Diverse Role of Diazotrophs in the Rhizosphere
Over the last several years, many diazotrophs have been isolated in our laboratories from nodules of legumes, plant roots, and rhizospheric soil (Hafeez et al., 2005; 2006). In addition to fixing N2 (0.02–153 μmol mg−1 protein h−1), many can solubilize phosphate (189–204 (μ/mL). Some strains can produce indole acetic acid at concentrations as high as 977 μg/mL. Such strains, when used as bio-inoculants for cereals, enhance root growth and significantly increase the N and P contents in grain and straw. Some diazotrophs degrade ACC deaminase and result in lower ethylene synthesis. Other selected diazotrophs, such as K1, ER20, JCM 1270 and Ca18, successfully mobilize Zn, e.g., Enterobacter sp. 3.1.1.C solubilized both Zn3(PO4)2 and ZnO in plate assays. Some diazotrophs produce antifungal metabolites, like antibiotics, siderophores, and HCN and have biocontrol activity against fungal pathogens. Greenhouse evaluations suggest that Pseudomonas putida StS3, Pseudomonas sp. SPR7, P. fluorescens Mst 8.2, and Enterobacter sp. 3.1.1.C have potential as biocontrol agents with decreased disease incidence of potato black scurf, rice sheath blight, and wheat root rot. Other diazotrophs produce the broad spectrum antifungal 2,4-diacetylphloroglucinol (to 100 ppm). Some (4.1.1.B, 6.1.1.B, S4, S10, 17A1, 18 Al, etc.) produce chitinase, indicating mycolytic activity. Others inhibit growth of less competitive bacteria by producing bacteriocins, the genes for which have been identified. One consortium of diazotrophs has great potential for promoting rice plant growth and vigor with consequent suppression of weed competition. The most effective of these diazotrophs are used as the biofertilizer ‘Biopower’ in Pakistan. The use of ‘Biopower’, plus half the recommended N-fertilizer, resulted in considerable yield increase. These results increase our understanding of the diversity and role of diazotrophs in the rhizosphere and will help improve the quality of beneficial microorganism as inoculants for sustainable agriculture.