Repository logo

Theses and Dissertations

Permanent URI for this collection

https://hdl.handle.net/10217/100514

Browse

Browsing Theses and Dissertations by Subject "1-aminocyclopropane-1-carboxylic acid"

Now showing 1 - 1 of 1
  • Thumbnail Image
    ItemOpen Access
    Genetic diversity of ACC-deaminase positive bacteria in Colorado soil under winter wheat cultivars (Triticum aestivum L.)
    (Colorado State University. Libraries, 2018) Elamari, Asma A., author; Stromberger, Mary E., advisor; Weir, Tiffany, committee member; Byrne, Patrick, committee member; Newman, Steven, committee member
    ACC-deaminase positive bacteria (ACC+) promote plant growth and development by lowering abiotic stress ethylene levels through deamination of 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene. During drought stress, ACC+ bacteria can help plants better tolerate drought stress in arid and semi-arid areas such as Colorado. The purpose of this study was to assess the capability of ACC+ bacteria to support winter wheat cultivar growth and production under drought stress conditions. In a field study, the relative abundance and genetic diversity of ACC+ bacteria associated with four winter wheat cultivars (Baca, Hatcher, Ripper, and RonL) were assessed under dryland, limited irrigation, and fully irrigated conditions in Colorado. The abundance of ACC deaminase positive bacteria was relatively high, with numbers ranging between 1.69 × 107 and 3.28 × 109 CFU's g-1 soil. At anthesis, the abundance and relative percent of ACC+ bacteria were greater under dryland and limited irrigation compared to full irrigation, and greater under RonL than other cultivars. The composition of rhizosphere ACC+ bacteria was distinct under RonL compared to Ripper. The majority of ACC+ bacteria enriched from these soils were Proteobacteria, specifically Pseudomonas spp. These results suggest that cultivar was a stronger driver of community composition of ACC+ bacteria than irrigation practice. The second study was conducted using a total of 55 bacterial isolates from the original experimental soils (RonL, Ripper, and Hatcher), which were selected to assay for Plant-Growth Promoting (PGP) traits: ACC-deaminase activity, Indole Acetic Acid (IAA) production, osmotic stress tolerance, phosphorus solubilization, and siderophore production. Most isolates were identified as species of Pseudomonas, but other species such as Arthrobacter, Variovorax, Agrobacterium, Rhizobium, Ochrobacterium, Micrococcus, Rahnella, and Bacillus were represented as well. Out of 55 isolates, 47 bacterial isolates tolerated osmotic stress when tested at an osmotic stress of -0.73 MPa water potential, 42 isolates demonstrated ACC-deaminase activity (potential ACC-deaminase activity ranged from 0.012 to 4.36 nmoles mg-1 protein h-1), 16 isolates were capable of solubilizing rock phosphate, one isolate produced IAA in the presence of 200 µg mL-1 tryptophan, and none of the isolates produced siderophores in vitro. Seven bacterial isolates that exhibit multiple PGPRs traits were selected for a root elongation assay using winter wheat seeds. The isolates selected were three Pseudomonas sp. isolated from RonL rhizospheres in fully irrigated plots (RLF6, RLF9, and RLF12); one Variovorax sp. isolated from Ripper rhizosphere in a dryland plot (RD12), one Pseudomonas sp. from Ripper rhizosphere of a fully irrigated plot (RF12), and two Pseudomonas sp. isolated from Hatcher rhizospheres (HD8 and HF1). The results showed that 6 days after seeds were inoculated, Ripper seedlings had the longest roots. In addition, isolates RLF6 and RF12, both Pseudomonas sp., were found to be the most effective in increasing root length as compared to sterile inoculum (control). The effect of inoculation on root length, however, was dependent on cultivar type. A subsequent field plot study was conducted to test the effectiveness of Pseudomonas sp. RLF9 to improve wheat growth and productivity under drought stress. Inoculum was prepared in the form of alginate bead and distributed in trenches next to winter wheat plants (at stem elongation growth phase), in replicated field plots under irrigated and non-irrigated treatments. Six weeks after inoculation, numbers of culturable Pseudomonas sp. were greater (P ≤ 0.05) in soil inoculated with RLF9 than sterile inoculant, except for the RonL/Ripper bioculture treatment. Overall, soil under RonL accumulated the highest ACC-deaminase positive bacterial populations (1.8×105 CFUs g-1 soil) compared to other cultivar treatments. Soil ACC deaminase activity was highly variable but tended to be greatest in soil planted to monocultures of RonL and Ripper, and bicultures of RonL and Ripper. There was no significant correlation between inoculant abundance in soil and soil ACC-deaminase activity, perhaps because of ACC deaminase activity of indigenous soil bacteria. The field season was marked with abundant rainfall during the spring, followed by outbreaks of pathogens, including stripe rust, that affected all field treatments and confounded late season drought and heat stress. As a result, there were no irrigation treatment effects or inoculation effects on wheat yield indices. The findings of this study are very attractive for further field studies, which would be helpful in extending isolate use as PGP inocula for wheat production, especially in Colorado and perhaps other semi-arid regions. Specifically, inoculation of winter wheat cultivars with PGPR's containing ACC-deaminase and other PGP traits could be an effective approach for successful crop production. Selection of PGPR inoculants and wheat cultivar combinations for obtaining optimum responses should be considered in future studies. The ability of potential inoculants to survive in the rhizosphere, compete with indigenous microbial communities, and colonize roots should also be studied. Based on laboratory studies of potential PGPR activities, Pseudomonas sp. RLF9 and possibly Pseudomonas sp. RLF6 could be good inocula selection to improve wheat growth and productivity under drought stress. Further research is needed to demonstrate the effectiveness of RLF6 and RLF9 as effective inoculants in the field.