The Pseudomonas syringae pv. tomato DC3000 PSPTO_0820 multidrug transporter is involved in resistance to plant antimicrobials and bacterial survival during tomato plant infection

Predatory potentials of novel Bdellovibrio isolates against multidrug-resistant and extremely drug-resistant bacterial pathogens of animals and plants

Bdellovibrio and like organisms (BALOs) predate on Gram-negative bacteria selectively, which holds promise as an alternate to combat against multidrug-resistant bacterial pathogens. In this study, two Bdellovibrio strains designated MPR17 and MPR18 were isolated from water samples by double-layer agar technique using P. fluorescens as prey and investigated for their predatory potentials against six pathogenic Gram-negative, MDR and XDR bacteria. Both the BALOs preyed on all the tested bacteria including P. syringae, B. glumae, V. parahaemolyticus, P. mirabilis, M. bovis, and A. schindleri with different killing rates. Against A. schindleri, Bdellovibrio sp. MPR18 produced a maximum of 2.11 × 107 PFU/ml, but MPR17 did not produce any detectable PFUs. TEM analysis of predator-prey interactions recorded different life stages including attachment, bdelloplast formation, prey lysis, and release. Morphologically, both the Bdellovibrio isolates MPR17 and MPR18 were small vibrioid in shape with a long single polar flagellum and measured of about < 500 nm diameter in size, suggesting that they are the smallest ever described Bdellovibrio. Predatory activity was further assessed in liquid co-cultures that evidenced time-dependent decrease in the prey cell density from 24 h with significant reduction at 72 h with different degrees of predation with respective prey bacteria. Bdellovibrio sp. MPR17 and MPR18 exhibited strong bacteriolytic activity in liquid cultures, which was equivalent to the bactericidal activities of standard antibiotics especially for M. bovis (99%), P. syringae (90%) and A. schindleri (90%). The findings of this study signify the potential of novel Bdellovibrio sp. MPR17 and MPR18 as a biocontrol agent with multiple host range to eradicate different MDR and XDR bacterial infections, which could contribute positively for resolving the crisis of antimicrobial resistance.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-025-04230-8.

Leucine rich repeat-malectin receptor kinases IGP1/CORK1, IGP3 and IGP4 are required for arabidopsis immune responses triggered by β-1,4-D-Xylo-oligosaccharides from plant cell walls

Pattern-Triggered Immunity (PTI) in plants is activated upon recognition by Pattern Recognition Receptors (PRRs) of Damage- and Microbe-Associated Molecular Patterns (DAMPs and MAMPs) from plants or microorganisms, respectively. An increasing number of identified DAMPs/MAMPs are carbohydrates from plant cell walls and microbial extracellular layers, which are perceived by plant PRRs, such as LysM and Leucine Rich Repeat-Malectin (LRR-MAL) receptor kinases (RKs). LysM-RKs (e.g. CERK1, LYK4 and LYK5) are needed for recognition of fungal MAMP chitohexaose (β-1,4-D-(GlcNAc)6, CHI6), whereas IGP1/CORK1, IGP3 and IGP4 LRR-MAL RKs are required for perception of β-glucans, like cellotriose (β-1,4-D-(Glc)3, CEL3) and mixed-linked glucans. We have explored the diversity of carbohydrates perceived by Arabidopsis thaliana seedlings by determining PTI responses upon treatment with different oligosaccharides and polysaccharides. These analyses revealed that plant oligosaccharides from xylans [β-1,4-D-(xylose)4 (XYL4)], glucuronoxylans and α-1,4-glucans, and polysaccharides from plants and seaweeds activate PTI. Cross-elicitation experiments of XYL4 with other glycans showed that the mechanism of recognition of XYL4 and the DAMP 33-α-L-arabinofuranosyl-xylotetraose (XA3XX) shares some features with that of CEL3 but differs from that of CHI6. Notably, XYL4 and XA3XX perception is impaired in igp1/cork1, igp3 and igp4 mutants, and almost not affected in cerk1 lyk4 lyk5 triple mutant. XYL4 perception is conserved in different plant species since XYL4 pre-treatment triggers enhanced disease resistance in tomato to Pseudomonas syringae pv tomato DC3000 and PTI responses in wheat. These results expand the number of glycans triggering plant immunity and support IGP1/CORK1, IGP3 and IGP4 relevance in Arabidopsis thaliana glycans perception and PTI activation.

Significance Statement

The characterization of plant immune mechanisms involved in the perception of carbohydrate-based structures recognized as DAMPs/MAMPs is needed to further understand plant disease resistance modulation. We show here that IGP1/CORK1, IGP3 and IGP4 LRR-MAL RKs are required for the perception of carbohydrate-based DAMPs β-1,4-D-(xylose)4 (XYL4) and 33-α-L-arabinofuranosyl-xylotetraose (XA3XX), further expanding the function of these LRR-MAL RKs in plant glycan perception and immune activation.

Antibiotic Resistance in Plant Pathogenic Bacteria: Recent Data and Environmental Impact of Unchecked Use and the Potential of Biocontrol Agents as an Eco-Friendly Alternative

The economic impact of phytopathogenic bacteria on agriculture is staggering, costing billions of US dollars globally. Pseudomonas syringae is the top most phytopathogenic bacteria, having more than 60 pathovars, which cause bacteria speck in tomatoes, halo blight in beans, and so on. Although antibiotics or a combination of antibiotics are used to manage infectious diseases in plants, they are employed far less in agriculture compared to human and animal populations. Moreover, the majority of antibiotics used in plants are immediately washed away, leading to environmental damage to ecosystems and food chains. Due to the serious risk of antibiotic resistance (AR) and the potential for environmental contamination with antibiotic residues and resistance genes, the use of unchecked antibiotics against phytopathogenic bacteria is not advisable. Despite the significant concern regarding AR in the world today, there are inadequate and outdated data on the AR of phytopathogenic bacteria. This review presents recent AR data on plant pathogenic bacteria (PPB), along with their environmental impact. In light of these findings, we suggest the use of biocontrol agents as a sustainable, eco-friendly, and effective alternative to controlling phytopathogenic bacteria.

Multifaceted roles of flavonoids mediating plant-microbe interactions

Plant-microbe interactions dynamically affect plant growth, health, and development. The mechanisms underpinning these associations are-to a large extent-mediated by specialized host-derived secondary metabolites. Flavonoids are one of the most studied classes of such metabolites, regulating both plant development and the interaction with commensal microbes. Here, we provide a comprehensive review of the multiple roles of flavonoids in mediating plant-microbe interactions. First, we briefly summarize the general aspects of flavonoid synthesis, transport, and exudation in plants. Then, we review the importance of flavonoids regulating plant-microbe interactions and dynamically influencing the overall community assembly of plant-root microbiomes. Last, we highlight potential knowledge gaps in our understanding of how flavonoids determine the interactions between plants and commensal microbes. Collectively, we advocate the importance of advancing research in this area toward innovative strategies to effectively manipulate plant-microbiome composition, in this case, via flavonoid production and exudation in plant roots. Video Abstract.

Cell wall-derived mixed-linked β-1,3/1,4-glucans trigger immune responses and disease resistance in plants

Pattern-triggered immunity (PTI) is activated in plants upon recognition by pattern recognition receptors (PRRs) of damage- and microbe-associated molecular patterns (DAMPs and MAMPs) derived from plants or microorganisms, respectively. To understand better the plant mechanisms involved in the perception of carbohydrate-based structures recognized as DAMPs/MAMPs, we have studied the ability of mixed-linked β-1,3/1,4-glucans (MLGs), present in some plant and microbial cell walls, to trigger immune responses and disease resistance in plants. A range of MLG structures were tested for their capacity to induce PTI hallmarks, such as cytoplasmic Ca²⁺ elevations, reactive oxygen species production, phosphorylation of mitogen-activated protein kinases and gene transcriptional reprogramming. These analyses revealed that MLG oligosaccharides are perceived by Arabidopsis thaliana and identified a trisaccharide, β-d-cellobiosyl-(1,3)-β-d-glucose (MLG43), as the smallest MLG structure triggering strong PTI responses. These MLG43-mediated PTI responses are partially dependent on LysM PRRs CERK1, LYK4 and LYK5, as they were weaker in cerk1 and lyk4 lyk5 mutants than in wild-type plants. Cross-elicitation experiments between MLG43 and the carbohydrate MAMP chitohexaose [β-1,4-d-(GlcNAc)₆ ], which is also perceived by these LysM PRRs, indicated that the mechanism of MLG43 recognition could differ from that of chitohexaose, which is fully impaired in cerk1 and lyk4 lyk5 plants. MLG43 treatment confers enhanced disease resistance in A. thaliana to the oomycete Hyaloperonospora arabidopsidis and in tomato and pepper to different bacterial and fungal pathogens. Our data support the classification of MLGs as a group of carbohydrate-based molecular patterns that are perceived by plants and trigger immune responses and disease resistance.

Arabinoxylan-Oligosaccharides Act as Damage Associated Molecular Patterns in Plants Regulating Disease Resistance

Immune responses in plants can be triggered by damage/microbe-associated molecular patterns (DAMPs/MAMPs) upon recognition by plant pattern recognition receptors (PRRs). DAMPs are signaling molecules synthesized by plants or released from host cellular structures (e.g., plant cell walls) upon pathogen infection or wounding. Despite the hypothesized important role of plant cell wall-derived DAMPs in plant-pathogen interactions, a very limited number of these DAMPs are well characterized. Recent work demonstrated that pectin-enriched cell wall fractions extracted from the cell wall mutant impaired in Arabidopsis Response Regulator 6 (arr6), that showed altered disease resistance to several pathogens, triggered more intense immune responses than those activated by similar cell wall fractions from wild-type plants. It was hypothesized that arr6 cell wall fractions could be differentially enriched in DAMPs. In this work, we describe the characterization of the previous immune-active fractions of arr6 showing the highest triggering capacities upon further fractionation by chromatographic means. These analyses pointed to a role of pentose-based oligosaccharides triggering plant immune responses. The characterization of several pentose-based oligosaccharide structures revealed that β-1,4-xylooligosaccharides of specific degrees of polymerization and carrying arabinose decorations are sensed as DAMPs by plants. Moreover, the pentasaccharide 33-α-L-arabinofuranosyl-xylotetraose (XA3XX) was found as a highly active DAMP structure triggering strong immune responses in Arabidopsis thaliana and enhancing crop disease resistance.

Epigallocatechin-3-gallate enhances tomato resistance to tobacco mosaic virus by modulating RBOH1-dependent H2O2 signaling

Tobacco mosaic virus (TMV) is one of the most damaging plant viruses from an economic and research point of view. Epigallocatechin-3-Gallate (EGCG), a flavonoid type secondary metabolite can selectively improve plant defense against pathogens; however, the effect of EGCG on plant defense against TMV and the underlying mechanism(s) remain elusive. In this study, exogenous EGCG application increased plant resistance to TMV as revealed by significantly decreased transcript levels of TMV-coat protein (CP) in tomato leaves. A time-course of H₂O₂ concentrations in tomato leaves showed that TMV inoculation rapidly increased the H₂O₂ accumulation, reaching its peak at 3 days post-inoculation (dpi) which remained the highest until 6 dpi. However, the combined treatment of EGCG and TMV remarkably decreased the concentrations of H₂O₂ at 3 and 6 dpi. Meanwhile, the transcript levels of RESPIRATORY BURST OXIDASE HOMOLOG 1 (SlRBOH1) were significantly increased by either EGCG or TMV inoculation, but the EGCG treatment along with TMV caused a further upregulation in the SlRBOH1 transcripts compared with that in only TMV-inoculated plants. Chemical scavenging of H₂O₂ or silencing SlRBOH1 both compromised the EGCG-induced enhanced resistance to TMV. Furthermore, EGCG-induced elevation in the activity of antioxidant enzymes was abolished by SlRBOH1 silencing, suggesting that EGCG enhanced defense against TMV by increasing the antioxidant enzyme activity via RBOH1-dependent H₂O₂ signaling. Taken together, our results suggest that EGCG functioned to maintain a delicate balance between ROS signaling and ROS scavenging via RBOH1, which enhanced tomato resistance to TMV.

Genome-Wide Transposon Screen of a Pseudomonas syringae mexB Mutant Reveals the Substrates of Efflux Transporters

Bacteria express numerous efflux transporters that confer resistance to diverse toxicants present in their environment. Due to a high level of functional redundancy of these transporters, it is difficult to identify those that are of most importance in conferring resistance to specific compounds. The resistance-nodulation-division (RND) protein family is one such example of redundant transporters that are widespread among Gram-negative bacteria. Within this family, the MexAB-OprM protein complex is highly expressed and conserved among Pseudomonas species. We exposed barcoded transposon mutant libraries in isogenic wild-type and ΔmexB backgrounds in P. syringae B728a to diverse toxic compounds in vitro to identify mutants with increased susceptibility to these compounds. Mutants with mutations in genes encoding both known and novel redundant transporters but with partially overlapping substrate specificities were observed in a ΔmexB background. Psyr_0228, an uncharacterized member of the major facilitator superfamily of transporters, preferentially contributes to tolerance of acridine orange and acriflavine. Another transporter located in the inner membrane, Psyr_0541, contributes to tolerance of acriflavine and berberine. The presence of multiple redundant, genomically encoded efflux transporters appears to enable bacterial strains to tolerate a diversity of environmental toxins. This genome-wide screen performed in a hypersusceptible mutant strain revealed numerous transporters that would otherwise be dispensable under these conditions. Bacterial strains such as P. syringae that likely encounter diverse toxins in their environment, such as in association with many different plant species, probably benefit from possessing multiple redundant transporters that enable versatility with respect to toleration of novel toxicants.IMPORTANCE Bacteria use protein pumps to remove toxic compounds from the cell interior, enabling survival in diverse environments. These protein pumps can be highly redundant, making their targeted examination difficult. In this study, we exposed mutant populations of Pseudomonas syringae to diverse toxicants to identify pumps that contributed to survival in those conditions. In parallel, we examined pump redundancy by testing mutants of a population lacking the primary efflux transporter responsible for toxin tolerance. We identified partial substrate overlap for redundant transporters, as well as several pumps that appeared more substrate specific. For bacteria that are found in diverse environments, having multiple, partially redundant efflux pumps likely allows flexibility in habitat colonization.

Genome-Wide Transposon Screen of a Pseudomonas syringae mexB Mutant Reveals the Substrates of Efflux Transporters

Bacteria express numerous efflux transporters that confer resistance to diverse toxicants present in their environment. Due to a high level of functional redundancy of these transporters, it is difficult to identify those that are of most importance in conferring resistance to specific compounds. The resistance-nodulation-division (RND) protein family is one such example of redundant transporters that are widespread among Gram-negative bacteria. Within this family, the MexAB-OprM protein complex is highly expressed and conserved among Pseudomonas species. We exposed barcoded transposon mutant libraries in isogenic wild-type and ΔmexB backgrounds in P. syringae B728a to diverse toxic compounds in vitro to identify mutants with increased susceptibility to these compounds. Mutants with mutations in genes encoding both known and novel redundant transporters but with partially overlapping substrate specificities were observed in a ΔmexB background. Psyr_0228, an uncharacterized member of the major facilitator superfamily of transporters, preferentially contributes to tolerance of acridine orange and acriflavine. Another transporter located in the inner membrane, Psyr_0541, contributes to tolerance of acriflavine and berberine. The presence of multiple redundant, genomically encoded efflux transporters appears to enable bacterial strains to tolerate a diversity of environmental toxins. This genome-wide screen performed in a hypersusceptible mutant strain revealed numerous transporters that would otherwise be dispensable under these conditions. Bacterial strains such as P. syringae that likely encounter diverse toxins in their environment, such as in association with many different plant species, probably benefit from possessing multiple redundant transporters that enable versatility with respect to toleration of novel toxicants.IMPORTANCE Bacteria use protein pumps to remove toxic compounds from the cell interior, enabling survival in diverse environments. These protein pumps can be highly redundant, making their targeted examination difficult. In this study, we exposed mutant populations of Pseudomonas syringae to diverse toxicants to identify pumps that contributed to survival in those conditions. In parallel, we examined pump redundancy by testing mutants of a population lacking the primary efflux transporter responsible for toxin tolerance. We identified partial substrate overlap for redundant transporters, as well as several pumps that appeared more substrate specific. For bacteria that are found in diverse environments, having multiple, partially redundant efflux pumps likely allows flexibility in habitat colonization.