Boron deficiency affects rhizobia cell surface polysaccharides important for suppression of plant defense mechanisms during legume recognition and for development of nitrogen-fixing symbiosis

Abstract

Boron (B) deficiency negatively affects legume–rhizobia symbiotic interactions and the development of N2-fixing nodules. Many described alterations are related to plant-derived carbohydrates involved in plant–microbe interactions; however, the effects of B on the bacterial polysaccharides that are crucial for correct symbiosis are unknown. Exopolysaccharide (EPS) production in several rhizobial strains grown in B-free media was analyzed following acetone extraction and silver-stained electrophoretic profiles of lipopolysaccharide (LPS). Moreover, the effects of B deficiency and mutations of the pathogenesis-related ABR17 protein on rhizobia cell surface polysaccharides on legume root colonization, nodulation, nitrogen fixation, and induction in pea nodules were investigated. B-deficiency led to a 65–80 % reduction in the amount of EPS and to modifications of LPS in all strains tested. B-deficient rhizobia were not affected in the degree of adsorption to roots. However, nodulation and nitrogen fixation were reduced or inhibited by B starvation or in plants inoculated with EPS or LPS defective mutants, and ABR17 was induced. The results provide evidence that B is important for production of rhizobia cell surface polysaccharides essential to establish a symbiotic rather than a pathogenic-like interaction, and for development of the N2-fixing legume root nodule.