Elevated atmospheric CO2 concentration triggers redistribution of nitrogen to promote tillering in rice.

Elevated atmospheric CO₂ concentration (eCO₂) often reduces nitrogen (N) content in rice plants and stimulates tillering. However, there is a general consensus that reduced N would constrain rice tillering. To resolve this contradiction, we investigated N distribution and transcriptomic changes in different rice plant organs after subjecting them to eCO₂ and different N application rates. Our results showed that eCO₂ significantly promoted rice tillers (by 0.6, 1.1, 1.7, and 2.1 tillers/plant at 0, 75, 150, and 225 kg N ha^-1 N application rates, respectively) and more tillers were produced under higher N application rates, confirming that N availability constrained tillering in the early stages of growth. Although N content declined in the leaves (-11.0 to -20.7 mg g^-1) and sheaths (-9.8 to -28.8 mg g^-1) of rice plants exposed to eCO₂, the N content of newly emerged tillers on plants exposed to eCO₂ equaled or exceeded the N content of tillers produced under ambient CO₂ conditions. Apparently, the redistribution of N within the plant per se was a critical adaptation strategy to the eCO₂ condition. Transcriptomic analysis revealed that eCO₂ induced less extensive alteration of gene expression than did N application. Most importantly, the expression levels of multiple N-related transporters and receptors such as nitrate transporter NRT2.3a/b and NRT1.1a/b were differentially regulated in leaf and shoot apical meristem, suggesting that multiple genes were involved in sensing the N signal and transporting N metabolites to adapt to eCO₂. The redistribution of N in different organs could be a universal adaptation strategy of terrestrial plants to eCO₂.