Cenchrus setigerus Vahl. secretes root agglutinins to promote colonization by plant growth-promoting rhizobacteria

Cenchrus setigerus Vahl. is a heat- and salinity-tolerant forage grass of immense value in ecological restoration of degraded lands. Plant growth-promoting rhizobacteria (PGPR) associated with the rhizosphere of grasses play a significant role by aiding development of the plant or by controlling phytopathogens. Biochemical and physiological characterization of PGPR isolated from the rhizosphere of C. setigerus was carried out, and the rhizobacteria were identified as belonging to the genera Azospirillum and Pseudomonas. A predominantly N-acetylglucosamine-binding protein (Cenchrus root agglutinin: CRA) that agglutinated buffalo erythrocytes was isolated from the roots of C. setigerus, by ammonium sulphate precipitation and ion-exchange chromatography. Immunoblot analyses of CRA indicated that it shared no similarity with other monocot and dicot lectins tested. Hemagglutination assays of CRA with buffalo erythrocytes were competitively inhibited in the presence of either Azospirillum Tarrand or Pseudomonas Migula, indicating that CRA specifically binds to surface antigens of these rhizobacteria. Further, lipopolysaccharides (LPS) were isolated from the rhizobacteria and used in a double diffusion assay to show that CRA reacted specifically with the LPS from Azospirillum and Pseudomonas. Our data illustrate the mechanism by which colonization of rhizobacteria around roots of C. setigerus is mediated through the interaction of bacterial LPS and CRA. The data also highlight the potential of C. setigerus and its associated PGPRs in restoration of degraded ecosystems.