SUMMARYMost terrestrial plants engage into arbuscular mycorrhizal (AM) symbiosis with fungi of the phylum Glomeromycota. The initial recognition of the fungal symbiont results in the activation of a symbiosis signalling pathway that is shared with the root nodule symbiosis (common SYM pathway). The subsequent intracellular accommodation of the fungus, and the elaboration of its characteristic feeding structures, the arbuscules, depends on a genetic programme in the plant that has recently been shown to involve the VAPYRIN gene in Medicaco truncatula. We have previously identified a mutant in Petunia hybrida, penetration and arbuscule morphogenesis 1 (pam1), that is defective in the intracellular stages of AM development. Here, we report on the cloning of PAM1, which encodes a VAPYRIN homologue. PAM1 protein localizes to the cytosol and the nucleus, with a prominent affinity to mobile spherical structures that are associated with the tonoplast, and are therefore referred to as tonospheres. In mycorrhizal roots, tonospheres were observed in the vicinity of intracellular hyphae, where they may play an essential role in the accommodation and morphogenesis of the fungal endosymbiont.

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BackgroundLegumes establish with rhizobial bacteria a nitrogen-fixing symbiosis which is of the utmost importance for both plant nutrition and a sustainable agriculture. Calcium is known to act as a key intracellular messenger in the perception of symbiotic signals by both the host plant and the microbial partner. Regulation of intracellular free Ca2+ concentration, which is a fundamental prerequisite for any Ca2+-based signalling system, is accomplished by complex mechanisms including Ca2+ binding proteins acting as Ca2+ buffers. In this work we investigated the occurrence of Ca2+ binding proteins in Mesorhizobium loti, the specific symbiotic partner of the model legume Lotus japonicus.ResultsA soluble, low molecular weight protein was found to share several biochemical features with the eukaryotic Ca2+-binding proteins calsequestrin and calreticulin, such as Stains-all blue staining on SDS-PAGE, an acidic isoelectric point and a Ca2+-dependent shift of electrophoretic mobility. The protein was purified to homogeneity by an ammonium sulfate precipitation procedure followed by anion-exchange chromatography on DEAE-Cellulose and electroendosmotic preparative electrophoresis. The Ca2+ binding ability of the M. loti protein was demonstrated by 45Ca2+-overlay assays. ESI-Q-TOF MS/MS analyses of the peptides generated after digestion with either trypsin or endoproteinase AspN identified the rhizobial protein as ferredoxin II and confirmed the presence of Ca2+ adducts.ConclusionsThe present data indicate that ferredoxin II is a major Ca2+ binding protein in M. loti that may participate in Ca2+ homeostasis and suggest an evolutionarily ancient origin for protein-based Ca2+ regulatory systems.Electronic supplementary materialThe online version of this article (doi:10.1186/s12866-015-0352-5) contains supplementary material, which is available to authorized users.