Type III Secretion System-Mediated Induction of Systemic Resistance by Pseudomonas marginalis ORh26 Enhances Sugar Beet Defence Against Pseudomonas syringae pv. aptata

The increasing demand for sustainable agricultural practises has sparked interest in microbes that promote plant immunity. Among these, <i>Pseudomonas</i> 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 <i>Pseudomonas</i> species and on a systemic level remains largely unexplored. We show for the first time that the T3SS of a plant-beneficial <i>Pseudomonas</i> strain induces ISR by root colonisation. T3SS-positive <i>Pseudomonas</i> isolates were applied to the roots of sugar beet (<i>Beta vulgaris</i> L.) and systemic effects on plant immunity were assessed in leaves exposed to the pathogen <i>P. syringae</i> pv. <i>aptata</i> P21. Our results show that <i>P. marginalis</i> ORh26 reduced lesion size and pathogen proliferation in sugar beet leaves. ORh26 activated peroxidase and phenylalanine ammonia-lyase and upregulated <i>NPR1</i> and <i>MYC2</i> 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.