The Unique Properties of the Symbiotic Anabaena Azollae in the Water Fern Azolla: Metabolism and Intercellular Recognition

Abstract

Cultured isolates of Anabaena azollae provide a suitable tool for studying the metabolism, transport, cell composition and antigenic properties of the cells. The isolates of Anabaena azollae from Azolla caroliniana and Azolla filiculoides exhibit antigenic homology in the quantitative immunoassays employed, suggesting that they may stem from the same parental Anabaena. These isolates were also similar in respect to the fructose supported N2-fixation activity: the fructose is taken up actively by the cells of Anabaena azollae and although it does not support the growth of Anabaena, it carries a series of changes involving cell composition, differentiation and metabolism. Respiration was facilitated, the amount of storage products increased, and the frequency of heterocysts was increased. Sucrose and glucose stimulated respiration, but showed limited effect on N2-fixation. The uptake of fructose, dependent on ATP synthesis, is recovered faster in starved cells than N2-fixation activity. Fructose carrier seems to be constitutive in the cultured isolate of Anabaena azollae grown autotrophically. The lectins identified in Anabaena azollae Newton cells are constitutive as well, and their actual role in the symbiosis with Azolla has not been verified yet. They may be involved in the regulation and control of the development of Anabaena in the leaf cavity and, possibly, trigger the exchange of metabolites between the host Azolla and the N2-fixing Anabaena.