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Biology (M.S.)




Ailanthus altissima (Simaroubaceae) is an invasive tree from China that has spread over much of North America. A number of characteristics contribute to its success, notably tolerance of nutrient-poor soils, rapid growth rates, prolific production of wind dispersed seeds, escape from predation, and the production of allelopathic compounds. That Ailanthus altissima frequently co-occurs with legumes in nutrient poor soils alludes to a possible relationship between Ailanthus altissima and nitrogen fixing legumes. Legumes increase the biospheres supply of bio-available nitrogen through facilitating the fixation of atmospheric nitrogen within structures on roots called nodules. Thus, increased nodulation of legumes caused by Ailanthus altissima may explain its ability to grow rapidly on poor soil. Previous unpublished studies reported increased nodulation in Trifolium pratense but could not conclude whether increased nodulation was due to Ailanthus altissima removing nutrients from the soil or adding soil leachates.

I designed a greenhouse experiment to determine if Ailanthus altissima affects Trifolium pratense total biomass, shoot to root ratio, in addition to any effects on nodulation of Trifolium pratense. The experimental design included fertilizer applications to both Ailanthus altissima and Trifolium pratense to determine if Ailanthus altissima impacted Trifolium pratense by altering nutrient availability or by releasing compounds into the soil. Ailanthus altissima seedlings were grown in soil and leachates were collected weekly by pouring water through the soil. The collected leachates were then applied to Trifolium pratense seedlings weekly over an eight-week period. I predicted that if Ailanthus altissima soil leachates increase nodulation of Trifolium pratense then increased nodulation would have occurred independent of Trifolium pratense’s access to nutrients. Indeed, Trifolium pratense treated with Ailanthus altissima soil leachates were more than twice as likely to be nodulated regardless of any fertilizer treatment applied to either Trifolium pratense or Ailanthus altissima. Based on the results, I was able to conclude that increased rates of Trifolium pratense nodulation was due to the addition of Ailanthus altissima soil leachates and not due to nutrient interactions. Further, total biomass of Trifolium pratense was not consistently influenced by application of Ailanthus altissima soil leachates; however, a significant interaction term between the application of Ailanthus altissima soil leachates and fertilizing the Ailanthus altissima suggests a link between nutrient resources and allelopathic interactions.

Two gene expression experiments were performed to explicate the mechanism by which Ailanthus altissima increased nodulation of Trifolium pratense. I monitored changes in the transcript abundance of two Trifolium pratense genes encoding target enzymes – chalcone synthase (CHS) and chalcone isomerase (CHI) – which are integral in the formation of flavonoid signaling molecules that solicit nitrogen-fixing bacteria: rhizobia. The first experiment was designed to confirm the role of the target enzymes by monitoring transcript abundance in Trifolium pratense roots grown in nitrogen limited conditions. Using quantitative reverse transcription polymerase chain reaction (qRT-PCR), I determined that nitrogen-limited conditions tended to cause an increase in levels of CHS and CHI transcript, presumably reflecting their role in the formation of rhizobia soliciting flavonoids.

The second gene expression experiment monitored transcript abundance of CHS and CHI in root tissues of Trifolium pratense treated with Ailanthus altissima soil leachates. Considering the increased nodulation observed in the greenhouse portion of this study, I expected Ailanthus altissima soil leachates would cause Trifolium pratense roots to express increased levels of CHS and CHI transcripts. Contrary to expectation, qRT-PCR revealed that Ailanthus altissima soil leachates caused levels of CHS transcript to decrease at all harvest times and CHI transcript abundance decreased at all harvest times except at six days after treatment. The decreased transcript abundance for CHS and CHI in response to Ailanthus altissima soil leachates may indicate that increased nodulation of Trifolium pratense exposed to Ailanthus altissima may be a response to unsolicited rhizobia. I propose that Ailanthus altissima is not inducing increased nitrogen fixation in neighboring legumes, but is instead altering the relationship between legumes and microbes – both parasitic and symbiotic. Based on the importance of the flavonoid pathway in regulating microbial relationships, any decrease in production of CHS and CHI could lead to a compromised ability of Trifolium pratense to interact with pathogens. Both plant-plant and plant-microbe interactions are site and species specific; however, results from my experiments further implicate allelopathy as a factor aiding the invasiveness of Ailanthus altissima.

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