Auxin Response Factors and Auxin-Regulated Gene Expression

生长素是一种在调节植物生长和发育过程中起关键作用的激素，包括器官形成、侧根发生、茎和根伸长、维管组织形成和分化、顶端优势和向性。 这些生长素调节的过程中的许多可能涉及生长素浓度的局部和瞬时变化，其导致生长素反应基因的激活或抑制。生长素反应基因的激活被认为会引发下游基因表达的级联反应，最终导致生长和发育的变化。据认为，两种类型的转录因子，称为生长素应答因子或ARF和Aux/IAA蛋白，在调节生长素应答基因表达中相互作用。ARF是转录激活因子或抑制因子，已显示其在体外与生长素应答基因启动子中的生长素应答元件[ARF]结合。Aux/IAA蛋白是不直接结合于ARF的转录阻遏物，但被认为通过与位于这些启动子元件上的ARF激活剂二聚化而被募集到ARF。 ARF激活子和Aux/IAA抑制子之间的二聚化被认为是在细胞中生长素浓度低时发生的，导致生长素反应基因的抑制。在生长素浓度升高时，Aux/IAA阻遏物变得不太稳定，导致它们从细胞中耗尽，随后去阻遏或激活生长素反应基因。大量的间接证据支持ARF激活剂与Aux/IAA阻遏物相互作用以调节生长素应答基因;然而，缺乏支持ARF在体内靶向Aux/IAA蛋白并将Aux/IAA蛋白募集到生长素应答启动子的直接证据。此外，虽然有相当数量的证据表明，生长素在体内调节Aux/IAA阻遏物的丰度，但仍有待确定ARF激活物的水平是如何调节的，以及特定ARF的水平是否决定它们是否以及何时调节特定的生长素反应基因。 本研究旨在为ARF和Aux/IAA在生长素反应基因启动子上的相互作用提供直接证据。此外，该研究旨在确定哪些ARF蛋白与生长素反应基因中的ARF结合，以及ARF蛋白水平如何影响生长素反应基因的表达。 拟议活动的更广泛影响。 这些实验将为本科生、研究生和博士后提供培训。强调本科生的参与[即，荣誉论文为密苏里大学本科生和夏季实验室的经验，为两个密苏里大学和其他本科生通过密苏里大学生命科学本科生研究机会计划]。 了解ARF和Aux/IAA蛋白质如何在机制上发挥作用，为调节绕过生长素信号传导并导致表型变化的生长素反应基因提供了新的策略（例如，增加侧根形成）。 在基因表达水平上对生长素作用机制的理解以及对ARF和Aux/IAA蛋白的操纵可能会在农业、农林和园艺中带来益处。

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.