STUDIES ON THE EFFECTS OF PLANT VARIETY AND ROOT EXUDATE COMPOUNDS ON THE SOIL MICROBIAL COMMUNITY

Plants modify the soil environment through their root system, changing its physical properties and exuding compounds that promote or inhibit the growth of certain microorganisms. Therefore the structure of the soil microbial community is different in the rhizosphere than in the bulk soil. This dissertation introduces three research projects that investigated the effects of specific root exudate compounds on the soil microbial community structure, and plant cultivar specific differences in the rhizosphere microbiota. The progenitor of maize is Balsas teosinte (Zea mays subsp. parviglumis). In a greenhouse experiment we compared the structure and function of its bacterial and fungal rhizosphere community with that of domesticated corn cultivars sweet corn and popping corn by terminal restriction fragment length polymorphism, fatty acid methyl ester analysis, and soil enzyme assays. The results allude to functional and structural differences in the rhizosphere microbial communities of the corn varieties that could lead to useful discoveries on how corn domestication has altered rhizosphere processes. To study how root exudate flavonoids 7,4′-dihydroxyflavone and naringenin influence the soil bacterial community structure we constructed model systems to approximate the flavonoid exudation of Medicago sativa roots. Soil samples from the model systems were subjected to ATP assays and 16S rRNA gene amplicon sequencing. Our results suggest that 7,4′-dihydroxyflavone can interact with a diverse range of soil bacteria, including members of Acidobacteria subdivision 4, Gaiellales, Nocardioidaceae, and Thermomonosporaceae and may have other functions in the rhizosphere in addition to nod-gene induction in the legume–rhizobia symbiosis. Hydroxyproline is the most common amino acid in plant cell wall proteins and serves as an important carbon and nitrogen source for soil bacteria. We treated soil with the L or the D enantiomer of hydroxyproline and collected samples for 16S rRNA gene amplicon sequencing three and seven days after the treatment. The L- and D-hydroxyproline treatments induced very similar responses in the bacterial community structure, but there were several differentially abundant groups. Our results inform about the role of hydroxyproline in shaping the soil microbial community in the rhizosphere and about the catabolism of its enantiomers in the soil

AH-1S communication switch integration program

The C-6533/ARC communication system as installed on the test AH-1E Cobra helicopter was modified to allow discrete radio selection of all aircraft radios at the cyclic radio/intercommunication system switch. The current Cobra-fleet use of the C-6533 system is cumbersome, particularly during low-altitude operations. Operationally, the current system C-6533 configuration and design requires the pilot to estimate when he can safely remove his hand from an active flight control to select radios during low-altitude flight. The pilot must then physically remove his hand from the flight control, look inside the cockpit to select and verify the radio selection and then effect the selected radio transmission by activating the radio/ICS switch on the cyclic. This condition is potentially hazardous, especially during low-level flight at night in degraded weather. To improve pilot performance, communications effectiveness, and safety, manprint principles were utilized in the selection of a design modification. The modified C-6533 design was kept as basic as possible for potential Cobra-fleet modification. The communications system was modified and the design was subsequently flight-tested by the U.S. Army Aeroflightdynamics Directorate and NASA at the NASA Ames Research Center, Mountain View, California. The design modification enables the Cobra pilot to maintain hands-on flight controls while selecting radios during nap-of-the-Earth (NOE) flight without looking inside the cockpit which resulted in reduced pilot workload ratings, better pilot handling quality ratings and increased flight safety for the NOE flight environment

The Effect of Root Exudate 7,4\u27-Dihydroxyflavone and Naringenin on Soil Bacterial Community Structure

Our goal was to investigate how root exudate flavonoids influence the soil bacterial community structure and to identify members of the community that change their relative abundance in response to flavonoid exudation. Using a model system that approximates flavonoid exudation of Medicago sativa roots, we treated a soil with 7,4′-dihydroxyflavone and naringenin in two separate experiments using three different rates: medium (equivalent to the exudation rate of 7,4′-dihydroxyflavone from M. sativa seedlings), high (10× the medium rate), and low (0.1× the medium rate). Controls received no flavonoid. Soil samples were subjected to ATP assays and 16S rRNA gene amplicon sequencing. The flavonoid treatments caused no significant change in the soil ATP content. With the high 7,4′-dihydroxyflavone treatment rate, operational taxonomic units (OTUs) classified as Acidobacteria subdivision 4 increased in relative abundance compared with the control samples, whereas OTUs classified as Gaiellales, Nocardioidaceae, and Thermomonosporaceae were more prevalent in the control. The naringenin treatments did not cause significant changes in the soil bacterial community structure. Our results suggest that the root exudate flavonoid 7,4′-dihydroxyflavone can interact with a diverse range of soil bacteria and may have other functions in the rhizosphere in addition to nod gene induction in legume—rhizobia symbiosis