The Brn~i~rhi=ohiuni sp.. isolated from the root nodules of Acnciu uirriculIforttzis A. CUNN.. a member of Mitnosuceae, produced 17.0 pg/ml of indole acetic acid (IAA) from tryptophan in culture. The bacteria preferred D-isomer over UL-or 1.-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%), MnSO, (0.1 pg/ml), glutamine (O.lu/,) and sodium dodecyl sulfate (SDS, 1.0 pg/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 Rhizohitmi spp. drew great attention due to its involvement in the formation and growth of the nodules (KEFFORD et al. 1960al. , NUTMAN 1977)). Rhizohiurn spp. are known to produce IAA from tryptophan in culture ( DULLAART 1970, BHATTACHARYYA andBASU 1991). But most of the information is restricted to Rhizabiirni spp. isolated from the root nodules of the herbs of Fnhaceue. 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 Pongnniia pirmaru (SINHA and BASU 1981). Moreover. most of the leguminous plants so far studied belong to the family Fuhnceue or Cnesulphri~rc~~ne. The report OR rhizobial IAA production in a tree of the family Mittzosaceac is very much scanty. Actrcitr uuricirliforniis is a leguminous tree of the neglected and unexplored family Mimosaceue. The mature nodule of A . cruricul~fort~iis contained 5.0 pg/g I A A (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 thc 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 Brn~idzi:ohiziru sp. and lo direct an effort to optimizc the cultural requirement of the incubation medium for maximum IAA production by different supplements. This invcstigation yielded some interesting and tempting results which would be helpful to have an insight on the legume-rhizobial symbiosis.
Methods
Organisms: The microsymbiont was isolated from the healthy mature root nodules of .4caci~ crtir~cit~~formis A. CUNN.. and was identified f o ~~o w i n g JORDAN ( 1984) as a specics of B~Nc!i,rlri=ohiurtt. Medium and growth conditions: The basal medium for incubation was yeast extract mineral medium of SKERMAN (1959) with 1 YO mannitol (YEM) and having 0.01% CaCI2 x 2 H 2 0 instead of NaCl and CaCO, at p H 6.8, and supplemented with different concentrations of I.-or DL-or D-tryptophan. The bacteria were incubated in 20 ml medium in 100 ml ERLENMEYER flasks in three replicates at 30 2 "C on a rotary shaker for 150 hr (the time for maximum IAA production by this bacteria). Growth was measured turbidimetrically by an EEL (EVANS Electroselinium Ltd., U K ) colorimeter at 540 nm.
Determination of IAA production: T o estimate the amount of IAA produced, the medium was centrifuged at 5000 g and the cell frec supernatant was used for IAA extraction following SINHA and BASU (1981). The IAA containing extract was purified and identified by comparing the Rf (0.85) with 16'