Calcium/calmodulin-mediated microbial symbiotic interactions in plants.

Cytoplasmic calcium (Ca) transients and nuclear Ca oscillations act as hubs during root nodulation and arbuscular mycorrhizal symbioses. Plants perceive bacterial Nod factors or fungal signals to induce the Ca oscillation in the nucleus of root hair cells, and subsequently activate calmodulin (CaM) and Ca/CaM-dependent protein kinase (CCaMK). Ca and CaM-bound CCaMK phosphorylate transcription factors then initiate down-stream signaling events. In addition, distinct Ca signatures are activated at different symbiotic stages: microbial colonization and infection; nodule formation; and mycorrhizal development. Ca 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 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/CaM-mediated signaling in fine-tuning both local and systemic symbiotic events.