Bacterial medium-chain 3-hydroxy fatty acid metabolites trigger immunity in Arabidopsis plants

In plants, cell-surface immune receptors sense molecular non–self-signatures. Lipid A of Gram-negative bacterial lipopolysaccharide is considered such a non–self-signature. The receptor kinase LIPOOLIGOSACCHARIDE-SPECIFIC REDUCED ELICITATION (LORE) mediates plant immune responses to Pseudomonas and Xanthomonas but not enterobacterial lipid A or lipopolysaccharide preparations. Here, we demonstrate that synthetic and bacterial lipopolysaccharide-copurified medium-chain 3-hydroxy fatty acid (mc-3-OH-FA) metabolites elicit LORE-dependent immunity. The mc-3-OH-FAs are sensed in a chain length– and hydroxylation-specific manner, with free ( R )-3-hydroxydecanoic acid [( R )-3-OH-C10:0] representing the strongest immune elicitor. By contrast, bacterial compounds comprising mc-3-OH-acyl building blocks but devoid of free mc-3-OH-FAs—including lipid A or lipopolysaccharide, rhamnolipids, lipopeptides, and acyl-homoserine-lactones—do not trigger LORE-dependent responses. Hence, plants sense low-complexity bacterial metabolites to trigger immune responses.