The plant hormone auxin controls both cell growth and cell division and also has a crucial role in determining overall morphology. The molecular-level details of auxin signal perception have been studied for decades. An auxin receptor complex located in the nucleus has been associated with many auxin-regulated processes. It is not clear, however, that this complex can explain the full range of auxin-regulated phenomena, especially those occurring along the cell membrane. Thus, the search for a cell surface-located auxin signaling mechanism continues. This project will extend previous work showing that auxin activates major signal mediators that function along the inner surface of the cell membrane called RAC/ROP GTPases. These important proteins may act as mediators activating surface-located responses. The current project will test whether the transmembrane protein FERONIA could be the missing link connecting extracellular auxin signals to this system. Specifically, this project will determine whether the extracellular domain of FERONIA directly interacts, alone or in a complex, with a low molecular weight extracellular matrix protein called LORELEI-like glucosylphosphatidylinositol-anchored protein1. Biochemical methods such as protein co-immunoprecipitation, pull-down assays and cell biological methods will be used to test for these interactions in plant cells and determine how auxin affects them. Direct auxin-binding studies will be used to test for receptor-ligand interactions between auxin and FERONIA or FERONIA complexes. These crucial experiments testing the potential of FERONIA as a cell surface auxin signaling apparatus will provide training for one graduate student and one postdoctoral fellow.
Normal 0 false false false EN-US X-NONE X-NONE This project focuses on understanding the mechanisms of FERONIA receptor kinase (RK) a cell membrane protein with important roles in growth, development and reproduction and interactions with pathogens in the model plant Arabidopsis. Our ultimate goal is to elucidate how FERONIA, as one protein, can be so broadly and crucially involved in plant cell processes. As a RK, FERONIA has an extracellular domain (FERecd), a transmembrane domain, and a cytoplasmic domain that regulates the activities of potentially many cellular proteins so as to affect different cell processes. As a cell surface RK, FERecd is expected to interact with molecules on the cell surface, which in turn modifies the activity of its cytoplasmic domain, achieving a signal transduction function. This project specifically examines the functional relationship between FER and another cell surface molecule, LLG1. We found that plants defective in either the Fer or Llg1 gene have similar growth and developmental phenotype, and that FERecd and LLG1 physically interact on the cell surface. FER and LLG1 also interact with RHO GTPases, which are major molecular switches located along the cytoplasmic side of the cell membrane, controlling multiple cellular processes. Our results further demonstrate that LLG1 is required for FER to function as a cell surface signal receptor and mediator. In llg1 mutant plants, FER protein cannot be localized on the cell membrane, but is instead trapped inside the cell in the endoplasmic reticulum, the first compartment for cell surface-located proteins to pass through on their way to the cell membrane. In addition, our results show that FER and LLG1 also interact in the endoplasmic reticulum. These findings are consistent with FER and LLG1 as having co-functional roles and by interacting with FER already in the endoplasmic reticulum, LLG1 directs and ensures the deposition of FER on the cell membrane. They also explain why the phenotypes in llg1 mutant plants are exactly like those in fer mutants: in llg1 mutants, presence of FER protein at its proper functional location is reduced or even abrogated, similar to the situation in loss-of-function fer mutants. The finding the FER depends on another protein to assume its biological role is also novel. FER-LLG1 interaction provides another molecule whose activity could be differently regulated as that for FER. This might offer additional ways to fine-tune the ultimate FER functions, enabling it to fulfill the multiple roles it is known to play. FERONIA is a member of a small protein family in Arabidopsis and FER homologs are also found throughout the plant kingdom. Some of these FER-related proteins are also known to be involved in plant growth and defenses. Knowledge from FER will provide a foundation from which broader investigations into how this family of proteins together contributes to overall plant growth and survival. Undergraduate and graduate students and postdoctoral scientists have been trained in this project. Some of their results are basis for publications for broad dissemination among the scientific community. Materials and concepts generated from this project were used in a STEM workshop for high school teachers; high school interns were also trained using materials and experiments from this project.
