EXTENT OF HIGH-AFFINITY IRON TRANSPORT-SYSTEMS IN-FIELD ISOLATES OF RHIZOBIA

A Uruguayan rhizobia collection (67 isolates) obtained from nodules of Medicago sativa, Melilotus albus, Medicago polymorpha, Trifolium subt erraneum, Trifolium repens, Trifolium vesiculosum, Lotus corniculatus, Lotus subbiflorus, Lotus pedunculatus, Ornithopus sp. and Adesmia sp. has been examined to assess the occurrence of high affinity iron upta ke systems. CAS (Chrome-azurol S)-assay results suggested that most of the free-living form of these microsymbionts may produce siderophores . The highest siderophore production was observed among Medicago and T rifolium microsymbionts whereas no siderophore expression or moderate positive results were found among Lotus microsymbionts; suggesting tha t microsymbionts of legumes growing on neutral or alkaline soils may e xpress in vitro enhanced siderophore production. Electrophoretic patte rns of outer-membrane protein enriched fractions revealed that iron-li mited microsymbionts of Medicago sativa, Lotus corniculatus, Lotus sub biflorus, Trifolium repens, Trifolium subterraneum and Ornithopus sp. produced high molecular weight proteins (ranging from 64 to 94 kDa) co mpared to cells grown in iron-sufficient media.