Importance of rhizospheric processes in plant-plant interactions.

摘要

Rhizospheric interactions have been a largely overlooked area of plant biology until the past decade when the importance and abundance of plant-plant and plant-microbe associations began to catch the scientific community's attention. Negative plant-plant interactions such as those involved in the advance of invasive species were well investigated and root secretions, a vast array of molecules exuded from roots, were implicated in the process. Additionally, intra-species interactions determining the ability of a plant to sense self and non-self plants was well established in many plant species. However, it was not until recently that it was established that a plant species, Cakile edentula, could interact with pot neighbors and alter its growth according to the relatedness of the neighbor (whether the neighbor was a genetically related kin or a non-related stranger). Based on this study, this dissertation investigated whether Arabidopsis thaliana was also capable of recognizing kin and stranger rhizospheric neighbors; however we used an in vitro approach to also determine if root secretions are involved in the process. Our results supported that A. thaliana has the ability to recognize kin and stranger plants based on a secreted chemical determined by increased lateral root growth when in the presence of stranger secretions versus when grown in the presence of own or kin plant secretions. To further investigate the role of root secretions in the kin recognition process in A. thaliana, we tested a known root secretion inhibitor (and ABC transport inhibitor) sodium orthovanadate, and found that it eliminated the increase in lateral root growth in plants exposed to stranger secretions and therefore is involved in blocking the chemical signal involved in kin and stranger recognition. We then investigated the role of ABC transporters (AtPGP1, AtATH1, AtATH10) in kin recognition since sodium orthovanadate, which eliminated the recognition response, is a known ABC transport inhibitor. In parallel, we also tested four additional ABC transport inhibitors to see if they caused a similar response in the elimination of the stranger recognition response. We found that all of the inhibitors had a similar response on root growth patterns, but did not change gene expression patterns for all three ABC genes in the same manner. Likewise, use of ABC T-DNA insertion mutants showed that the ABC transporters were involved but perhaps to different degrees. Therefore, it is possible that the kin recognition signal could be due to more than one compound, controlled by more than one ABC transporter, or that it could be one compound controlled by one of many ABC transporters, why AtATH1, AtATH10, and AtPGP1 have varying involvement in the kin recognition process. Finally, to shed light on why these A. thaliana plants have evolved the ability to recognize kin versus stranger plants, we attempted to determine the trade-off cost in plants exposed to own, kin and stranger secretions after pathogen infection. We investigated the gene expression levels of two known pathogen response genes PR1 and PDF1.2 and found that uninfected plants exposed to stranger secretions had significantly higher expression of PR1 as compared to plants exposed to own or kin secretions, but that there was no-change in PDF1.2 expression when comparing secretion treatments. The same pattern of increased gene expression of PR1 was found in infected plants but there was no increased pathogen resistance associated with this elevated level of the defense related gene. Therefore, we speculate that this increase in the PR1 expression may be due to partitioning of resources for competition as elevated PR1 expression can be linked to other plant growth processes. In summary, this dissertation has investigated a little known niche in plant biology and determined that A. thaliana is capable of recognizing kin and stranger plants based on a root secreted chemical and that ABC transporter genes are involved in this process. The knowledge herein contributes to understanding root-secretions, plant-plant interactions and plant community interactions as well as plant biology in general.