The interaction of boron with glycolipids is required to increase tolerance to stresses in Anabaena PCC 7120

Abstract

Boron (B) is an essential nutrient for heterocystous cyanobacteria growing under diazotrophic conditions. Under B-deficient conditions, the heterocyst envelope is highly disorganized, and the glycolipid layer is predominantly lost. Therefore, we examined whether B is implicated in the regulation of synthesis or processing and/or stability of glycolipids in Anabaena PCC 7120. RT-PCR analysis indicated that the expression of hglE was not significantly changed under B deficiency, suggesting that the synthesis of glycolipids during heterocyst formation was not compromised. In contrast, the overexpression of devB and hepA, encoding a glycolipid and a carbohydrate transporter, respectively, results in the instability of the envelope under B-deficient conditions. The capacity of borate to bind and stabilize molecules is considered the basis of any B biological function. Using a borate-binding-specific resin and thin layer chromatography, we detected the glycolipids that interact with B. Several heterocyst-specific glycolipids were detected as putative B ligands, suggesting a role for B in stabilizing the heterocyst envelope. Moreover, the glycolipids of Anabaena growing in non-diazotrophic conditions were also detected as putative B ligands. Although B is not essential for Anabaena under non-N2-fixing conditions, the presence of this micronutrient increased the tolerance of Anabaena to detergent treatment, salinity and hyperosmotic conditions. Taken together, the results of the present experiment suggest a beneficial role for B in environmental adaptation. Furthermore, we discuss the nutrient requirement for living organisms growing in nature and not under laboratory conditions.