Do Root Exudates of Specific Winter Wheat Cultivars Recruit ACC Deaminase-Positive Bacteria?.

Root exudates are important for establishing plant-microbial associations because they are chemical signals that promote chemotaxis, initiate symbiotic interactions, and/or act as substrates promoting the growth of some microbes. We hypothesize that root exudates are utilized by winter wheat to recruit ACC deaminase positive (ACC+) bacteria. These bacteria degrade the “stress” ethylene precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), and promote plant growth under drought stress.   Our previous studies have found that the abundance, diversity, and species composition of these bacteria vary under different wheat cultivars in Colorado, possibly in response to different root exudate profiles.  In this study, we characterized the root exudate profiles of two contrasting wheat cultivars, Ripper and RonL, and measured the response of ACC+ bacteria to exudates and exudate fractions added to soil. Root exudates of Ripper and RonL were collected under hydroponic growth conditions over a six-week growth period. Exudates were analyzed by HPLC, and were qualitatively different between Ripper and RonL. A subsample of exudate from each cultivar was kept whole while the remainder was fractionated into aqueous, chloroform, and ethyl acetate fractions. Whole exudates, individual exudate fractions, sterile DI water (control), or sterile Hoagland plant nutrient solution (hydroponic medium control) were added to pre-incubated, plant-free soil on a daily basis for seven days. After seven days, the abundance of ACC+ bacteria and total heterotrophic bacteria were enumerated by plate count methods. Bacteria were also quantified by qPCR using universal bacterial primers and primers specific to ACC+ bacteria. We found that the ethyl acetate fraction of RonL root exudate doubled the relative abundance of ACC+ bacteria in plant-free soil, whereas ACC+ bacteria were unresponsive to Ripper root exudates. DNA will be pyrosequenced to determine bacterial compositional responses to root exudates.