The abundance of ACC deaminase-positive bacteria and their interaction with winter wheat in a Colorado soil
Date
2012
Authors
Abduelafez, Ibrahem, author
Stromberger, Mary, advisor
Weir, Tiffany, committee member
Moragues Canela, Marc, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Plant growth-promoting rhizobacteria (PGPRs) are known as beneficial bacteria for plant growth and yield. One PGPR group are the ACC deaminase-positive (ACC+) bacteria which degrade 1-aminocyclopropane-1-carboxylic acid (ACC), the plant-produced precursor to ethylene. Plants produce ethylene in elevated quantities under environmental stress ("stress ethylene"), and studies have shown that ACC+ bacteria, in association with plant roots, can improve plant growth under abiotic stress (e.g., drought, salinity, heavy metals) by reducing concentrations of stress ethylene. There are few studies that have examined the natural abundance and distribution of these bacteria as affected by plant genotype, plant growth stage, and agricultural management practice; and no studies have been conducted in the western United States. The objectives of my research were to determine the influence of winter wheat genotype and irrigation practice on the abundance of culturable ACC+ bacteria in a Colorado soil, and to determine the plant-growth-promoting effect of selected ACC+ on winter wheat varieties ranging in drought sensitivities under greenhouse conditions. A field study was conducted at the Limited Irrigation Research Farm (LIRF) in Greeley, Colorado. Roots and root-associated soil (0-20 cm depth) were collected with a shovel under four winter wheat varieties (Triticum aestivum L. "Baca", "Hatcher", "Ripper" and "RonL") managed by three different irrigation regimes: full irrigation, limited irrigation (irrigation commencing at the anthesis stage), and dryland. Samples were collected at four physiological growth stages during the 2010-2011 growing season: emergence (November 2010), green-up (March 2011), anthesis (May 2011) and mid-grain filling (June 2011). Total culturable bacteria were enumerated on TSB agar, and culturable ACC+ bacteria were enumerated on DF minimal salts agar media containing ACC as the sole N source. The abundance of ACC+ bacteria was relatively high in the Colorado soil (1.69 × 107- 3.28 × 109 CFU's g-1) and varied according by an interaction between sampling date and irrigation practice (P < 0.0001). Abundance of ACC+ bacteria in the fully irrigated plots peaked in March and declined throughout the rest of the growing season, whereas ACC+ bacterial numbers increased significantly in the late growth stages for the dryland and limited irrigation treatments. The percentage of ACC+ bacteria, relative to total culturable bacteria, was significantly affected by the three-way interaction between wheat variety × irrigation treatment × sampling date (P= 0.0095). Whereas few differences in % ACC+ bacteria were observed among treatments in November and March, % ACC+ bacteria in May and June were generally greater under dryland or limited irrigation compared to full irrigation management. Wheat variety had no effect on % ACC bacteria under full irrigation, whereas % ACC bacteria generally was lowest under Baca and greatest under RonL under limited irrigation or dryland. A greenhouse study was conducted to determine how interactions among specific ACC+ bacterial strains, winter wheat genotype, and water stress affect winter wheat productivity. Two strains of Pseudomonas brassicacearum (HD6 and HF1) were selected for inocula because of their high ACC-deaminase activity and numerical dominance in a Colorado soil. Two winter wheat varieties (Ripper and RonL) were grown under non-stressed (soil maintained at 80-100% water holding capacity) or water-stressed (soil maintained at 40-60% water holding capacity) conditions, with different ACC+ bacterial inoculum treatments (HD6, HF1, or a combination of HD6 + HF1). Inoculation with ACC+ bacteria increased stem height of both wheat varieties, and increased the biomass, number of seeds, and number of fertile heads of RonL. There was a negative effect of the strain HD6 on Ripper productivity under non-stressed conditions in the greenhouse study. In conclusion, these studies demonstrate that the abundance of ACC+ bacteria and the potential effects of these bacteria on winter wheat productivity are dependent on winter wheat genotype and irrigation practice. Certain wheat varieties are able to accumulate more numbers of ACC+ bacteria in their rhizospheres under water stress, and certain strains of ACC+ bacteria can positively (RonL) or negatively (Ripper) impact winter wheat depending on the level of water stress.