The implication of calcium as intracellular messenger in the arbuscular mycorrhizal (AM) symbiosis has not yet been directly demonstrated, although often envisaged. We used soybean (Glycine max) cell cultures stably expressing the bioluminescent Ca2+ indicator aequorin to detect intracellular Ca2+ changes in response to the culture medium of spores of Gigaspora margarita germinating in the absence of the plant partner. Rapid and transient elevations in cytosolic free Ca2+ were recorded, indicating that diffusible molecules released by the mycorrhizal fungus are perceived by host plant cells through a Ca2+-mediated signaling. Similar responses were also triggered by two Glomus isolates. The fungal molecules active in generating the Ca2+ transient were constitutively released in the medium, and the induced Ca2+ signature was not modified by the coculture of germinating spores with plant cells. Even ungerminated spores were able to generate the signaling molecules, as proven when the germination was blocked by a low temperature. The fungal molecules were found to be stable to heat treatment, of small molecular mass (<3 kD), and, on the basis of extraction with an organic solvent, partially lipophilic. Evidence for the specificity of such an early fungal signal to the AM symbiosis is suggested by the lack of a Ca2+ response in cultured cells of the nonhost plant Arabidopsis (Arabidopsis thaliana) and by the up-regulation in soybean cells of genes related to Medicago truncatula DMI1, DMI2, and DMI3 and considered essential for the establishment of the AM symbiosis.

Plant organelle function must constantly adjust to environmental conditions, which requires dynamic coordination. Ca²⁺ signaling may play a central role in this process. Free Ca²⁺ dynamics are tightly regulated and differ markedly between the cytosol, plastid stroma, and mitochondrial matrix. The mechanistic basis of compartment-specific Ca²⁺ dynamics is poorly understood. Here, we studied the function of At-MICU, an EF-hand protein of Arabidopsis thaliana with homology to constituents of the mitochondrial Ca²⁺ uniporter machinery in mammals. MICU binds Ca²⁺ and localizes to the mitochondria in Arabidopsis. In vivo imaging of roots expressing a genetically encoded Ca²⁺ sensor in the mitochondrial matrix revealed that lack of MICU increased resting concentrations of free Ca²⁺ in the matrix. Furthermore, Ca²⁺ elevations triggered by auxin and extracellular ATP occurred more rapidly and reached higher maximal concentrations in the mitochondria of micu mutants, whereas cytosolic Ca²⁺ signatures remained unchanged. These findings support the idea that a conserved uniporter system, with composition and regulation distinct from the mammalian machinery, mediates mitochondrial Ca²⁺ uptake in plants under in vivo conditions. They further suggest that MICU acts as a throttle that controls Ca²⁺ uptake by moderating influx, thereby shaping Ca²⁺ signatures in the matrix and preserving mitochondrial homeostasis. Our results open the door to genetic dissection of mitochondrial Ca²⁺ signaling in plants.

Calcium (Ca²⁺) is among the most important intracellular messengers in living organisms. Understanding the players and dynamics of Ca²⁺ signalling pathways in plants may help to unravel the molecular basis of their exceptional flexibility to respond and adapt to different stimuli. In the present review, we focus on new tools that have recently revolutionized our view of organellar Ca²⁺ signalling as well as on the current knowledge regarding the pathways mediating Ca²⁺ fluxes across intracellular membranes. The contribution of organelles and cellular subcompartments to the orchestrated response via Ca²⁺ signalling within a cell is also discussed, underlining the fact that one of the greatest challenges in the field is the elucidation of how influx and efflux Ca²⁺ transporters/channels are regulated in a concerted manner to translate specific information into a Ca²⁺ signature.

Calcium is commonly involved as intracellular messenger in the transduction by plants of a wide range of biotic stimuli, including signals from pathogenic and symbiotic fungi. Trichoderma spp. are largely used in the biological control of plant diseases caused by fungal phytopathogens and are able to colonize plant roots. Early molecular events underlying their association with plants are relatively unknown.