Mutant clover reveals the connections between carbon and nitrogen fixation for legumes

Getting nitrogen is a problem for some plants, but not for legumes that can fix nitrogen in the soil, thanks to root nodules. Getting enough carbon to process the nitrogen is more of a problem for legumes, so they control their number of nodules through an autoregulation mechanism. Some mutant plants ‘super-nodulate’ and these are thought to be carbon-limited. So what will happen to them with elevated carbon concentrations of carbon dioxide in the atmosphere? Yunfa Qiao and colleagues in China and Australia, compared the responses of Medicago truncatula super-nodulation mutants (sunn-4 and rdn1-1) and wild type to five CO₂ levels (300–850 μmol mol⁻¹), to find out.

Getting to know how plants respond to elevated carbon dioxide is important as it may stimulate photosynthesis. But if plants cannot access other nutrients, the increased photosynthesis might cause other problems. A puzzle is how will legumes, with their ability to develop nodules to take advantage of increased carbon dioxide, respond to future conditions?

Qiao and colleagues compared two super-nodulation mutants of M. truncatula or barrelclover, sunn-4 and rdn1-1 and one wild type to see how nitrogen fixation varied in response to elevated carbon dioxide concentrations. They grew the plants in five different concentrations of carbon dioxide. They then examined nodule formation and nitrogen fixation at eighteen and forty-two days after sowing.

The plants all increased biomass, nodule numbers and fixed nitrogen with increasing carbon dioxide – up to 700 parts per million. But the mutants fared differently. The rdn1-1 mutant tended to perform a little better than the wild type. But the sunn-4 mutant performed worse.

“The most striking difference was seen in the inability of the sunn-4 mutant to compensate for its reduced shoot biomass under eCO₂, while the rdn1-1 mutant exceeded the shoot biomass increases seen in the WT A17,” write and colleagues. “Similarly, the rdn1-1 mutant showed the highest total N₂ fixation, especially under eCO₂ (e.g. 700 μmol mol⁻¹). This was strongly correlated with the increase in shoot biomass… In contrast, the sunn-4 mutant struggled to increase shoot biomass with increasing N₂ fixation, as well as being characterized by lower total N₂ fixation per plant. Therefore, we conclude that the phenotype of the sunn-4 mutant, similar to the respective mutants in soybean…, is not related to C supply due to the specific mutation in the SUNN/NARK gene.”

RESEARCH ARTICLE

Qiao Y, Miao S, Jin J, Mathesius U, Tang C. 2021. Differential responses of the sunn4 and rdn1-1 super-nodulation mutants of Medicago truncatula to elevated atmospheric CO2. Annals of Botany 128: 441–452. https://doi.org/10.1093/aob/mcab098