Negative Immune Regulator <scp>CAD7</scp> Functions as a Small‐Molecule Aldehyde Reductase and Increases Histamine Accumulation in Arabidopsis

Negative immune regulator CAD7 is a non-canonical member of the CAD (cinnamyl alcohol dehydrogenase) family in plants. However, little is known on its biochemical functions and underlying mechanisms of immune regulation. Here, we show that Arabidopsis thaliana AtCAD7 harbours substrate-binding residues divergent from lignin-forming CADs (AtCAD4/5), being conserved with bacterial alcohol dehydrogenases EcYahK and EcYjgB. Comparative enzymatic analysis revealed that AtCAD7 exhibits a broad substrate preference for diverse small-molecule aldehydes, including histamine-derived intermediates, being distinct from the canonical lignin-forming AtCAD5. Metabolomic analyses revealed that AtCAD7-overexpression transformants were affected in the biosynthesis and metabolism of amino acids, whereas AtCAD7-silencing lines were activated in the phenylpropanoid pathway and increased in flavonoid accumulation. Histamine was elevated in AtCAD7-overexpression lines and functional validation revealed its promoted effect on plant susceptibility to Phytophthora parasitica. In contrast, phytoalexins scopolin and chlorogenic acid were enriched in AtCAD7-silencing lines, accompanied by upregulated expression of phenylpropanoid pathway-related genes. Functional validation demonstrated that scopolin and chlorogenic acid enhanced plant resistance to P. parasitica. Collectively, our study uncovers that CAD7 functions as a metabolic hub linking small-molecule aldehyde reductase activity to immune suppression, providing a potential novel target for developing crops with enhanced resistance to Phytophthora pathogens.