Chemical Activation of EDS1/PAD4 Signaling Leading to Pathogen Resistance in Arabidopsis

In a chemical screen we identified thaxtomin A (TXA), a phytotoxin from plant pathogenic Streptomyces scabies, as a selective and potent activator of FLAVIN-DEPENDENT MONOOXYGENASE1 (FMO1) expression in Arabidopsis (Arabidopsis thaliana). TXA induction of FMO1 was unrelated to the production of reactive oxygen species (ROS), plant cell death or its known inhibition of cellulose synthesis. TXA-stimulated FMO1 expression was strictly dependent on ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1) and PHYTOALEXIN DEFICIENT4 (PAD4) but independent of salicylic acid (SA) synthesis via ISOCHORISMATE SYNTHASE1 (ICS1). TXA induced the expression of several EDS1/PAD4-regulated genes, including EDS1, PAD4, SENESCENCE ASSOCIATED GENE101 (SAG101), ICS1, AGD2-LIKE DEFENSE RESPONSE PROTEIN1 (ALD1) and PATHOGENESIS-RELATED PROTEIN1 (PR1), and accumulation of SA. Notably, enhanced ALD1 expression did not result in accumulation of the product pipecolic acid (PIP), which promotes FMO1 expression during biologically induced systemic acquired resistance. TXA treatment preferentially stimulated expression of PAD4 compared with EDS1, which was mirrored by PAD4 protein accumulation, suggesting that TXA leads to increased PAD4 availability to form EDS1-PAD4 signaling complexes. Also, TXA treatment of Arabidopsis plants led to enhanced disease resistance to bacterial and oomycete infection, which was dependent on EDS1 and PAD4, as well as on FMO1 and ICS1. Collectively, the data identify TXA as a potentially useful chemical tool to conditionally activate and interrogate EDS1- and PAD4-controlled pathways in plant immunity.