|STROMBERGER, MARY - Colorado State University|
|ABDUELAFEZ, IBRAHEM - Colorado State University|
|BYRNE, PATRICK - Colorado State University|
|ELAMARI, ASMA - Colorado State University|
|MORAGUES, MARK - University Of California|
|WEIR, TIFFANY - Colorado State University|
Submitted to: Applied Soil Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/28/2017
Publication Date: 3/9/2017
Citation: Stromberger, M., Abduelafez, I., Byrne, P., Elamari, A., Manter, D.K., Moragues, M., Weir, T. 2017. Genotype-specific enrichment of ACC deaminase-positive bacteria in winter wheat rhizospheres. Applied Soil Ecology. doi:10.2136/sssaj2016.12.0437.
Interpretive Summary: Among the many ecosystem services provided by soil bacteria, perhaps none are as important within agricultural systems as supporting plant growth. Soil bacteria that influence plant growth positively are called plant growth promoting rhizobacteria (PGPR). These bacteria reside within root tissues, on the root surface, and in the rhizosphere soil, where they can improve plant growth by increasing availability of nutrients to plants, producing phytohormones that regulate plant growth, and antagonizing and outcompeting plant pathogens in the soil environment. Water scarcity is among the most difficult of the challenges facing agricultural sustainability and food security. Of growing interest is the ability of certain PGPRs to reduce the effects on plants of abiotic stress in the environment, including drought. Under abiotic stress (e.g., drought, salinity, and heavy metal stress), plants produce the phytohormone ethylene, which induces defense responses such as reduced root and shoot growth and reduced productivity. ACC deaminase-positive (ACC+) bacteria degrade the ethylene precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), through the action of ACC deaminase. The ACC substrate is released by plant tissues, then absorbed and degraded by ACC+ microbes. This results in lower plant ethylene concentrations, continued root elongation and greater resistance to water stress. To date, studies conducted on these bacteria have primarily focused on their isolation and subsequent testing as inocula for crops grown in the greenhouse or field. Rarely have studies examined the natural abundance and diversity of indigenous ACC+ populations in rhizospheres, and none have studied the indigenous populations of the semi-arid Great Plains region of the United States, despite the potential for their positive influence on plant growth under water stress. In this paper, we showed that (i) the abundance and composition of indigenous ACC deaminase-positive bacteria varied under different wheat genotypes, (ii) ACC deaminase enzyme activity correlated with predicted abundance of ACC deaminase-positive bacteria, and (iii) potential for ACC+ bacteria to promote drought resistance in winter wheat may be genotype dependent.
Technical Abstract: Bacteria that produce ACC deaminase promote plant growth and development by lowering levels of the stress hormone ethylene through deamination of 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene. Therefore, it is hypothesized that ACC deaminase positive (ACC+) bacteria can help plants tolerate drought stress in arid and semi-arid areas. The purpose of this study was to assess the abundance and genetic diversity of ACC+ bacteria associated with different winter wheat genotypes grown under dryland, limited irrigation, or fully irrigated conditions at two Colorado research stations in Greeley and Fort Collins. At Greeley, the abundance of culturable ACC+ bacteria in rhizosphere soil was relatively high, with numbers ranging from 1.69 × 107 to 3.28 × 109 CFU's g-1. At anthesis, the abundance of ACC+ bacteria, relative to total culturable bacteria, was greater in rhizospheres of wheat under dryland (7-18%) and limited irrigation (4-14%) when compared with fully irrigated wheat (1-2%), and greatest under the wheat genotype RonL (as high as 18%) compared to other genotypes. The genetic diversity and composition of ACC+ bacteria associated with wheat genotypes Ripper and Ron L under dryland and fully irrigated conditions at Greeley was assessed after enrichment in selective media using 454 pyrosequencing. There were no differences in diversity estimates according to wheat genotype and irrigation regime; however bacterial community membership was significantly affected by wheat genotype and to a lesser degree, by irrigation regime. Dominant genera of ACC+ bacteria included Pseudomonas (35-54% of OTUs), Sphingobacterium (3-13%), Chryseobacterium (5-10%), Buttiauxella (4-11%), Stenotrophomonas (~4%), and Acinetobacter (0-5%). Compared to Ripper, RonL had greater proportions ofspecies from Sphingobacterium (p=0.041), most closely aligned with Sphingobacterium multivorum. A culture-independent study conducted at Fort Collins also confirmed enrichment of ACC+ bacteria under RonL compared to 11 other wheat genotypes, with RonL rhizosphere soil having a greater ACC deaminase activity potential and predicted abundance of ACC+ bacteria, based on PICRUSt analysis of 16S rDNA sequences. In conclusion, The relative abundance and composition of cultivable and predicted communities of ACC+ bacteria differ according to winter wheat genotype, while soil moisture status alters the absolute and relative abundance of these bacteria over the course of the growing season. Therefore, we speculate that under water stress, the potential for ACC+ bacteria to promote drought resistance in winter wheat may be genotype dependent.