Nuclear calcium changes at the core of symbiosis signalling

Many plants acquire a significant proportion of their nutrient requirements through mutualistic symbiotic interactions with micro-organisms. Legumes in particular acquire the macronutrients nitrogen and phosphorus, and most likely an array of micro-nutrients, from interactions with nitrogen-fixing bacteria and with mycorrhizal fungi. Although the structures formed to support these interactions are different (nodules compared with arbuscules), there is conservation in early signalling between these two symbioses. It is likely that different receptors for rhizobial or mycorrhizal signals induce responses that feed into a common signalling pathway. In the nodulation signalling pathway, calcium plays an essential role as a secondary messenger, and the component that probably transduces the calcium signal is a unique calcium-activated kinase that is required for both mycorrhization and nodulation. The nodulation signalling pathway contains transcriptional regulators downstream of the calcium-activated kinase that are not required for the mycorrhizal symbiosis. This suggests that different symbiosis-specific signalling pathways are activated downstream of the calcium-activated kinase, and raises the question of how specificity of gene induction can be achieved in two pathways that are both likely to use calcium and a unique calcium-activated kinase to induce different downstream events.