The increasing demand for sustainable agricultural practises has sparked interest in microbes that promote plant immunity. Among these, Pseudomonas species have shown the potential to enhance induced systemic resistance (ISR) in crops. While type III secretion systems (T3SSs) in pathogenic bacteria have been widely studied for their role in local immunosuppression, their function in beneficial Pseudomonas species and on a systemic level remains largely unexplored. We show for the first time that the T3SS of a plant-beneficial Pseudomonas strain induces ISR by root colonisation. T3SS-positive Pseudomonas isolates were applied to the roots of sugar beet (Beta vulgaris L.) and systemic effects on plant immunity were assessed in leaves exposed to the pathogen P. syringae pv. aptata P21. Our results show that P. marginalis ORh26 reduced lesion size and pathogen proliferation in sugar beet leaves. ORh26 activated peroxidase and phenylalanine ammonia-lyase and upregulated NPR1 and MYC2 defence genes. Remarkably, a T3SS-deficient mutant of ORh26 failed to induce these effects. Genomic analysis identified T3SS structural genes and effector proteins, including a pectate lyase and an effector of the HopJ family, that may mediate these responses. This study reveals a previously uncharacterised role of T3SS in the induction of ISR and improves our understanding of plant–microbe interactions.

The type III secretion system (T3SS) has traditionally been studied for its role in bacterial virulence. However, recent research emphasizes its dual role in beneficial interactions between bacteria and plants. This review examines the immunomodulatory functions of T3SS beyond pathogenicity and focuses on how T3SS effectors manipulate plant immune responses to promote symbioses. By comparing T3SS mechanisms in pathogenic and non-pathogenic bacteria, we aim to understand how this system enables beneficial microbes to colonize plants and improve plant growth and stress resilience. We also investigate the potential of T3SS to trigger induced systemic resistance in plants, a mechanism that could be utilized in agriculture to improve crop resistance to pathogens. The review concludes with an outlook on future research and emphasizes the need for comprehensive studies on T3SS effectors in non-pathogenic bacteria and their interactions with plant hosts.

Endophytes, as an integral part of plants, form unique relationships with their hosts that go beyond classical definitions of symbiosis and influence plant development, immunity, and stress responses. The pepper endophyte strain Pseudomonas putida A32 has several plant growth-promoting properties and increases the tolerance of pepper to drought, but its biocontrol potential is unknown. In this study, we investigated the protective role of P. putida A32 against infection with the pathogenic bacterium P. syringae pv. aptata P21 in two pepper genotypes in laboratory experiments. The percentage of lesion reduction in genotype 26 treated with P. putida A32 was 46.62%. The results showed a significant reduction in hydrogen peroxide and malondialdehyde levels by 29.45 and 20.22%, respectively, in infected genotype 26. The treated but uninfected controls showed a significant increase in superoxide dismutase activity in genotype 26 by 41.26% and ascorbate peroxidase activity in genotype 19 by 40.28% in the treated infected plants. The tolerant genotype 19 was much less dependent on the bacterial treatment under stress conditions than the susceptible genotype 26. Future research will investigate the role of P. putida A32 in the induced systemic resistance of different pepper genotypes to protect against pathogens.

Using beneficial bacteria from the plant microbiome to combat pathogens is an environmentally friendly strategy for biological control. Although significant progress has been made in characterizing microorganisms with biocontrol potential, the optimal methods for applying such biological preparations to achieve maximum effectiveness against plant pathogens remain insufficiently defined. Our goal was to select rhizobacteria from the sugar beet microbiome and analyze their biocontrol capacity in both indirect and direct applications to protect the plant from Pseudomonas syringae pv. aptata P21. The methodological approach differed: indirect application involved seed priming with selected strains, Bacillus safensis MRh275, B. pseudomycoides JRh226, Stenotrophomonas maltophilia JRh266, or the T2 consortium (MRh275 and JRh266), while direct application involved simultaneous treatment of both the pathogen and the biocontrol strain. Although the direct approach resulted in a greater reduction in lesions and a lower concentration of H₂O₂, the indirect approach showed higher activity of peroxidase and superoxide dismutase as antioxidant enzymes, as well as phenylalanine ammonia-lyase, which is involved in the phenylpropanoid pathway and plant defense mechanisms. Infected plants showed higher expression of NPR1, MYC2, and LOX defense-related genes only under indirect biocontrol with all three strains, except in the T2 application. The T2 consortium performed best in direct biocontrol, where it most effectively reduced lesions. Since encounters between plants and pathogens cannot be accurately predicted, and the application of biological preparations should be easy and accessible for farmers, this study highlights the use of indirect biocontrol through seed priming to enhance the plant’s intrinsic defense capacity.