Auxin Response Factors and Auxin-Regulated Gene Expression

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Auxin is a hormone that plays key roles in regulating a wide range of plant growth and developmental processes, including organ patterning, lateral root initiation, stem and root elongation, vascular tissue formation and differentiation, apical dominance, and tropisms. Many of these auxin-regulated processes likely involve localized and transient changes in auxin concentration that result in activation or repression of auxin response genes. Activation of auxin response genes is thought to trigger a cascade of downstream gene expression, ultimately leading to changes in growth and development. It is thought that two types of transcription factors, referred to as Auxin Response Factors or ARFs and Aux/IAA proteins, interact in regulating auxin response gene expression. ARFs are transcriptional activators or repressors that have been shown to bind in vitro to Auxin Response Elements [AuxREs] in promoters of auxin response genes. Aux/IAA proteins are transcriptional repressors that do not bind directly to AuxREs, but are thought to be recruited to AuxREs by dimerizing with ARF activators that reside on these promoter elements. Dimerization between ARF activators and Aux/IAA repressors is thought to occur when auxin concentrations in a cell are low, resulting in repression of auxin response genes. At elevated auxin concentrations, Aux/IAA repressors become less stable, resulting in their depletion from the cell and subsequent derepression or activation of auxin response genes. A considerable amount of indirect evidence supports ARF activators interacting with Aux/IAA repressors to regulate auxin response genes; however, direct evidence that supports the targeting of ARFs to AuxREs in vivo and the recruitment of Aux/IAA proteins to auxin responsive promoters is lacking. Furthermore, while there is a considerable amount of evidence that auxin regulates the abundance of Aux/IAA repressors in vivo, it remains to be determined how the levels of ARF activators are regulated and if levels of specific ARFs determine if and when they regulate specific auxin response genes. The proposed research is focused on providing direct evidence for or against ARF and Aux/IAA interactions on promoters of auxin response genes. Furthermore, the research is aimed at determining which ARF proteins bind to AuxREs in the auxin response gene and how ARF protein levels affect the expression of auxin response genes. Broader Impacts of the Proposed Activity. The experiments will provide training for undergraduates, graduate students, and postdoctoral associates. The involvement of undergraduates is emphasized [i.e., honors theses for U MO undergraduates and summer laboratory experiences for both U MO and other undergraduates through the U MO Life Sciences Undergraduate Research Opportunity Program]. Understanding how ARF and Aux/IAA proteins function mechanistically is providing novel strategies to regulate auxin response genes that bypass auxin signaling and result in phenotypic changes (e.g., increased lateral root formation). A mechanistic understanding of auxin action at the gene expression level and the manipulation of ARF and Aux/IAA proteins is likely to lead to benefits in agriculture, agriforestry, and horticulture.