Bacterial-induced calcium oscillations are common to nitrogen-fixing associations of nodulating legumes and nonlegumes

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 (Ca2+), 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 Ca2+ oscillations, including nonleguminous species within the nitrogen-fixing clade. We showed that Ca2+ oscillations are a common feature of legumes in their association with rhizobia, while Cercis, a non-nodulating legume, does not show Ca2+ oscillations in response to Nod factors from Sinorhizobium fredii NGR234. Parasponia andersonii, a nonlegume 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 Ca2+ 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 Ca2+ oscillations in response to symbiotic bacteria is a common feature of nodulating species within the nitrogen-fixing clade.