Not AvailableIn our endeavor to improve the nitrogen fixation efficiency of a soil diazotroph that would be unaffected by synthetic nitrogenous fertilizers, we have deleted a part of the negative regulatory gene nifL and constitutively expressed the
positive regulatory gene nifA in the chromosome of Azotobacter chroococcum
CBD15, a strain isolated from the local field soil. No antibiotic resistance gene or
other foreign gene was present in the chromosome of the engineered strain.
Wheat seeds inoculated with this engineered strain, which we have named Azotobacter chroococcum HKD15, were tested for 3 years in pots and 1 year in the
field. The yield of wheat was enhanced by 60% due to inoculation of seeds by
A. chroococcum HKD15 in the absence of any urea application. Ammonium only
marginally affected acetylene reduction by the engineered Azotobacter strain.
When urea was also applied, the same wheat yield could be sustained by using
seeds inoculated with A. chroococcum HKD15 and using 85 kg less urea (40
kg less nitrogen) than the usual 257 kg urea (120 kg nitrogen) per hectare.
Wheat plants arising from the seeds inoculated with the engineered Azotobacter
strain exhibited far superior overall performance, had much higher dry weight
and nitrogen content, and assimilated molecular 15N much better. A nitrogen
balance experiment also revealed much higher total nitrogen content. Indole-3-
acetic acid (IAA) production by the wild type and that by the engineered strain
were about the same. Inoculation of the wheat seeds with A. chroococcum
HKD15 did not adversely affect the microbial population in the field rhizosphere
soil.
INot Availabl