METABOLISM OF TRYPTOPHAN IN SOIL

Production of auxins in soil is stimulated by the addition Of L-trypto phan (L-TRP) as a precursor. Metabolism Of L-TRP results in the format ion of many products including niacin (kynurenine pathway) and seroton in (hydroxylation) in addition to auxins and is regulated by two disti nct enzymes, L-TRP-2,3-dioxygenase (TDO) and indole-2,3-dioxygenase (I DO). The roles of IDO and TDO in L-TRP metabolism and indole-3-acetic acid (IAA) formation in soil were studied by the use of bactericides a nd specific enzyme inhibitors. The addition of a broad spectrum bacter icide, chloramphenicol, to soil resulted in a significant decrease of L-TRP catabolism and stimulated hydroxylation Of L-TRP. There was no e vidence in this work to support the view that L-TRP is transformed thr ough extracellular reactions in soil. Inhibition of TDO by the additio n of benzaldehyde and norharman (a potent inhibitor of TDO and IDO) al so reduced L-TRP catabolism in soil. Although IAA was detected in all soils treated with bactericides or enzyme inhibitors, production of th is secondary metabolite (IAA) was not reduced by blocking the kynureni ne pathway or hydroxylation. Auxins were not detected in soils tested with the addition of intermediates of the TDO and IDO pathways (kynure nine and 5-hydroxytryptophan, respectively). Incubation of D-TRP as a possible auxin precursor in California soils resulted in a significant decrease in metabolism compared with L-TRP additions as measured by C O2 evolution and HPLC analysis. Rhizosphere bacteria isolated from let tuce (Lactuca sativa) were found to produce more IAA derived from L-TR P after they had been cultured on a medium containing L-alanine, L-asp aragine and L-lysine compared with cultures that had been exposed to L -TRP or nutrient agar and then grown on a L-TRP supplemented minimal s alts liquid medium. A soil which had extremely low yields of L-TRP-der ived IAA increased production of auxin derivatives, including TRP-deri ved IAA, when incubated for 1 week with the amino acids, L-lysine, L-a sparagine and L-alanine. The results suggest that synthesis of IAA in soil may not be the result of direct cataboliSM Of L-TRP but may be li nked to the cometabolism Of L-TRP by the broad speCifiCity Of L-aminot ransferases present in soil.