Calcium/calmodulin-mediated microbial symbiotic interactions in plants

Cytoplasmic calcium (Ca(2+)) transients and nuclear Ca(2+) oscillations act as hubs during root nodulation and arbuscular mycorrhizal symbioses. Plants perceive bacterial Nod factors or fungal signals to induce the Ca(2+) oscillation in the nucleus of root hair cells, and subsequently activate calmodulin (CaM) and Ca(2+)/CaM-dependent protein kinase (CCaMK). Ca(2+) and CaM-bound CCaMK phosphorylate transcription factors then initiate down-stream signaling events. In addition, distinct Ca(2+) signatures are activated at different symbiotic stages: microbial colonization and infection; nodule formation; and mycorrhizal development. Ca(2+) acts as a key signal that regulates a complex interplay of downstream responses in many biological processes. This short review focuses on advances in Ca(2+) signaling-regulated symbiotic events. It is meant to be an introduction to readers in and outside the field of bacterial and fungal symbioses. We summarize the molecular mechanisms underlying Ca(2+)/CaM-mediated signaling in fine-tuning both local and systemic symbiotic events.