Bacterial‐induced calcium oscillations are common to nitrogen‐fixing associations of nodulating legumes and non‐legumes

Summary Plants that form root‐nodule symbioses are within a monophyletic ‘nitrogen‐fixing’ clade and associated signalling processes are shared with the arbuscular mycorrhizal symbiosis. Central to symbiotic signalling are nuclear‐associated oscillations in calcium ions (Ca 2+ ), occurring in the root hairs of several legume species in response to the rhizobial Nod factor signal. In this study we expanded the species analysed for activation of Ca 2+ oscillations, including non‐leguminous species within the nitrogen‐fixing clade. We showed that Ca 2+ oscillations are a common feature of legumes in their association with rhizobia, while Cercis , a non‐nodulating legume, does not show Ca 2+ oscillations in response to Nod factors from Sinorhizobium fredii NGR 234. Parasponia andersonii , a non‐legume that can associate with rhizobia, showed Nod factor‐induced calcium oscillations to S. fredii NGR 234 Nod factors, but its non‐nodulating sister species, Trema tomentosa , did not. Also within the nitrogen‐fixing clade are actinorhizal species that associate with Frankia bacteria and we showed that Alnus glutinosa induces Ca 2+ oscillations in root hairs in response to exudates from Frankia alni , but not to S. fredii NGR 234 Nod factors. We conclude that the ability to mount Ca 2+ oscillations in response to symbiotic bacteria is a common feature of nodulating species within the nitrogen‐fixing clade.