Vascular plants have a long-distance transport system consisting of phloem and xylem. The phloem provides the path for the distribution of photosynthates and for the translocation of proteins and mRNAs involved in plant growth and development. Phloem tissu e is composed of several specialised cells. The sieve tube is the main conducting unit in the phloem tissue. Sieve tubes are highly specialised cells that are connected symplastically through sieve plates to form continuous columns. We will use a combined genetic and genomic approach to study the processes and factors that control sieve tube differentiation. We will take advantage of an Arabidopsis promoter-trap line, which shows expression of a marker gene (UidA) in loci of phloem differentiation. We have subjected this line to EMS mutagenesis and will identify and characterise mutants with altered expression patterns of the marker gene. We have also isolated the regulatory sequence that is responsible for the marker gene expression in the promoter-trap lin e and fused it to the gene for green fluorescent protein (GFP). We will use the resulting plants for the isolation of cells that undergo phloem differentiation. Subsequently, we aim to investigate the transcriptome of these cells using microarray technique s. The objective of this proposal is to investigate the factors and genes that control phloem development and to obtain a complete picture of phloem cell differentiation.