The symplastic route composed of plasmodesmata (PD) channels and the transporting phloem tissue (rich in PD) is the major pathway for carbon allocation in plants. How the symplastic transport route is formed during plant ontogeny and what is its significance in conducting and distributing morhogenetic signals to the growing organs is poorly understood at the moment and is addressed here. My laboratory has recently made a breakthrough that facilitates the analysis of symplastic communication. In a genetic screen we identified gain-of-function mutations in a locus that codes for a CALLOSE SYNTHASE isoform CALS3. The cals3-d mutations result in restricted symplastic trafficking through the PD. Using the cals3-d mutations in a vector system that allows cell type specific and inducible control of expression of the transgene, icals3m, we have been able to construct a molecular tool, with which we can regulate the passage of the various signaling molecules between the neighboring cells. This tool has already opened several new lines of research on symplastic communication concerning understanding of the regulation of PD channels, phloem development and symplastically moving signals. By a combination of experimental approaches at molecular, genetic, imaging and theoretically levels we will investigate here:(1) How is symplastic trafficking regulated?(2) What are the (symplastic) signals specifying phloem development?(3) How do the signals emanating from the phloem control root development?(4) Can we predict new regulatory factors controlling symplastic trafficking in space and time, based on the experimental data (on the distribution of symplastic channels, symplastically controlled genes and symplastically mobile molecules)?