Enhancing pasture legume performance in a changing climate: the case for supernodulating varieties

<p>Pasture legumes play a vital role in sustainable and productive agricultural systems by providing grazing forage and facilitating nitrogen cycling within the pasture sward. Through the process of symbiotic nitrogen fixation (SNF), legumes increase nitrogen availability by establishing a mutualistic association with rhizobia bacteria housed within root nodules, which fix atmospheric nitrogen in exchange for plant-derived carbohydrates. In response to this high carbohydrate demand from nodules, legumes display autoregulation of nodulation (AON) to restrict nodules number to the minimum required to sustain nitrogen supply under current photosynthetic levels. Mutations to the AON pathway can result in super-nodulating legumes, which typically grow smaller than wild-type plants, due to the high carbohydrate cost of producing and maintaining excessive nodule numbers. Recent research has demonstrated that these altered AON super-nodulating mutants are more responsive to elevated CO₂ (eCO₂) conditions, out-performing their non-mutant counterparts in biomass production and nutritional content (Zhang et al., 2023). It is thought that super-nodulating mutants are carbon-limited and can perform better at eCO₂ through improved photosynthesis, facilitated by the re-investment of nitrogen assimilates into photosynthetic machinery. As eCO₂ conditions are predicted to increase in future climates, harnessing the benefits of symbiotic nitrogen fixation through AON mutants has the potential to improve the CO₂ fertilisation effect, without protein yield penalties. This paper critically examines our current understanding of super-nodulating legumes and identifies the necessary steps for translating this research into practical applications within pasture systems.</p>