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The nitrogen-fixing actinobacterium Frankia establishes a symbiotic association with the roots of several dicotyledonous plants collectively called "actinorhizals". These plants form root nodules in which nitrogen-fixation takes place, thus permitting them to thrive in pioneer soils poor in nitrogen and organic matter such as glacial moraines, lava fields, forest burnouts or anthropogenic sites such as mine spoils orhydrodam dykes (Benson and Silvester, 1993).The different Frankia lineages form a coherent cluster at the root of the actinobacteria phylum (Sen et al., 2014), and Frankia alni in particular is in symbiosis with alder species (Nouioui et al., 2016).
The interaction has evolved over several millions of years with a sophisticated dialogue that does not imply Nod factors (Normand et al., 2007). The Frankia determinants of symbiosis are still poorly known, which is for the most part due to the lack of a genetic transformation system. Transcriptomics has shown genes coding for nitrogenase (nif), hydrogenase uptake (hup), hopanoids (shc, hpn), iron-sulfur (suf) clusters wereamong the most up-regulated(Alloisio et al., 2010). Proteomics has also been usedto analyse the symbiosis (Mastronunzio and Benson, 2010) and to identify up-regulated peptides (Alloisio et al., 2007) and it has shown the presence of Nif, Hup, Suf, Hop proteins as expected but also several transporters, regulators and various proteins involved in stress responses.
Osmotic stress is a constant challenge facing soil bacteria that live in soils ranging from low salinity in rainy cold latitudes to highly saline ones in warm and dry parts of the world. It also affects symbionts that must alternate between two main biotopes, the soil and the root tissues. Soils, especially poor soils have low osmotic potential while plant tissues have much a higher osmotic potential. Most studies on the effect of salinity on actinorhizal symbionts have focussed on the plant, especially Casuarina to illustrate various adaptations such as sodium partitioning or prolineaccumalation (Selvakesavan et al., 2016). Only a few recent studies on Frankia have shown the effects of salinity on ammonium assimilaiton(Srivastava et al., 2014).Alnus-,Casuarina- and Elaeagnus-infectiveisolates grow best at 50mM but are nevertheless able togrowwell in medium containing up to 200 mM NaCl, but not at 500 mM (Dawson and Gibson, 1987). The effect of NaCl on one strain, CcI6, was apparently less severe than that of another osmolyte, sucrose (Oshone et al., 2013). Nevertheless, little is known about the molecular adaptations and the present study was undertaken to better understand how Frankia coped with osmolytes using proteogenomics, an approach recently used to better decipher how Geodermatophilaceae coped with desiccation (Sghaier et al., 2015).