Suppression and restoration of lesion formation in arabidopsis Isd mutants

Systemic acquired resistance (SAR) is a broad-spectrum, systemic defense response that is activated in many plant species after pathogen infection. We have previously described Arabidopsss mutants constitutively SAR and concomitantly develop lesions simulating disease (lsd). Here, we describe two new mutants, lsd6 and lsd7, that develop spontaneous necrotic lesions possess elevated levels of salicylic acid (SA) as well as heightened disease resistance, similar to the previously characterized lsd and accelerated cell death (acd2) mutants. Genetic analysis of lsd6 lsd7 that the mutant phenotypes as simple dominant traits. When crossed with transgenic Arabidopsis plants containing the SA-degrading enzyme salicylate hydroxylase, the F₁ progeny showed suppression of both SAR gene expression and resistance. In addition, salicylate hydroxylase suppressed lesion formation in the F₁ progeny, suggesting that SA or some SA-dependent process may have a role in pathogen-associated cell death. Surprisingly, lesions were restored in the lsd6 F₁ progeny after the application of either 2,6-dichloroisonicotinic acid or SA. Lesions were not restored by with either compound in the lsd7 F₁ plants. Our findings demonstrate that steps early in the signal transduction pathway leading to SAR and disease resistance are potentiated by later events, suggesting feedback control of lesion formation.