Endophytic Bacteria Gluconacetobacter diazotrophicus Regulates Nitrogen Uptake in Sugarcane Roots

Minimizing the nitrogen (N) fertilizer input for the expanding production of biofuel crops is a major challenge to reduce environmental impacts. Biological nitrogen fixation (BNF) associated with endophytic bacteria may have a significant contribution for the success of sugarcane yields in Brazil. In addition, plants acquire inorganic N sources from the soil. However, the regulation of N uptake during the sugarcane-endophytic association remains unknown. Here, we performed the physiological and the molecular characterization of N uptake in high nitrogen-fixing sugarcane genotype 'SP70-1143' in association with endophytic bacteria. To evaluate the N uptake in sugarcane roots, short-term influx of ¹⁵N-labeled ammonium or nitrate was determined in plantlets grown in association or not with Gluconacetobacter diazotrophicus. Significant differences in ¹⁵N-ammonium acquisition in sugarcane roots were found 10d after inoculation when compared to non-inoculated plants grown under N sufficiency, while ¹⁵N-nitrate influxes showed a transient increase after 3d under association with bacteria. These results indicate a distinct regulation of N uptake during endophyte-plant interaction in which nitrate acquisition peaked earlier and ammonium later. The high affinity nitrogen uptake relies on orthologous of plasma membrane proteins AMMONIUM TRANSPORTERS (AMT) and NITRATE TRANSPORTERS (NRT). The identification of major NRTs and AMTs responsible for N acquisition showed that up regulation of ScAMT1;3 and ScNRT2;1 followed the high N uptake rates during inoculation with endophytes. Taken together, the data suggest that the temporal regulation of N transporters is a part of differential signaling triggered by Gluconacetobacter diazotrophicus coordinating N acquisition in sugarcane roots.