Plant beneficial microbes mediate biocontrol of diseases by interfering with pathogens or via strengthening the host. Although phytohormones, including cytokinins, are known to regulate plant development and physiology as well as plant immunity, their production by microorganisms has not been considered as a biocontrol mechanism. Here we identify the ability of Pseudomonas fluorescens G20-18 to efficiently control P. syringae infection in Arabidopsis, allowing maintenance of tissue integrity and ultimately biomass yield. Microbial cytokinin production was identified as a key determinant for this biocontrol effect on the hemibiotrophic bacterial pathogen. While cytokinin-deficient loss-of-function mutants of G20-18 exhibit impaired biocontrol, functional complementation with cytokinin biosynthetic genes restores cytokinin-mediated biocontrol, which is correlated with differential cytokinin levels in planta. Arabidopsis mutant analyses revealed the necessity of functional plant cytokinin perception and salicylic acid-dependent defence signalling for this biocontrol mechanism. These results demonstrate microbial cytokinin production as a novel microbe-based, hormone-mediated concept of biocontrol. This mechanism provides a basis to potentially develop novel, integrated plant protection strategies combining promotion of growth, a favourable physiological status and activation of fine-tuned direct defence and abiotic stress resilience.

Cátedra de Microbiología Agrícola Facultad de Agronomía Universidad de Buenos Aires Av San Martín 4453 Buenos Aires 1417 Argentina

Department of Biology Irving K Barber School of Arts and Sciences University of British Columbia Okanagan Campus 3333 University Way Kelowna BC V1V 1V7 Canada

Department of Plant and Environmental Sciences Copenhagen Plant Science Centre University of Copenhagen Højbakkegård Allé 13 2630 Taastrup Denmark Department of Plant Physiology Institute of Plant Sciences University of Graz Schubertstraße 51 8010 Graz Austria

Department of Plant and Environmental Sciences Copenhagen Plant Science Centre University of Copenhagen Højbakkegård Allé 13 2630 Taastrup Denmark Department of Plant Physiology Institute of Plant Sciences University of Graz Schubertstraße 51 8010 Graz Austria Global Change Research Centre Czech Globe AS CR v v i Drásov 470 Cz 664 24 Drásov Czech Republic

Department of Plant Physiology Institute of Plant Sciences University of Graz Schubertstraße 51 8010 Graz Austria

Department of Plant Physiology Institute of Plant Sciences University of Graz Schubertstraße 51 8010 Graz Austria Laboratorio de Biología Funcional y Biotecnología CICBA INBIOTEC CONICET Facultad de Agronomía de Azul UNCPBA Av República de Italia 780 7300 Azul Buenos Aires Argentina Cátedra de Microbiología Facultad de Agronomía de Azul UNCPBA Av República de Italia 780 7300 Azul Buenos Aires Argentina

Laboratory of Growth Regulators Centre of the Region Haná for Biotechnological and Agricultural Research Institute of Experimental Botany ASCR and Faculty of Science Palacký University Olomouc Czech Republic

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