What is it about?

Calcium is an important second messenger and calcium signaling regulates many important physiological functions in eukaryotes. In the symbiotic interaction between legumes and rhizobia or mycorrhizal fungi, the nuclear calcium oscillation signal in root cells is the core signaling event. As an early response to Nod factors, which are the primary symbiotic signal, nuclear calcium oscillations regulate the expression of downstream symbiosis genes to mediate infection and promote nodule formation. This study revealed the molecular mechanism by which DMI1 and CNGC15, two calcium channel proteins located in the nuclear envelope, co-encode the symbiotic nuclear calcium signal.

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Why is it important?

Understanding the molecular mechanisms involved in generating and interpretating the calcium signal fingerprint during beneficial symbiotic interactions between microbes and plants is of great significance for the sustainable development of agriculture. Despite being recognized as the central event in symbiotic signaling between legumes and symbiotic microorganisms, how nuclear calcium oscillations are encoded is poorly understood. DMI1 and CNGC15, which localize and interact in the nuclear envelope, are calcium channel proteins involved in producing the nuclear and perinuclear symbiotic calcium oscillations (Charpentier et al., 2016). This study describes a gain-of function mutation in DMI1 that induces spontaneous calcium oscillations and spontaneous nodule formation. We show that expression of the DMI1 gain-of-function mutations and CNGC15s in mammalian HEK293T cell lines is sufficient to recapitulate symbiotic-like calcium oscillations. We further demonstrate that this nuclear-associated DMI1-CNGC15 calcium channel complex can be activated by conformational changes in DMI1. This symbiotic calcium channel complex is currently the only known mechanism for nuclear membrane calcium release in plants, and represents a completely different mechanism from that described in animal systems.

Perspectives

I am honored to be the first author in this study. It's an exciting story. In this study, we investigated the phenotype of the gain-of-function mutant spd1, which was the first spontaneous nodulation mutant in Medicago truncatula, it gives us a breakthrough point to analyze the function mechanism of symbiotic calcium channel proteins. The calcium channel activation model of DMI1 was analyzed from the perspective of protein conformational changes. This study elucidated currently the only known mechanism of symbiosis calcium channel complex in encoding nuclear calcium signal. The symbiosis ability of legumes is so remarkable that the encoding mechanism of nuclear calcium signal, the core event of legumes, deserves more attention. I believe that this work can be a catalyst for even better stories to come.

Haiyue Liu
Chinese Academy of Sciences Center for Excellence in Molecular Plant Science

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This page is a summary of: Constitutive activation of a nuclear-localized calcium channel complex in Medicago truncatula, Proceedings of the National Academy of Sciences, August 2022, Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.2205920119.
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