Leveraging radiotracers in the model grass, Setaria viridis, to unravel the physiological and metabolic basis for biological nitrogen uptake via associative N­2-fixing rhizobacteria

While nitrogen-fixing rhizobacteria have been shown to promote plant growth, it remains unclear whether biological nitrogen fixation (BNF) plays a major role in this process. This talk will address new insights into the physiological and metabolic basis for understanding plant growth promotion that leveraged a combination of radiotracers, Azospirillum brasilense and Herbaspirillum seropedicae bacteria, and the growth-responsive C4 grass, Setaria viridis (A10.1). ¹¹CO₂ (t_½ 20.4 min), fixed within source leaves, was used to quantify carbon input and allocation of ¹¹C-photoassimilates to roots including exudation, providing a comprehensive look at how a plant uses its new carbon resources under different growth regimes, including nitrogen limitation with or without bacterial inoculation. Radiometabolite analyses also provided insight into how a plant’s metabolic landscape is altered by these conditions. When stressed by nitrogen limitation, plants rapidly re-program their physiology and metabolism to support compensatory root growth for survival. However, inoculation with bacteria re-instates “normal” host physiological and metabolic behavior suggesting that BNF is indeed important.  This hypothesis was tested using ¹³N₂ (t_½ 9.97 min) which provided direct evidence for ¹³N incorporation into inoculated plants.  In fact, using a hyper N₂-fixing strain of A. brasilense (Hm053), we showed that all the plant’s nitrogen demands could be met by BNF. Finally, using the ¹³NO₃^- tracer, we mapped nitrate uptake kinetics against the growth condition to show that the Hm053 strain impairs nitrate transporter function relative to non-inoculated plants, suggesting that BNF is the primary means for host nitrogen input.  Research was supported by DOE-BER.