Rhizosphere interactions: the root microbiome
Rout, Marnie E. .
What makes a plant invasive? It might be a matter of microbes.
Invasive plants can profoundly alter ecosystem processes. Attributes like faster growth rates, increased biomass, and enhanced phytochemistry have been documented in many invasive plants; these are traits frequently allowing invaders to out-compete native communities. Current theories suggest these are plant-regulated; however, our research shows that bacterial endophytes can regulate these traits in the invasive grass Sorghum halepense. Using culture and molecular approaches, we demonstrated that S. halepense harbors several bacterial endophytes within its roots and rhizomes. Five endophytes were isolated from rhizomes and identified using16S-rRNA gene sequencing. Physiological functions of these plant-associated bacteria, confirmed using in vitro studies, included N2-fixation,iron siderophore production, phosphate solubilization, and production of the plant-growth hormone indole-3-acetic acid (IAA). Long-term field studies documented alterations to several soil biogeochemical cycles across an invasion gradient. Heavily invaded soils had increased plant-available forms of essential macronutrients (N, P, K, Mg) and trace metals (Cu, Fe, Mn, Zn) compared to moderately- and non-invaded soils. Deep 16S rRNA gene sequencing of these soils showed significant differences in microbial community structure, with decreases in soil microbial alpha and beta diversity as a function of plant invasion. Using a novel antibiotic-based approach, bacterial activities were restricted within the plant to assess bacterial contributions to invasive plant traits. We showed that bacterial endophytes significantly increase S.halepense biomass and alter resource allocation enhancing rhizome growth,and plants with active endophytes significantly inhibit growth of a native prairie grass frequently displaced by the invader. Restricting bacterial activity completely removed these competitive effects. HPLC analysis of growth medium showed the invader produced significantly lower concentrations of the allelochemical sorgoleone when endophytes were inhibited. Conversely, endophyte activity increased production of the herbivore-defense compound,dhurrin, in S. halepense leaves. Leaves from plants with active bacterial endophytes were fed to a generalist insect herbivore, causing growth inhibition and high mortality. Bacterial inhibition caused a 6-fold decrease in dhurrin, in conjunction with significant increases in insect growth and survival. Collectively, these results suggest that microbial endophytes significantly contribute to S. halepense invasions via physiological contributions to soil nutrient cycles and by enhancing various invasive plant traits (growth rate, biomass, rhizome production, competitive effects through allelochemicals, and increased herbivore-defense production). This work indicates these invasive plant traits are microbially-mediated in this grass. This invasion strategy, Microbially Enhanced Competitive Ability (MECA), where microbial associations significantly contribute to altered ecosystem processes and enhance several invasive plant traits, alters traditional plant-invasion paradigms.
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1 - Fort Keogh LARRL, USDA-Agricultural Research Service, 243 Fort Keogh Rd, Miles City, MT, 59301, USA
plant growth-promoting bacteria
Presentation Type: Symposium or Colloquium Presentation
Location: Delaware D/Hyatt
Date: Tuesday, July 10th, 2012
Time: 11:15 AM