Date of Award

Spring 2020

Degree Type

Masters Thesis

Degree Name

Master of Science (MS)


Biological, Environmental, and Earth Sciences

Committee Chair

Dr. Dmitri V. Mavrodi

Committee Chair School

Biological, Environmental, and Earth Sciences

Committee Member 2

Dr. Janet Donaldson

Committee Member 2 School

Biological, Environmental, and Earth Sciences

Committee Member 3

Dr. Micheal Davis

Committee Member 3 School

Biological, Environmental, and Earth Sciences


Pseudomonas synxantha 2-79 is a biocontrol agent that represents beneficial indigenous rhizobacteria that are broadly distributed in the Pacific Northwest, USA and flourish in the rhizosphere of commercially grown wheat under surprisingly arid conditions. The molecular adaptation of 2-79-like bacteria to plants growing in dry soils is poorly understood. We hypothesized that the ability of 2-79 to colonize and persist in the rhizosphere of water-stressed plants is underpinned by the formation of hydrating biofilms and the utilization of root exudates that contain plant-derived osmoprotectants called quaternary ammonium compounds (QACs). We tested this hypothesis by identifying waters stress response pathways in the 2-79 genome and then constructing a series of isogenic mutants deficient in one or more biofilm matrix and QAC transporters. The analysis of mutants revealed that under water-replete conditions, the QAC transporters function differentially and redundantly in the uptake of quaternary amines as nutrients. In contrast, under water stress, the QACs choline, betaine, and carnitine are accumulated preferentially for osmoprotection. Under drought stress, a mutant devoid of all known QACs transporters was less competitive in the colonization of the Brachypodium rhizosphere than its wild-type parental strain. The analysis of alg, psl, and eps biofilm matrix pathways revealed their role in the protection of 2-79 from desiccation. These pathways also contributed redundantly to the competitive colonization of plant roots. Our results confirm the importance of plant-derived QACs for microbial adaptation to the rhizosphere lifestyle and support the idea that the exchange of metabolites between plant roots and microorganisms profoundly affects and shapes the belowground plant microbiome under water stress.