Growth Promotion of Winter Wheat Under Drought Stress by ACC Deaminase-Positive Bacteria.

Under abiotic stress such as drought, plants produce increased levels of “stress” ethylene that decreases plant growth and productivity. ACC deaminase-positive bacteria are plant growth promoting rhizobacteria (PGPRs) that degrade 1-aminocyclopropane-1-carboxylic acid (ACC), the precursor to ethylene, and therefore promote continued productivity under drought stress. The objectives of this study were to determine the 1) ACC deaminase enzyme activity of several ACC deaminase-positive bacteria isolated from winter wheat roots, and 2) determine the plant growth-promoting effect of selected ACC deaminase-positive bacterial isolates in a greenhouse study. ACC-deaminase bacteria were isolated from the rhizosphere and roots of winter wheat grown under fully irrigated and dryland conditions. Isolates were identified by 16S rRNA sequencing and included Pseudomonas flourescens (most abundant), three other Pseudomonas sp., Rahnella aquatilis, and one Bacillus sp. ACC deaminase activity of each bacterial isolate was determined by measuring the amount of α-ketobutyrate produced when the enzyme ACC deaminase reacted with ACC substrate. Three isolates with contrasting ACC deaminase enzyme activity were selected for further study in the greenhouse. Winter wheat seeds of Ripper (drought tolerant variety) and RonL (drought sensitive variety) were inoculated with ground peat containing either individual species or a mixture of three species. An uninoculated control was included as well. Wheat was grown under conditions of water stress or no water stress, and at physiological maturity, aboveground biomass was measured for plant height, biomass and grain yield. This study will determine if ACC deaminase positive bacteria from a Colorado soil are functionally redundant in terms of PGPR effect, or if PGPR activity is influenced by interactions among bacterial species, wheat variety, and environmental conditions.