The impact of salinity on the symbiosis between Casuarina glauca Sieb. ex Spreng. and N2-fixing Frankia bacteria based on the analysis of Nitrogen and Carbon metabolism

Casuarina glauca is an actinorhizal plant that establishes root-nodule symbiosis with N-2-fixing bacteria of the genus Frankia. This plant is highly recalcitrant to extreme environmental conditions such as salinity and drought. The aim of this study was to evaluate the impact of salt stress on the symbiotic relationship between C. glauca and Frankia Thr, focusing on N and C metabolism. Symbiotic and non-symbiotic plants were exposed to 0, 200, 400 and 600 mM NaCl. The following analyses were performed: stable carbon (delta C-13) and nitrogen (delta N-15) isotope signature; nitrogenase activity in nodules (acetylene reduction assay); and gene expression of a set of genes involved in nodule infection and N/C metabolism (qRT-PCR). Data were analysed using two-way ANOVA. Salt stress induced an enrichment in delta C-13 and delta N-15, reflecting a negative impact of salt in the relative water content and N-2 fixation, respectively. Furthermore, nitrogenase activity in nodules was insignificant already at 200 mM NaCl, consistent with the expression patterns of nifH as well as of plant genes involved in nodule induction and metabolism. The ability of C. glauca to thrive under highly saline environments is not dependent on the symbiosis with Frankia.