Inhibition of protoporphyrinogen oxidase expression in Arabidopsis causes a lesion-mimic phenotype that induces systemic acquired resistance

Summary We have used an antisense expression technology inArabidopsisbased on the yeast GAL4/UAS transactivation system (Guyeret al.Genetics1998; 149:633–639) to reduce levels of protoporphyrinogen IX oxidase (PPO), the last common enzyme of the biosynthesis of the haem group and chlorophyll. Plants expressing the antisensePPOgene presented growth alterations and their leaves showed necrotic lesions that appeared similar to lesions characteristic of the pathogen-induced hypersensitive reaction, and seen in the so-called lesion-mimic mutants. Plants expressing the antisense gene also had high endogenous salicylic acid levels, constitutive expression of thePR-1gene, and were resistant toPeronospora parasitica, consistent with the activation of systemic acquired resistance (SAR). Treatment of wild-type plants with sublethal concentrations of herbicides that inhibit PPO also induced defence responses that conferred enhanced tolerance toP. parasitica. This effect was not observed in NahG andnim1plants, which are compromised in their ability to activate SAR. These results demonstrate that genetic or chemical disruption of a metabolic pathway can lead to the induction of a set of defence responses including activation of SAR.