This paper originates from an address at the 8th International Symposium on Nitrogen Fixation with Non-Legumes, Sydney, NSW, December 2000 Different Azospirillum strains and some other plant growth-promoting rhizobacteria (PGPR) were screened for the occurrence of genes coding for denitrification and nitrogenase reductase ( nifH ) using polymerase chain reaction (PCR)-based techniques. All PGPR examined were nitrogenase-positive. Azospirillum strains were remarkably dissimilar with respect to denitrification capabilities, in particular with respect to genes of the dissimilatory nitrite reductase. A. brasilense , A. lipoferum and A. halopraeferens strains possess a cytochrome cd 1-containing nitrite reductase with low sequence similarities among them. A. irakense and A. doebereinerae have a Cu-containing nitrite reductase and A. amazonense is unable to denitrify. The molecular data were corroborated by activity measurements. The current results indicate that the inability to perform denitrification is unlikely a selective advantage for Azospirillum spp. and other associative bacteria for forming an association with plant roots.
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Kloos et al. (2001) studied this question.