This paper originates from an address at the 8th International Symposium on Nitrogen Fixation with Non-Legumes, Sydney, NSW, December 2000
Different Azospirillumstrains 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
cd1-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.