Characterization of plant growth promoting bacteria from crops in Bolivia

The use of plant growth promoting bacteria (PGPB) is an economically and environmental friendly alternative to the application of chemical fertilizers resp. pesticides in agriculture. To obtain novel bacterial strains that could be used for plant growth promotion (PGP) in the agriculture of Bolivia, plant associated bacteria derived from horseradish tree (Moringa oleifera), sorghum (Sorghum vulgare), sunflower (Helianthus annuus), and safflower (Carthamus tinctorius) were screened for direct and indirect in vitro PGP traits. Subsequently, most promising strains were selected for ad planta studies. According to in vitro experiments, out of 59 tested isolates, 19% were declared as diazotrophs, 41% as solubilizers of phosphorus, 10% were able to reduce the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), and 17% exhibited phytohormone (IAA) synthesis. Only a small proportion of rhizobacterial strains (7% out of 276) showed in vitro antagonism against plant pathogenic Colletotrichum sp., whereas none of isolated stem and leaf endophytes inhibited growth of Verticillium chlamydosporum or Bipolaris maydis in dual culture. Based on results of in vitro screens, the rhizospheric strains Pectobacterium cypripedii M56, Pantoea agglomerans M72, and P. agglomerans M81 were selected for ad planta applications. Results revealed significant increases in number of beans per black bean plant resp. diameters of flower heads of sunflower plants compared to controls by P. agglomerans M72. We conclude that the assessment and selection of plant associated bacteria based on traits conferring theoretically PGP can provide the basis for the development of new microbial inoculants for agricultural purposes.