Mechanism of resistance to Cucumber mosaic virus elicited by inoculation with Bacillus subtilis subsp. subtilis

Rhizobacterial Colonization and Management of Bacterial Speck Pathogen in Tomato by Pseudomonas spp

Plants and soil microorganisms interact at every stage of growth. Pseudomonas spp. are highly regarded for their ability to increase crop production and protection from diseases. The aim of this study is to understand the mechanisms of the rhizobacterial colonization of tomato roots via chemotaxis assay and the activation of tomato resistance against the pathogenic bacterium, Pseudomonas syringae pv. tomato DC3000 (Pst). The capillary assay was used to evaluate the chemotaxis response of PGPRs (plant growth-promoting rhizobacteria). The activities of defense enzymes and the expressions of PR (pathogenesis-related) genes were measured using real-time qPCR. Chemotactic responses to malic and citric acids (the most important root exudates found in different plant species) at low concentrations varied substantially among the rhizobacterial isolates (63 species). Beneficial isolates including Pseudomonas resinovorans A5, P. vranovensis A30, P. resinovorans A28, P. umsongensis O26, P. stutzeri N42, and P. putida T15 reacted well to different concentrations of root exudates. P. putida T15 demonstrated the most potent anti-Pst activity. At three and six days after inoculation, the greatest levels of polyphenol oxidase and peroxidase activity were reported in the A5 and T15 groups. In tomato, transcript levels of four PR (pathogenesis-related) genes were elevated by rhizobacterial treatments. PGPR isolates alone or in combination with BABA (β-amino butyric acid) up-regulated the transcriptions of PR1, PR2, LOX, and PAL genes. Treatments with N42 and T15 resulted in the greatest improvements in tomato growth and yield traits. In conclusion, the results explain the mechanisms of rhizobacterial colonization for the improved management of Pst. Rhizobacterial isolates play a role in tomato's resistance to Pst via salicylic acid and jasmonic acid pathways.

Efficiency of Bacillus thuringiensis and Bacillus cereus against Rhynchophorus ferrugineus

The Red Palm Weevil (Rhynchophorus ferrugineus (Oliv.) (Coleoptera, Dryophthoridae) is a well-known palm tree pest that has caused enormous economic damage all over the globe. Insecticides are still the primary method of controlling this pest at this period. However, field populations of RPW have been shown to be resistant to pesticides. Using Bacillus spp. might be one of the options for controlling R. ferruginous. In this study, 23 species of Bacillus spp. were isolated from the rhizosphere of date palm trees in Al Ahsa Oasis, Saudi Arabia. The isolates were identified using 16S rRNA gene sequencing. R. ferrugineus larvae and adults were tested on sugarcane pieces that were treated with the B. thuringiensis strain PDC-AHSAA1 and B. cereus strains (PDC-AHSAA2, PDC-AHSA3 and PDC-AHSA4). The LC₅₀ values for larvae and adults were quite low when they were compared with those of the other isolated strains. The B. thuringiensis strain PDC-AHSAA1 was more effective against both the larvae and adults. The determined LC₅₀ values for B. thuringiensis ranged from 4.19 × 10⁷-3.78 × 10⁹. After 21 days, the data on larval mortality and body weight were evaluated. The surviving larvae that were treated with the bacterial isolates did not acquire a substantial weight. For the RPW larvae and adults, the mortality and corrected mortality death rates were increased by increasing the concentration of B. thuringiensis. In conclusion, Bacillus-treated diets negatively influenced the growth and development of the RPW. This research reported on the interaction between the RPW and the rhizosphere Bacillus spp. and highlighted the tremendous potential for the development of microbial resource-based control strategies for this pest.

Protective and Curative Activities of Paenibacillus polymyxa against Zucchini yellow mosaic virus Infestation in Squash Plants

The use of microbial products as natural biocontrol agents to increase a plant's systemic resistance to viral infections is a promising way to make agriculture more sustainable and less harmful to the environment. The rhizobacterium Paenibacillus polymyxa has been shown to have strong biocontrol action against plant diseases, but its antiviral activity has been little investigated. Here, the efficiency of the culture filtrate of the P. polymyxa strain SZYM (Acc# ON149452) to protect squash (Cucurbita pepo L.) plants against a Zucchini yellow mosaic virus (ZYMV, Acc# ON159933) infection was evaluated. Under greenhouse conditions, the foliar application of the culture filtrate of SZYM either in protective or curative treatment conditions enhanced squash growth, reduced disease severity, and decreased ZYMV accumulation levels in the treated plants when compared to the non-treated plants. The protective treatment group exhibited the highest inhibitory effect (80%), with significant increases in their total soluble carbohydrates, total soluble protein content, ascorbic acid content, and free radical scavenging activity. Furthermore, a considerable increase in the activities of reactive oxygen species scavenging enzymes (superoxide dismutase, polyphenol oxidase, and peroxidase) were also found. In addition, the induction of systemic resistance with a significant elevation in the transcriptional levels of polyphenolic pathway genes (CHS, PAL, and C3H) and pathogenesis-related genes (PR-1 and PR-3) was observed. Out of the 14 detected compounds in the GC-MS analysis, propanoic acid, benzenedicarboxylic acid, tetradecanoic acid, and their derivatives, as well as pyrrolo [1,2-a] pyrazine-1,4-dione, hexahydro-3-(2-methylpropyl) were the primary ingredient compounds in the ethyl acetate extract of the SZYM-culture filtrate. Such compounds may act as elicitor molecules that induce systemic resistance against viral infection. Consequently, P. polymyxa can be considered a powerful plant growth-promoting bacterium (PGPB) in agricultural applications as well as a source of bioactive compounds for sustainable disease management. As far as we know, this is the first time that P. polymyxa has been shown to fight viruses in plants.

Induction of Systemic Resistance against Sheath Blight in Rice by Different Pseudomonas Isolates

Sheath blight disease is a fungal pathogen that causes leaf blight in rice plants, resulting in significant yield losses throughout the growing season. Pseudomonas spp. have long been used as biocontrol agents for a variety of plant diseases. Four Pseudomonas isolates were tested for their ability to promote rice growth and generate systemic resistance to Rhizoctonia solani, the causal pathogen of sheath blight disease. In vitro, Pseudomonas isolates produced the growth hormone indole acetic acid (0.82–1.82 mg L⁻¹). Additionally, seed treatment with Pseudomonas putida suspension outperformed P. brassicacearum, P. aeruginosa and P. resinovorans in terms of germination and vigor evaluation. The maximum seed germination of 89% was recorded after seed treatments with a fresh suspension of P. putida, followed by 87% germination in P. aeruginosa treatment, compared with only 74% germination in the untreated controls. When compared with the infected control plants, all Pseudomonas isolates were non-pathogenic to rice and their co-inoculation considerably enhanced plant growth and health by reducing the disease index to 37% and improving plant height (26%), fresh weight (140%) and dry weight (100%). All Pseudomonas isolates effectively reduced sheath blight disease incidence, as well as the fungicide carbendazim, which is recommended for field management of R. solani. In comparison to untreated control seedlings, treatment with Pseudomonas isolates enhanced the production of peroxidase and polyphenol oxidase enzymes and the expression of the phenylalanine ammonia lyase (PAL) and NPR1 genes, which could be involved in disease incidence reduction. In conclusion, the use of Pseudomonas spp. has been demonstrated to improve rice growth and resistance to R. solani while also providing an environmentally acceptable option to the agroecosystems.