Multilevel analysis of Azospirillum biofilms and cystlike forms and characterization of their possible role in wheat drought tolerance

Abstract

The type strains Azospirillum brasilense Sp7 and A. baldaniorum Sp245, capable of penetrating plant roots, formed mono- and multilayered biofilms on the wheat root surface both in sterile aqueous medium and in nonsterile soil. The biofilms formed on an abiotic surface under hydroponic conditions also were mono- and multilayered. In the wheat root system, the azospirilla produced biofilms mainly on the root apex, the root hairs, and the areas of lateral root development, regardless of the growth conditions. It was exactly in these root zones that a lectin identical to wheat germ agglutinin had previously been detected, and a hapten specific for this lectin was present in the biofilm matrix of the strains under study. The synthesis of osmoprotectants and glycopolymers and the formation of cystlike forms by Azospirillum contributed to biofilm survival during drying. The biofilm components played a part in increasing the resistance of the Azospirillum-inoculated plants to drought-simulating conditions in soil. The same components played a role in increasing the resistance of Azospirillum-inoculated plants to simulated soil drought. Drought was simulated by stopping soil watering for 10 days. The Sp7-derived phenotypic variant Sp7.8, which develops large cystlike forms with a complex morphology, not only remained viable during biofilm desiccation but also protected the plants from the negative effects of soil drought more successfully than did the parent strain. In biofilms, the phenotypic variant Sp7.8 formed 1.8 times more cystlike forms (under N-sufficient conditions) than did strain Sp7.