Two ferric uptake regulator proteins in <i>Ralstonia pseudosolanacearum</i> strain OE1-1 function cooperatively in response to the extracellular iron level under ferrous iron-rich conditions

The <i>Ralstonia solanacearum</i> species complex (RSSC) comprising soil-borne Gram-negative phytopathogenic bacteria causes bacterial wilt diseases of diverse crop plants. Considering that phylotype I strain OE1-1 enters iron-rich roots from iron-deficient soil during an infection of tomato plants, the mechanisms controlling strain OE1-1 gene expression in response to extracellular iron levels should be clarified. In this study, RSSC was revealed to have two ferric uptake regulator homologs (Fur1 and Fur2). Notably, Fur1 and Fur2 cooperatively repress the expression of genes related to siderophores (Fe³⁺-chelating compounds) as well as the extracellular Fe³⁺-chelating activity in the presence of sufficient amounts of extracellular Fe²⁺. Additionally, Fur1 and Fur2 contribute to the virulence of strain OE1-1 in tomato plants. These findings suggest that RSSC uses two Fur proteins to modulate extracellular Fe³⁺-chelating activities in response to extracellular iron levels to maintain virulence in crop plants.