Role of iron in rhizobacteria-mediated induced systemic resistance of cucumber

Seed treatment with the rhizosphere bacterium Serratia marcescens strain 90 -166 suppressed anthracnose of cucumber, caused by Colletotrichum orbicular e, through induced systemic resistance (ISR). When the iron concentration o f a planting mix was decreased by addition of an iron chelator, suppression of cucumber anthracnose by strain 90-166 was significantly improved. Strai n 90-166 produced 465 +/- 70 mg/liter of catechol siderophore, as determine d by the Rioux assay in deferrated King's medium B. The hypothesis that a c atechol siderophore produced by strain 90-166 may be responsible for induct ion of systemic resistance by this strain was tested by evaluating disease suppression by a mini-Tn5-phoA mutant deficient in siderophore production. Sequence analysis of genomic DNA flanking the mini-Tn5-phoA insertion ident ified the target gene as entA, which encodes an enzyme in the catechol side rophore biosynthetic pathways of several bacteria. Severity of anthracnose of cucumbers treated with the entA mutant was not significantly different ( P = 0.05) from the control, whereas plants treated with wild-type 90-166 ha d significantly less disease (P = 0.05) than the control. Total (internal a nd external) population sizes of 90-166 and the entA mutant on roots did no t differ significantly (P = 0.05) at any sample time, whereas internal popu lation sizes of the entA mutant were significantly lower (P = 0.05) than th ose of the wild-type strain at two sampling times. These data suggest that catechol siderophore biosynthesis genes in Serratia marcescens 90-166 are a ssociated with ISR but that this role may be indirect via a reduction in in ternal root populations.