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Involvement of IAA in the interaction between Azospirillum brasilense and Panicum miliaceum roots

AMALIA HARARI, J. KIGEL and Y. OKON
Plant and Soil
Vol. 110, No. 2 (August (II), 1988), pp. 275-282
Published by: Springer
Stable URL: http://www.jstor.org/stable/42937614
Page Count: 8
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Involvement of IAA in the interaction between Azospirillum brasilense and Panicum miliaceum roots
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Abstract

The possible involvement of IAA in the effect that Azospirillum brasilense has on the elongation and morphology of Panicum miliaceum roots was examined by comparing in a Petri dish system the effects of inoculation with a wild strain (Cd) with those of an IAA-overproducing mutant (FT-326). Both bacterial strains produced IAA in culture in the absence of tryptophan. At the stationary growth phase, production of IAA by FT-326 was ca. 12 times greater than that of Cd. When inoculation was made with bacterial concentrations higher than 10 6 colony forming units ml⁻¹ (CFU ml⁻¹), both strains inhibited root elongation to the same extent. At lower concentrations Cd enhanced elongation by 15-20%, while FT-326 was ineffective. Both strains promoted root-hair development, and root-hairs were produced nearer the root tip the higher the bacterial concentration (e.g. root elongation region was reduced). Effects of FT-326 on root-hair development were greater than those of Cd. Acidified ether extracts of Cd and FT-326 cultures had inhibitory or promoting effects on root elongation depending on the dilution applied. At low dilutions, extracts from FT-326 were more inhibitory for elongation than those from Cd. At higher dilutions root elongation was promoted, but FT-326 extracts had to be more diluted than those from Cd. Dilutions that promoted root elongation contained supra-optimal concentrations of IAA, 1-3 orders of magnitude higher than those required for optimal enhancement by synthetic IAA. It is suggested that the bacteria produce in culture an IAA-antagonist or growth inhibitor that decreases the effectiveness of IAA action. The large variability reported for the effects of Azospirillum on root elongation could be the result of the opposite effects on root elongation of IAA and other compounds produced by the bacteria.

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