A novel class of developmental mutants in Arabidopsis thaliana where lateral organs of the shoot fuse upon growing into contact have been identified by the PI. Studies of the original mutant, fdh1, indicate that organ fusion is mediated by epidermal cell interactions. Experimental evidence suggests that morphogenetic factors are produced that may induce fusion between epidermal cells and preliminary results indicate that this activity can be extracted by surface washes of fdh leaves. At least three genes that regulate organ fusion in Arabidopsis have been determined by the PI. Pollen from Arabidopsis and closely related species will germinate and grow on the fdh1 shoot surface. Wildtype pollen from tomato, tobacco and snapdragon plants, however, will not grow pollen tubes when applied to fdh shoot surfaces. These results suggest that fdh1 plants express factors that mediate pollen recognition, germination and growth. The PI does not yet know whether all organ fusion mutants support promiscuous pollen growth. Both the organ fusion and pollen growth phenotypes cosegregate with the original fdh1 gene mutation. Since only carpel epidermal cells fuse and interact selectively with pollen in wildtype plants, the PI hypothesizes that mutation of the FDH gene causes the constitutive expression of a carpel-specific program in all epidermal cells of the shoot. Although fdh organs and epidermal cells are morphologically normal, carpel-specific biological activity is expressed over the entire shoot surface. The availability of such mutants makes possible the identification of molecules involved in these biological processes. The objective of this research project is to understand how epidermal cells mediate organ fusion and identify ectopically expressed factors that may be important in stigma-pollen interactions. This will be accomplished using biochemical, molecular and genetic approaches. The PI proposes to characterize factors mediating organ fusion and pol len growth. In particular we will analyze the fdh mutant cell wall and cell wall constituents. Cytochemical techniques to determine the staining profile of fresh and cryopreserved fdh tissues and the porosity of fdh cell walls will also be conducted. In a collaborative effort, an analysis of the sugar and lipid monomer composition of fdh cell walls will be done. The expression of S-locus glycoprotein genes and the presence of JIM antibody binding species will also be tested for. Initial attempts at characterizing the morphogenetic factors will be made and tests for functional conservation between C. roseus and fdh factors will be conducted. In addition efforts toward isolating the FDH genes using one of three strategies will be initiated.
