Differential cross-utilization of heterologous siderophores by nodule bacteria of Cajanus cajan and its possible role in growth under iron-limited conditions

Siderophores are ferric specific ligands that are involved in receptor specific iron transport into bacteria. Partially purified siderophores from 37 different rhizospheric bacterial isolates, and 17 Cajanus cajan root nodule isolates were tested for their utilization by the 17 nodule isolates as test organisms. Significant variation in the siderophore cross-utilization pattern was observed. Catecholate siderophores were produced predominantly by nodule isolates whereas rhizospheric isolates produced catecholates, as well as hydroxamates. The quantity of hydroxamate produced was higher than that of catecholates. Considerable variation was observed with respect to siderophore cross-utilization amongst nodule isolates obtained from a particular location. Isolates collected from different nodules of same plant also showed considerable variation with respect to siderophore cross-utilization. Isolates, which showed, higher siderophores cross-utilizing ability, when tested for homologous (from other nodule bacteria) siderophores also gave good cross-utilization of heterologous (from non-nodule rhizospheric bacteria) siderophores. Isolate GHU(iii), the highest cross-utilizer of homologous and heterologous siderophores showed a significant increase in growth in the presence of exogenously added siderophores. Similar results were observed during the co-inoculation studies with rhizospheric isolate under iron-limited condition. Both isolates GAU(1)1 and AP(6)4, low cross-utilizers of siderophores, failed to show growth increase in response to co-inoculation and addition of siderophores. The iron regulated outer membrane protein (IROMP) profiles of strains showing maximum or least cross-utilization did not differ. The significance of these findings in context to survivability under natural conditions and the possible reasons accounting for high siderophore cross-utilizing status of isolate GHU(iii) is discussed.