Endophytes are microbes that reside within plants, forming a mutualistic relationship where they improve host physiology in exchange for plant carbohydrates. Inoculating plants with a cultivated consortia of endophytes originally isolated from trees in plant family Salicaceae is proposed as a sustainable strategy for increasing crop yield, plant health, and climate change resilience. However, this plant-microbe relationship may turn parasitic if endophytes become over abundant and their carbohydrate demand exceeds the plant’s capacity. Inoculation method is an important yet underexplored factor in predicting the outcome of this relationship and endophyte colonization success. This study investigates how inoculation methods influence the number of endophytes that colonize different plant tissues. A consortium of Salicaceae endophytes was introduced into hybrid poplar plants grown in sand through different inoculation methods. The methods tested are (1) no inoculation, (2) a pre-planting root soak inoculation, (3) a pre-planting unrooted stem soak inoculation, and (4) a post-planting media soak inoculation. After growth, endophyte density and distribution will be estimated by separately culturing endophytes isolated from leaf, stem, and root tissues and counting their colony forming units per gram. The carbon cost of hosting endophytes will be estimated through in vitro and in planta respiration measurements from the sampled tissue. Epidermal wounding from shoot cutting in the stem-soak inoculation method may increase the amount of entry sites for endophyte colonization. Therefore, it is expected that plants receiving inoculum via stem-soak will show increased endophyte distribution and density and higher respiration rates when compared with other inoculation methods. Comparisons of estimated in planta endophyte densities with plant physiological measurements will establish the relationship between endophyte abundance and host health in poplar. Further research will be needed to create a more generalized model of the effect of individual endophyte distribution and density on the benefits and costs to plant health.  

Endophytes are mutualistic microbes that promote plant growth and stress tolerance by improving host nutrient uptake and producing phytohormones. Plants grown under drought or nutrient limitations have benefited from inoculation with endophytes isolated from wild poplar. Previously, tissue culture-grown poplar has received a pre-planting inoculation to successfully promote growth under abiotic stress. However, a lag in plant growth and initial negative physiological and morphological response has also been observed in plants inoculated early in their development. These results suggest that a favorable plant-endophyte interaction may depend on the method of inoculation. To further understand the best method to maximize plant growth promotion with endophyte inoculation, I have explored how the timing of inoculum delivery impacts plant physiological processes. In a greenhouse experiment, hybrid poplar plants were split into three treatment groups: an uninoculated control group, a pre-planting group inoculated with an endophyte consortium via a root soak, and a post-planting group inoculated via a soil soak. Measuring changes in root system architecture, plant height, and biomass will determine how the plants allocate their resources in response to inoculation treatments. Leaf chlorophyll fluorescence will be measured to assess the stress put on plants from the association with endophytes, while measurements of leaf photosynthetic and root respiration rates will be used as a proxy for changes in the host’s carbon budget. I anticipate that allowing plants to acclimate to greenhouse conditions before inoculation will allow them to have greater above- and belowground productivity due to the photosynthetic cost of establishing symbiosis. Host plant physiology will be correlated with additional measurements of endophyte density and distribution throughout the plant to relate inoculation timing to estimated intracellular endophyte concentrations. This information will provide a greater understanding of the initial costs and benefits of endophyte-plant symbiosis and optimal timing of inoculation.