Accumulation of soluble carbohydrates, trehalase and sucrose synthase in effective (Fix+) and ineffective (Fix–) nodules of soybean cultivars that differentially nodulate with Bradyrhizobium japonicum

Abstract

Roots of soybeans have the ability to form symbioses with nitrogen-fixing rhizobial bacteria to form nitrogen-fixing (Fix⁺) nodules, thus allowing the plant to grow in the absence of mineral nitrogen. Several soybean cultivars from China nodulated normally with Bradyrhizobium japonicum USDA110 spc4, but developed only a few nodules with 61-A-101, another B. japonicum strain. When soybeans were infected with Rhizobium sp. NGR234, ineffective (Fix^–) nodules that do not fix nitrogen were formed. Plants infected with NGRΩnodD2, a mutant strain overproducing lipo-chitooligosaccharidic nodulation signals (Nod factors), showed significantly higher numbers of ineffective nodules. Nodules from the different plant–microsymbiont combinations were characterized with respect to their accumulation of soluble carbohydrates and their induction of trehalase and sucrose synthase. These two plant enzymes are known to be nodule-stimulated proteins. Pool sizes of soluble carbohydrates in nodules showed strain-specific alterations in sucrose and trehalose, whereas myo-inositol and pinitol were affected in a more cultivar-specific way. Immunoblots with nodulin-specific antiserum indicated that sucrose synthase is induced in Fix⁺ nodules, but undetectable in Fix^– nodules, indicating a strain-specific induction profile. Trehalase activity in nodules showed a similar strain-specific induction profile. High enzyme activity was measured for nodules harboring the Bradyrhizobium strains, whereas ineffective nodules containing NGR234 exhibited activities in the range of uninfected roots. Nodules induced by NGRΩnodD2 showed increased trehalase activity. A similar induction of trehalase was observed when uninfected roots were treated with Nod factors purified from NGR234. The data obtained are discussed in the context of carbohydrate allocation in nodules and the question of how rhizobial bacteria influence the carbohydrate metabolism of their host plant is addressed.