Photosynthetic Gains in Super-Nodulating Mutants of Medicago truncatula under Elevated Atmospheric CO 2 Conditions.

Legumes are generally considered to be more responsive to elevated CO 2 (eCO 2 ) conditions due to the benefits provided by symbiotic nitrogen fixation. In response to high carbohydrate demand from nodules, legumes display autoregulation of nodulation (AON) to restrict nodules to the minimum number necessary to sustain nitrogen supply under current photosynthetic levels. AON mutants super-nodulate and typically grow smaller than wild-type plants under ambient CO 2 . Here, we show that AON super-nodulating mutants have substantially higher biomass under eCO 2 conditions, which is sustained through increased photosynthetic investment. We examined photosynthetic and physiological traits across super-nodulating rdn1-1 (Root Determined Nodulation) and sunn4 (Super Numeric Nodules) and non-nodulating nfp1 (Nod Factor Perception) Medicago truncatula mutants. Under eCO 2 conditions, super-nodulating plants exhibited increased rates of carboxylation (V cmax ) and electron transport (J) relative to wild-type and non-nodulating counterparts. The substantially higher rate of CO 2 assimilation in eCO 2 -grown sunn4 super-nodulating plants was sustained through increased production of key photosynthetic enzymes, including Rieske FeS. We hypothesize that AON mutants are carbon-limited and can perform better at eCO 2 through improved photosynthesis. Nodulating legumes, especially those with higher nitrogen fixation capability, are likely to out-perform non-nodulating plants under future CO 2 conditions and will be important tools for understanding carbon and nitrogen partitioning under eCO 2 conditions and future crop improvements.