Enhancement of nitrogen fixation in legumes and production of antifungal compounds by rhizosphere-colonizing actinomycetes

Research described in this thesis examined the growth promoting and antifungal compound producing abilities of root-inhabiting Streptomyces isolated from rhizosphere soils. In experiments conducted in non-sterile soil, we observed that Streptomyces lydicus WYEC108, originally isolated from the rhizosphere of linseed, colonizes the roots and nodules of peas and soybeans. As compared to uncolonized controls, WYEC108 colonization leads to the production of larger, more numerous nodules in both plants. WYEC108 colonization of the nodules leads to improved bacteroid health and lifespan, enhanced nitrogen fixation rates by nodules, and improved plant growth. We hypothesized that the actinomycete secretes siderophores within the nodules to increase rhizosphere iron bioavailability to the bacteroids. We confirmed that enhanced siderophore production within the colonized nodules aided their assimilation of iron and molybdenum required for nitrogenase and leghemoglobin production by bacteriods, which led to significantly higher nitrogen fixation rates by WYEC108-colonized plants. Use of a 300 by WYEC108 specific gene probe confirmed that the actinomycete colonizing the nodules was S. lydicus WYEC108, not another soil actinomycete. We also investigated whether other siderophore-producing, root-colonizing Streptomyces have similar effects on peas as S. lydicus WYEC108. Some, but not all of the Streptomyces had similar effects. By measuring the effects of each Streptomyces on each of the parameters measured, total siderophore production by a strain was found to be the best indicator of its ability to enhance nitrogen fixation and plant growth. Pea roots and nodules collected from agricultural fields were also confirmed to be colonized by naturally occurring actinomycetes present in the soil. This is the first report of such a beneficial plant-microbe rhizosphere interaction involving Streptomyces and legumes, the first showing microbial siderophore production as a mechanism for aiding nodular assimilation of iron, and the first evidence to show that this is a natural beneficial plant-microbe interaction. We also studied the antifungal compounds produced by an actinomycete, Streptomyces strain RG, isolated from a habitat previously unstudied for antifungal compound producing actinomycetes, the rhizosphere of the desert shrub Big Sagebrush. This isolate was confirmed to be a novel Streptomyces species based on the sequence of its 16s rRDNA. Strain RG produced multiple low molecular weight, polar antifungal compounds. We characterized the compounds active against Saccharomyces cerevesiae and purified or partially purified multiple compounds. One of the compounds was a novel antifungal of a molecular weight 254 daltons. These data, along with results of ongoing research, show that the sagebrush rhizosphere is a source of previously undescribed actinomycetes that produce novel antifungal compounds of potential value as antimicrobial agents for treatment of fungal infections of plants and animals, including humans.