Production of Indole Acetic Acid in Culture by a Rhizobium Species from the Root Nodules of a Monocotyledonous Tree, Roystonea regia

The study of the rhizobial root nodules of the monocotyledonous tree Roystonea regia revealed that the Rhizobium sp. isolated from the root nodules produced high amounts (45.6 μg/ml) of indole acetic acid (IAA) from L-tryptophan supplemented basal medium. The IAA production reached its optimum using 3 mg/ml of L-tryptophan. The preferred carbon and nitrogen sources were glucose and KNO3 and the optimum concentrations 1% and 0.02%, respectively. FeSO4 × 7 H2O was found to be the only metal ion that increased IAA production. An optimum IAA production was also achieved when the basal medium was supplemented with glucose (1%), FeSO4 × 7 H2O (10 μg/ml), KNO3 (0.02%) as well as EDTA (5 μg/ml) and L-tryptophan (3 mg/ml). The possible role of IAA production in the monocotyledonous tree-Rhizobium symbiosis is discussed. Hormone production is shown to be the beneficial aspect of this symbiosis as shown earlier in dicotyledonous plants.