Denitrification within the genus Azospirillum and other associative bacteria

Different Azospirillum strains and some other plant growth-promoting rhizob acteria (PGPR) were screened for the occurrence of genes coding for denitri fication and nitrogenase reductase (nifH) using polymerase chain reaction ( PCR)-based techniques. All PGPR examined were nitrogenase-positive. Azospir illum strains were remarkably dissimilar with respect to denitrification ca pabilities, in particular with respect to genes of the dissimilatory nitrit e reductase. A. brasilense, A. lipoferum and A. halopraeferens strains poss ess a cytochrome cd(1)-containing nitrite reductase with low sequence simil arities among them. A. irakense and A. doebereinerae have a Cu-containing n itrite reductase and A. amazonense is unable to denitrify. The molecular da ta were corroborated by activity measurements. The current results indicate that the inability to perform denitrification is unlikely a selective adva ntage for Azospirillum spp. and other associative bacteria for forming an a ssociation with plant roots.