Auxin biosynthesis and denitrification in plant growth promoting bacteria

Abstract

An indole-3-pyruvate decarboxylase (=IPDC) is known to be involved in the production of indole-3-acetic acid (=IAA) in the root associated Enterobacter cloacae strain FERM BP-1529 and in Azospirillum brasilense. Segments of the IPDC gene could be amplified by PCR from several different enterobacterial species which convert tryptophan to IAA. Five of the cloned segments were sequenced. All sequences showed strong homology to the IPDC-gene but were significantly different from the published sequence from A. brasilense Sp 245. Conserved region, detected in the alignment of the IPDC-gene segments were used as primers for PCR to identify this gene in Zea mays. An amplification product and a positive hybridization signal were the first genetical hints for the existence of this gene in higher plants. As the ability to produce the plant hormone was found not to be limited to nitrogen fixing plant growth promoting bacteria, but occurred also in the non-nitrogen fixing, free living Klebsiella aerogenes, the bacterial production of IAA might have other functions than promoting plant growth. To study the distribution of the denitrification ability in the genus Azospirillum by DNA-hybridization, oligonucleotides were synthesized for the genes for the dissimilatory cytochrome cd1 nitrite reductase (nirS), the N2O reductase (nosZ) and the dissimilatory Cu-nitrite reductase (nirK). These primers were used for PCR with the different Azospirillum species. In case of A. irakense KA3 an amplificate was obtained with the nirK oligonucleotides. Cloning, sequencing and hybridization of the fragment established the identity of the segment as part of the Cu-nitrite reductase gene, the occurrence of which was unknown for Azospirillum. In case of A. brasilense Sp7, a segment of the cytochrome cd1 nitrite reductase gene, nirS, and a segment of the N2O reductase gene, nosZ, could be amplified by PCR with strong sequence homology to the corresponding genes from P. stutzeri. Whereas the nosZ segment cross-hybridized also with DNA from all other A. brasilense and A. lipoferum strains investigated and with A. halopraeferens, the nirS segment gave only positive hybridization signals for A. brasilense Sp7 and strain Cd. nirS could be localized on a cosmid of a genomic gene bank of strain Sp7.