Soil salinity is one of the main factors limiting agricultural development worldwide and has an adverse effect on plant growth and yield. To date, plant growth-promoting rhizobacteria (PGPR) are considered to be one of the most promising eco-friendly strategies for improving saline soils. The bacterium Bacillus megaterium ZS-3 is an excellent PGPR strain that induces growth promotion as well as biotic stress resistance and tolerance to abiotic stress in a broad range of host plants. In this study, the potential mechanisms of protection against salinity stress by B. megaterium ZS-3 in Arabidopsis thaliana were explored. Regulation by ZS-3 improved growth in A. thaliana under severe saline conditions. The results showed that ZS-3 treatment significantly increased the biomass, chlorophyll content and carotenoid content of A. thaliana. Compared to the control, the leaf area and total fresh weight of plants inoculated with ZS-3 increased by 245% and 271%, respectively; the chlorophyll a, chlorophyll b, and carotenoid contents increased by 335%, 146%, and 372%, respectively, under salt stress. Physiological and biochemical tests showed that ZS-3 regulated the content of osmotic substances in plants under salt stress. Compared to the control, the soluble sugar content of the ZS-3-treated group was significantly increased by 288%, while the proline content was significantly reduced by 41.43%. Quantification of Na+ and K+ contents showed that ZS-3 treatment significantly reduced Na+ accumulation and increased the K+/Na+ ratio in plants. ZS-3 also isolated Na+ in vesicles by upregulating NHX1 and AVP1 expression while limiting Na+ uptake by downregulating HKT1, which protected against Na+ toxicity. Higher levels of peroxidase and catalase activity and reduced glutathione were detected in plants inoculated with ZS-3 compared to those in uninoculated plants. In addition, it was revealed that ZS-3 activates salicylic acid (NPR1 and PR1) and jasmonic acid/ethylene (AOS, LOX2, PDF1.2, and ERF1) signaling pathways to induce systemic tolerance, thereby inducing salt tolerance in plants. In conclusion, the results of this study indicate that ZS-3 has the potential to act as an environmentally friendly salt tolerance inducer that can promote plant growth in salt-stressed environments.

Plant growth-promoting rhizobacteria are important for improving plant iron nutrition, but the interactions among inoculants, host plants and soil microorganisms have not been greatly explored. Rahnella aquatilis JZ-GX1 was applied to treat the increasingly serious iron deficiency chlorosis in Cinnamomum camphora, and the resulting improvement in chlorosis was determined by assessing the contents of chlorophyll, active iron, Fe2+ and antioxidant enzymes in leaves, the effects on the soil microbial community and the metabolism in the rhizosphere by high-throughput sequencing techniques and liquid chromatography–mass spectrometry (LC–MS). The results showed that inoculation with JZ-GX1 significantly increased the chlorophyll content of C. camphora, which promoted the redistribution of active iron in roots and leaves, increased the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX), and thus reduced membrane damage in iron-deficient C. camphora caused by reactive oxygen species. According to genome prediction and ultra-performance liquid chromatography–mass spectrometry (UPLC–MS) analysis, the JZ-GX1 strain could secrete desferrioxamine (DFO), and the concentration of DFO in C. camphora rhizosphere was 21-fold higher than that in uninoculated soil. The exogenous application of DFO increased the SPAD and Fe2+ contents in leaves. In addition, the inoculant affected the fungal community structure and composition in the C. camphora rhizosphere soil and increased the abundances of specific taxa, such as Glomus, Mortierella, Trichoderma, and Penicillium. Therefore, R. aquatilis JZ-GX1 application promoted iron absorption in C. camphora trees by secreting DFO and alleviated iron deficiency chlorosis through interactions with the local fungal community.

Burkholderia pyrrocinia JK-SH007 can effectively control poplar canker caused by pathogenic fungi. Its antifungal mechanism remains to be explored. Here, we characterized the functional role of CysB in B. pyrrocinia JK-SH007. This protein was shown to be responsible for the synthesis of cysteine and the siderophore ornibactin, as well as the antifungal activity of B. pyrrocinia JK-SH007. We found that deletion of the cysB gene reduced the antifungal activity and production of the siderophore ornibactin in B. pyrrocinia JK-SH007. However, supplementation with cysteine largely restored these two abilities in the mutant. Further global transcriptome analysis demonstrated that the amino acid metabolic pathway was significantly affected and that some sRNAs were significantly upregulated and targeted the iron–sulfur metabolic pathway by TargetRNA2 prediction. Therefore, we suggest that, in B. pyrrocinia JK-SH007, CysB can regulate the expression of genes related to Fe–S clusters in the iron–sulfur metabolic pathway to affect the antifungal activity of B. pyrrocinia JK-SH007. These findings provide new insights into the various biological functions regulated by CysB in B. pyrrocinia JK-SH007 and the relationship between iron–sulfur metabolic pathways and fungal inhibitory substances. Additionally, they lay the foundation for further investigation of the main antagonistic substances of B. pyrrocinia JK-SH007.

Background Plant crown gall disease caused by Agrobacterium tumefaciens causes significant losses in the cultivation of various ornamental and fruit trees. The emission of volatile organic compounds (VOCs) by biocontrol agents (BCAs) has garnered considerable attention due to their notable antagonistic effects. This study evaluated the biocontrol effects of VOCs produced by Pseudomonas chlororaphis subsp. aurantiaca ST-TJ4 against A. tumefaciens PX-1, the causal agent of cherry blossom crown gall.Results The VOCs released by P. chlororaphis subsp. aurantiaca ST-TJ4 significantly inhibited the colony size, cell viability, and swimming motility of A. tumefaciens PX-1, consequently impairing chemotaxis. Moreover, transmission and scanning electron microscopy revealed substantial severe morphological and ultrastructural changes in A. tumefaciens PX-1 cells, accompanied by a significant reduction in their ability to attach to plant roots. Furthermore, VOCs decreased the transcriptional expression levels of virulence-related genes (VirA, VirG, VirD2, VirE3) and three chemotaxis-related genes (CheW1, CheW2, CheA), which play pivotal roles in the pathogenicity of the bacteria. The observed downregulation of the superoxide dismutase (sod) gene indicated oxidative damage to A. tumefaciens PX-1 cells. These gene expression changes explained why A. tumefaciens PX-1 lost its early pathogenicity when inoculated on rose. In the antibacterial substance test, the VOCs of P. chlororaphis subsp. aurantiaca ST-TJ4 exhibited antagonistic effects on A. tumefaciens PX-1, with 2-undecone, 1-nonanol and 2-heptanone identified as the active compounds; among them, 1-nonanol and 2-heptanone exhibited the strongest antibacterial effect.Conclusions The VOCs produced by P. chlororaphis subsp. aurantiaca ST-TJ4 exhibited biocontrol potential against the tree crown gall pathogen A. tumefaciens.