The Arabidopsis <i>csb3</i> mutant reveals a regulatory link between salicylic acid-mediated disease resistance and the methyl-erythritol 4-phosphate pathway

We report on <i>constitutive subtilisin3 (csb3)</i>, an Arabidopsis mutant showing strikingly enhanced resistance to biotrophic pathogens. Epistasis analyses with <i>pad4, sid2, eds5, NahG, npr1, dth9</i> and <i>cpr1</i> mutants revealed that the enhanced resistance of csb3 plants requires intact salicylic acid (SA) synthesis and perception. <i>CSB3</i> encodes a 1-hydroxy-2-methyl-2-butenyl 4-diphosphate synthase, the enzyme controlling the penultimate step of the biosynthesis of isopentenyl diphosphate via the 2-C-methyl-d-erythritol-4-phosphate (MEP) pathway in the chloroplast. <i>CSB3</i> is expressed constitutively in healthy plants, and shows repression in response to bacterial infection. We also show the pharmacological complementation of the enhanced-resistance phenotype of <i>csb3</i> plants with fosmidomycin, an inhibitor of the MEP pathway, and propose that <i>CSB3</i> represents a point of metabolic convergence modulating the magnitude of SA-mediated disease resistance to biotrophic pathogens.