Arabidopsis SON1 Is an F-Box Protein That Regulates a Novel Induced Defense Response Independent of Both Salicylic Acid and Systemic Acquired Resistance

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Abstract
One of several induced defense responses in plants is systemic acquired resistance (SAR), which is regulated by salicylic acid and in Arabidopsis by the NIM1/NPR1 protein. To identify additional components of the SAR pathway or other genes that regulate SAR-independent resistance, we performed genetic suppressor screens of mutagenized nim1-1 seedlings, which are highly susceptible to infection by Peronospora parasitica. We isolated the son1 (suppressor of nim1-1) mutant, which shows full restoration of pathogen resistance without the induction of SAR-associated genes and expresses resistance when combined with a salicylate hydroxylase (nahG) transgene. These features indicate that son1-mediated resistance is distinct from SAR. Resistance is effective against both the virulent oomycete Peronospora and the bacterial pathogen Pseudomonas syringae pv tomato strain DC3000. We cloned SON1 and found it to encode a novel protein containing an F-box motif, an element found within the specificity determinant in the E3 ubiquitin-ligase complex. We propose the existence of a novel defense response that is independent of SAR and negatively regulated in Arabidopsis by SON1 through the ubiquitin-proteosome pathway.