides for the control of these pathogens, this study evaluated the efficiency of different emulsion formulations of Piper sarmentosum extracts. The emulsion formulations were demonstrated to be effective in controlling BLB and SBR of rice in in vitro plate assays and in planta under glasshouse conditions. The observed in vitro inhibition of the bacterial pathogens and significant disease suppression in planta indicate that these plant extract formulations represent promising alternatives to be adopted in management strategies for controlling rice diseases., Agricultural science; Microbiology; Plant biology; Natural product chemistry; Bacterial leaf blight; Biological control; Sheath brown rot; Piper sarmentosum; Rice; Plant-based formulations.

Piper sarmentosum is a cultivated plant that also grows wildly in the South East Asian region. It is well known for its medicinal properties and a variety of active chemical constituents. The control potential of the plant extract against bacterial leaf blight caused by Xanthomonas oryzae and sheath brown rot of rice caused by Pseudomonas fuscovaginae was evaluated. The aqueous methanol extracts of fruits and leaves of P. sarmentosum were chemically analyzed using gas-chromatography mass-spectrometry (GC-MS). The results gave the highest best matches of 28 compounds (80.2% of total extracts) in the leaf extract and 24 compounds (89.1% of total extracts) in the fruit extract. The flavonoid fraction 4H-pyran-4-one,2,3-dihydro-3,5-dihydroxy-6-methyl- (DDMP) was found to be the major component which constitutes a complex natural product that has been of recent medicinal interest. The extract of the fruit had IC50 of 28.08 and 8.41 mg ml−1 against P. fuscovaginae and X. oryzae, respectively, while the leaf extract had IC50 of 10.42 and 24.69 mg ml−1 against P. fuscovaginae and X. oryzae, respectively.