Studies on the root nodules of leguminous trees V. Production of indole acetic acid by a Bradyrhizobium sp. from the root nodules of a leguminous tree, Acacia auriculiformis A. CUNN

Abstract

AbstractThe Bradyrhizobium sp., isolated from the root nodules of Acacia auriculiformis A. CUNN., a member of Mimosaceae, produced 17.0 μg/ml of indole acetic acid (IAA) from tryptophan in culture. The bacteria preferred D‐isomer over DL‐ or L‐isomer of tryptophan for IAA production. The IAA production could be promoted in culture medium up to 4.350% over control by supplementing the medium with fructose (0.5%), MnSO4 (0.1 μg/ml), glutamine (0.1%) and sodium dodecyl sulfate (SDS, 1.0 μg/ml). The possible relationship between the rhizobial IAA production and legume‐rhizobia symbiosis is discussed. The root nodules of leguminous plants are the result of rhizobial infection and are highly specialized structure which provide a special environment for the biological reduction of dinitrogen by the microsymbiont. Formerly nitrogen fixation was thought to be the only function of the nodules. Later hormone production by Rhizobium spp. drew great attention due to its involvement in the formation and growth of the nodules (KEFFORD et al. 1960, NUTMAN 1977). Rhizobium spp. are known to produce IAA from tryptophan in culture (DULLAART 1970, BHATTACHARYYA and BASU 1991). But most of the information is restricted to Rhizobium spp. isolated from the root nodules of the herbs of Fabaceae. This may be due to the immense economic importance of the pulses. Probably the first report on rhizobial IAA production of a tree legume was from Pongamia pinnata (SINHA and BASU 1981). Moreover, most of the leguminous plants so far studied belong to the family Fabaceae or Caesalpiniaceae. The report on rhizobial IAA production in a tree of the family Mimosaceae is very much scanty. Acacia auriculiformis is a leguminous tree of the neglected and unexplored family Mimosaceae. The mature nodule of A. auriculiformis contained 5.0 μg/g IAA (BHATTACHARYYA and BASU 1986/87), but any information on the ability of IAA production by its microsymbionts is lacking. But such information may throw some new light on the better understanding of the symbiosis of the root nodules. In addition, this neglected tree is also an useful sand binder and the seeds of this plant contain an important fatty acid (AMBASTA 1986). The purpose of this study was to throw some light on the IAA‐synthesizing capacity of the Bradyrhizobium sp. and to direct an effort to optimize the cultural requirement of the incubation medium for maximum IAA production by different supplements. This investigation yielded some interesting and tempting results which would be helpful to have an insight on the legume‐rhizobial symbiosis.