Salicylic Acid Produced by Serratia marcescens 90-166 Is Not the Primary Determinant of Induced Systemic Resistance in Cucumber or Tobacco

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Abstract
The rhizobacterial strain Serratia marcescens 90–66 mediates induced systemic resistance (ISR) to fungal, bacterial, and viral pathogens. It was determined that strain 90–166 produced salicylic acid (SA), using the salicylateresponsive reporter plasmid pUTK21. High-pressure liquid chromatography analysis of culture extracts confirmedthe production of SA in broth culture. Mini-Tn5phoA mutants, which did not produce detectable amounts of SA, retained ISR activity in cucumber against the fungal pathogen Colletotrichum orbiculare. Strain 90–166 induced disease resistance to Pseudomonas syringae pv. tabaci in wild-type Xanthi-nc and transgenic NahG-10 tobacco expressing salicylate hydroxylase. Increasing ferric iron concentrations in vitro reduced SA production below detectable limits, and increasing ferric iron concentration in planta, applied as a root drench, significantly reduced the level of ISR observed in cucumber to C. orbiculare. An ISR¯ mutant (90-166-2882) still produced SA. The results of this study indicate that SA produced by 90–166 is not the primary bacterial determinant of ISR and that this bacterial-mediated ISR system is affected by iron concentration.