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Key message: A gene encoding a coproporphyrinogen III oxidase mediates disease resistance in plants by the salicylic acid pathway. Abstract: A number of genes that regulate powdery mildew resistance have been identified in Arabidopsis, such as ENHANCED DISEASE RESISTANCE 1 to 3 ( EDR1 to 3). To further study the molecular interactions between the powdery mildew pathogen and Arabidopsis, we isolated and characterized a mutant that exhibited enhanced resistance to powdery mildew. The mutant also showed dramatic powdery mildew-induced cell death as well as growth defects and early senescence in the absence of pathogens. We identified the affected gene by map-based cloning and found that the gene encodes a coproporphyrinogen III oxidase, a key enzyme in the tetrapyrrole biosynthesis pathway, previously known as LESION INITIATION 2 ( LIN2). Therefore, we designated the mutant lin2- 2. Further studies revealed that the lin2- 2 mutant also displayed enhanced resistance to Hyaloperonospora arabidopsidis ( H.a.) Noco2. Genetic analysis showed that the lin2- 2-mediated disease resistance and spontaneous cell death were dependent on PHYTOALEXIN DEFICIENT 4 ( PAD4), SALICYLIC ACID INDUCTION- DEFICIENT 2 ( SID2), and NONEXPRESSOR OF PATHOGENESIS- RELATED GENES 1 ( NPR1), which are all involved in salicylic acid signaling. Furthermore, the relative expression levels of defense-related genes were induced after powdery mildew infection in the lin2- 2 mutant. These data indicated that LIN2 plays an important role in cell death control and defense responses in plants. [ABSTRACT FROM AUTHOR]