Nitrogen metabolism and growth of wheat plant under diazotrophic endophytic bacteria inoculation

The plant growth-promoting bacteria (PGPB) improve nutrient uptake, crop yield, reduce fertilization cost and minimize the environmental pollution by decreasing nitrogen leaching. This study reports the impact of three diazotrophic endophytic bacteria on the metabolism, physiology and growth of wheat (Triticum aestivum hard L.) plants. A greenhouse experiment was carried out on an unsterilized soil with three levels of nitrogen fertilizer: no-fertilizer, half-recommended N-fertilizer rate and full recommended N-fertilizer rate; and with the inoculation of spontaneous antiobiotic resistant mutants strains, IAC-AT-8 (Azospirillum brasilense), IAC-HT-11 (Achromobacter insolitus) and IAC-HT-12 (Zoogloea ramigera). All the three bacteria were able to modify, in a specific way, the nitrate reductase and the glutamine synthetase activities and improve the chlorophyll content and promote root and shoot weight. All the strains produced IAA in pure cells culture and showed the presence of gene nifH. The N content and the N-use efficiency index were dependent on the endophytic bacteria strain and on the plant tissue assessed. The performance of the above parameters was affected by the nitrogen rate. The different responses suggest that the successful colonization and the growth promotion were achieved by distinct mechanisms. In general, the N metabolism and plant growth were positively impacted by the strains inoculation, mainly A. insolitus, strain IAC-HT-11. Moreover, up to now this is the first study showing Achromobacter insolitus as an endophyte bacterium able to promote wheat plant growth related to the plant N metabolism. The results confirm that the endophyte strains act as PGPB and have potential to be developed as inoculants to integrate crop management.