Communication and molecular transport between cells often occur through intercellular connections, termed gap junctions in animals and plasmodesmata (PD) in plants. While gap junctions have been extensively studied, the biochemical composition and molecular biology Of PD remain a biological black box even though their ultra-structure has been described. The elucidation of the molecular composition and regulatory mechanisms of PD would be a major advance in understanding cell-to-cell communication and its effects on plant growth and development. This proposal will study PD using plant viruses which spread between cells through PD. Tobacco mosaic virus (TMV), one of the best studied plant viruses, moves through PD with the help of the two of its proteins, the cell- to-cell movement protein (MP) and the coat protein (CP). MP mediates the local spread of the virus while both MP and CP are thought to potentiate the onset of the viral-systemic movement. The proposed research will utilize these virus-PD interactions to achieve three specific objectives: I. Functional characterization of a potential plant cellular receptor for TMV-MP. This receptor, designated MPR, has been identified using the yeast two-hybrid protein-protein interaction System. The full length MPR cDNA will be cloned and its expression characterized during plant development as well as during the viral infection process. MPR will be also used as a target in the two- hybrid assay to identify its cellular ligand, the putative and long- sought after plant homolog of TMV-MP. II. Cloning of an Arabidopsis gene involved in the systemic spread of a plant tobamovirus. A genetic assay for Arabidopsis mutants defective in viral systemic spread was developed and three mutants were isolated. Two mutants were obtained by chemical mutagenesis (EMS), and one was a result of the T-DNA insertion mutagenesis.The T-DNA tagged gene will be cloned and characterized and the other two mutations will be mapped. III. Cloning and characterization of the gene encoding the TMV-MP kinase. A tobacco cell wall-associated protein kinase that phosphorylates TMV-MP was purified to homogeneity. Preliminary data indicate that this enzyme, designated MP kinase, is localized to or in the vicinity of PD. The intercellular localization of the MP kinase will be studied in detail, its expression patterns will be examined during plant development, and the plant gene coding for this enzyme will be cloned.