9317381 Szymkowiak This is a collaborative project with Dr. Ian Sussex (9317262). Plant cells differentiate in a position- dependent and not a lineage-dependent manner. They must, therefore, receive signals that inform them of their position within the multicellular organism and differentiate accordingly. The nature of the signals, their origins and how they are transmitted is largely unknown. This collaborative research project will address these questions by examining how cells in the three layers of the shoot apical meristem maintain functional coordination over the course of meristem ontogeny. The generation and analysis of periclinal chimeras has demonstrated that interactions occur among the cell layers of the meristem. In general, the outer two meristem layers, L1 and L2, develop in a nonautonomous manner. That is, cells occupying these two layers do not develop according to their own genotype but according to a pattern of development determined by cells in the internal meristem layer, L3. Cells comprising the chimeras interact effectively both in the meristem and in the tissues and organs generated by the meristem. An interesting exception occurs during floral meristem development in certain chimeras between tomato and Solanum nigrum. The shoot meristems of these chimeras all function normally during vegetative development. Each chimera also makes the transition from vegetative meristem to inflorescence meristem. However, development is arrested in the floral meristems of four of the five cell layer combinations generated between tomato and S. nigrum. The tomato-S. nigrum chimeras provide an excellent opportunity to examine a developmental process in which cell-cell coordination has failed, in order to learn more about the signaling that is necessary for coordinated development. The main objective of this collaborative project is to use these periclinal chimeras to investigate possible mechanisms for coordinating cell layer function in t he meristem. To achieve this, Drs. Szymkowiak and Sussex propose to: (1) determine if the arrested development of chimeric floral meristems results from a breakdown in the coordination of the patterns of development in each cell layer by using gene probes specific for different stages of meristem ontogeny; (2) investigate the role of plasmodesmata in cell layer coordination by examining changes in plasmodesmata structure and size exclusion limits by using dye-injection studies in the aborting floral meristems before and during arrested development; and (3) generate additional chimeras using mutations that affect function in shoot meristems in order to characterize further processes requiring cellular interactions. Achievement of these goals will contribute to improved understanding of meristem organization and function. ***
