Capsular polysaccharides and exopolysaccharides from Gloeothece verrucosa under various nitrogen regimes and their potential plant defence stimulation activity

The freshwater and diazotrophic cyanobacterium Gloeothece verrucosa is well-studied for the circadian rhythm of its metabolism but the strain is also known for its production of exopolysaccharides. The impact of the carbon to nitrogen ratio fluctuation on the growth, photosynthetic and nitrogen metabolism and on exopolysaccharide production was investigated. Hence, 4 nitrogen sources at different concentrations were tested: NaNO₃, NH₄Cl, urea at 1, 2.5 and 17 mM and atmospheric N₂ as the sole nitrogen source. The characteristics and composition of the exopolysaccharides were examined. Especially in agriculture, the exopolysaccharides of Gloeothece verrucosa, represented a good alternative to petrochemical derived products due to their non-toxic and biodegradable characteristics. Therefore, the application of the exopolysaccharides as a plant biostimulant is investigated. The results indicated an increase in the growth correlated with an increase in nitrogen concentration, thus a reduction of the carbon to nitrogen ratio. Moreover, the nitrogenase activity of the selected strain is not sufficient for a high biomass production and mimic a nitrogen limiting condition. Exopolysaccharide content follows the inverse tendency with a higher production in the presence of higher carbon to nitrogen ratios. The polymer contains uronic acids, sulphated groups and proteins which reveal an extracellular polysaccharide rich in negative charges. Moreover, the monosaccharide composition is impacted by the nitrogen source. In vitro experimentation on Arabidopsis thaliana and foliar spraying treatment on tomato with various exopolysaccharide solutions indicates that the capsular polysaccharide fraction isolated in this research represents a good candidate for the biostimulation field as it induced an increase in phenylalanine ammonia lyase activity, free proline and anthocyanin content. All these components are known to be implicated in the plant defence mechanisms.