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

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 NGR234. Parasponia andersonii, a non‐legume that can associate with rhizobia, showed Nod factor‐induced calcium oscillations to S. fredii NGR234 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 NGR234 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.