Salicylic Acid Interferes with Tobacco Mosaic Virus Replication via a Novel Salicylhydroxamic Acid-Sensitive Mechanism

Salicylic acid (SA) induces resistance to all plant pathogens, including bacteria, fungi, and viruses, but the mechanism by which SA engenders resistance to viruses is not known. Pretreatment of tobacco mosaic virus (TMV)-susceptible (nn genotype) tobacco tissue with SA reduced the levels of viral RNAs and viral coat protein accumulating after inoculation with TMV. Viral RNAs were not affected equally, suggesting that SA treatment interferes with TMV replication. Salicylhydroxamic acid (SHAM), an inhibitor of the mitochondrial alternative oxidase, antagonized both SA-induced resistance to TMV in nn genotype plants and SA-induced acquired resistance in resistant (NN genotype) tobacco. SHAM did not inhibit induction of the PR-1 pathogenesis-related protein or induction of resistance to Erwinia carotovora or Botrytis cinerea by SA. This indicates that SA induces resistance to TMV via a novel SHAM-sensitive signal transduction pathway (potentially involving alternative oxidase), which is distinct from that leading to resistance to bacteria and fungi.