The integrity of the actin cytoskeleton is essential for plant immune signalling. Consequently, it is generally assumed that actin disruption reduces plant resistance to pathogen attack. Here, we demonstrate that actin depolymerization induced a dramatic increase in salicylic acid (SA) levels in Arabidopsis thaliana. Transcriptomic analysis showed that the SA pathway was activated due to the action of isochorismate synthase (ICS). The effect was also confirmed in Brassica napus. This raises the question of whether actin depolymerization could, under particular conditions, lead to increased resistance to pathogens. Thus, we explored the effect of pretreatment with actin-depolymerizing drugs on the resistance of Arabidopsis thaliana to the bacterial pathogen Pseudomonas syringae, and on the resistance of an important crop Brassica napus to its natural fungal pathogen Leptosphaeria maculans. In both pathosystems, actin depolymerization activated the SA pathway, leading to increased plant resistance. To our best knowledge, we herein provide the first direct evidence that disruption of the actin cytoskeleton can actually lead to increased plant resistance to pathogens, and that SA is crucial to this process.

Laboratory of Hormonal Regulations in Plants Institute of Experimental Botany of The Czech Academy of Sciences Rozvojova 263 165 02 Prague 6 Czech Republic

Laboratory of Pathological Plant Physiology Institute of Experimental Botany of The Czech Academy of Sciences Rozvojova 263 165 02 Prague 6 Czech Republic

Laboratory of Plant Biochemistry Department of Biochemistry and Microbiology University of Chemistry and Technology Prague Technicka 5 166 28 Prague 6 Czech Republic

Laboratory of Plant Biochemistry Department of Biochemistry and Microbiology University of Chemistry and Technology Prague Technicka 5 166 28 Prague 6 Czech Republic Laboratory of Pathological Plant Physiology Institute of Experimental Botany of The Czech Academy of Sciences Rozvojova 263 165 02 Prague 6 Czech Republic

Laboratory of Plant Biochemistry Department of Biochemistry and Microbiology University of Chemistry and Technology Prague Technicka 5 166 28 Prague 6 Czech Republic Laboratory of Pathological Plant Physiology Institute of Experimental Botany of The Czech Academy of Sciences Rozvojova 263 165 02 Prague 6 Czech Republic Department of Biochemistry Faculty of Science Charles University Prague Faculty of Science 128 44 Hlavova 2030 8 Prague 2 Czech Republic

Laboratory of Plant Biochemistry Department of Biochemistry and Microbiology University of Chemistry and Technology Prague Technicka 5 166 28 Prague 6 Czech Republic Laboratory of Pathological Plant Physiology Institute of Experimental Botany of The Czech Academy of Sciences Rozvojova 263 165 02 Prague 6 Czech Republic Ludwig Maximilians University of Munich Faculty of Biology Biocenter Department Genetics Grosshaderner Str 2 4 D 82152 Martinsried Germany

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