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

We have used an antisense expression technology in Arabidopsis based on the yeast GAL4/UAS transactivation system (Guyer et al., Genetics, 1998; 149:6 33-639) to reduce levels of protoporphyrinogen IX oxidase (PPO), the last c ommon enzyme of the biosynthesis of the haem group and chlorophyll. Plants expressing the antisense PPO gene presented growth alterations and their le aves showed necrotic lesions that appeared similar to lesions characteristi c of the pathogen-induced hypersensitive reaction, and seen in the so-calle d lesion-mimic mutants. Plants expressing the antisense gene also had high endogenous salicylic acid levels, constitutive expression of the PR-1 gene, and were resistant to Peronospora parasitica, consistent with the activati on of systemic acquired resistance (SAR). Treatment of wild-type plants wit h sublethal concentrations of herbicides that inhibit PPO also induced defe nce responses that conferred enhanced tolerance to P. parasitica. This effe ct was not observed in NahG and nim1 plants, which are compromised in their ability to activate SAP. These results demonstrate that genetic or chemica l disruption of a metabolic pathway can lead to the induction of a set of d efence responses including activation of SAR.