The presence of tryptophan in soil and auxin production by indigenous soil microbes are considered to be important natural plant growth-promoting factors. In order to elucidate the natural regulation of microbial auxin synthesis, we treated different soils by an air drying/rewetting cycle and measured pool sizes of auxins, auxin precursors, and degradation products of tryptophan together with a range of respiration parameters. Potential (tryptophan addition) microbial production of indole-3-acetic acid (auxin) was predominant in the equilibrated fresh soils. Auxin production depended on the soil nutrient content, and the size and metabolic status of the microbial biomass. Immediately after rewetting, potential auxin production was low, whereas potential indole-3-ethanol and anthranilic acid production as well as basal respiration were transitionally enhanced. This was concurrent with proliferation ofr-strategist microbes. After the respiration flush, the natural tryptophan contents increased, indicating cell lysis, probably caused by a rise in protozoan grazing on ther-strategists. Auxin production was high in fresh and in re-equilibrating rewetted soils, probably due to nutritional limitations under stationary conditions. Hence, this high production was attributed to theK-strategist component of the soil microflora. The differences observed in the recovery of auxin production between the different rewetted soils suggest that original activities can become re-established rapidly when the indigenous microbial community is pre-adapted to the stress. We propose that the release of tryptophan, microbial auxin, and the shift towards indole-3-ethanol production function as stimulants for root development induced by environmental fluctuations.
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