Year of Award

2011

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Organismal Biology and Ecology

Department or School/College

Division of Biological Sciences

Committee Chair

Ragan M. Callaway

Committee Co-chair

William E. Holben

Commitee Members

Anna Sala, Thomas Chrzanowski, Thomas H. DeLuca

Keywords

Sorghum halepense, bacterial endophytes, dhurrin, ecological invasions, allelopathy, biogeochemical cycling

Abstract

Invasive plants can profoundly alter ecosystem processes, and tremendous economic costs are often associated with these disturbances. Attributes like higher growth rates, increased biomass, and enhanced chemical defenses have been documented in many invasive plants. When expanding into new ranges, these traits frequently allow invasive plants to outcompete native plant communities. Current theories suggest these invasive attributes are plant-regulated; however, my work shows that bacterial endosymbionts can regulate these traits in the invasive grass Sorghum halepense. Using culture and molecular approaches, I found the invasive grass harbors several bacterial organisms inside the roots and rhizomes. These bacterial endosymbionts were isolated from within plant tissues and identified using 16S-rRNA gene sequencing. Numerous physiological functions of these plant-associated bacterial isolates were confirmed using in vitro studies, including the capacity for N2-fixation, iron siderophore production (iron chelation), phosphate solubilization, and production of the plant-growth hormone indole-3-acetic acid (IAA). In long-term field studies conducted within the Fort Worth Nature Center & Refuge spanning 46-months, alterations to several soil biogeochemical cycles across an S. halepense invasion gradient were documented. Heavily invaded soils had increased plant-available forms of essential macronutrients (nitrogen, phosphorus, potassium, and magnesium) and trace metals (copper, iron, manganese, and zinc) compared to moderately and non-invaded soils. Using a novel antibiotic approach, I restricted growth of the bacterial endosymbionts within the plant and found they significantly increased plant biomass, and altered resource allocation enhancing rhizomatous growth. Plants with endosymbionts significantly inhibited the growth of a native prairie grass, Schizachyrium scoaparium, which is frequently displaced by the invader in tallgrass prairie ecosystems. Restricting bacterial growth completely removed these competitive effects. Plants with bacterial endosymbionts also had increased production of the herbivore-defense compound, dhurrin, contained in leaves. When leaves from plants with bacterial endosymbionts were fed to a generalist insect herbivore (Tricoplusia ni), the insect could not grow and experienced significant mortality. Restricting bacterial growth resulted in a 6-fold decrease in dhurrin, in conjunction with significant increases in insect growth and survival. These results suggest microbial endosymbionts significantly contribute to S. halepense invasions by enhancing the plant traits of biomass, growth rate, competitive effects, and herbivore-defense. This works shows that these invasive plant traits are microbially-mediated. This novel invasion strategy is referred to as Microbially Enhanced Competitive Ability (MECA), in which microbial associations significantly contribute to a range of plant traits that directly correspond to invasion success.

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© Copyright 2011 Marnie Erin Rout