Department of Plant Pathology, Auburn University, AL 36849, U.S.A.
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.