The formation of the plant body requires the assembly of different tissue systems in an organized pattern to make organs. The aim of this project is to understand how the formation of complex tissue systems is genetically regulated and how has been acquired during plant evolution. A preliminary analysis of the variation in ground tissue anatomies in different species across the Brassicaceae has showed a remarkable diversity of ground tissue phenotypes in this family. We intend to use this diversity to investigate the developmental and evolutionary mechanisms underlying tissue complexity in plants. We will compare the ground tissue organization of A. thaliana with two other Brassicaceae species, Cardamine hirsuta and Thellungiella halophila. The double layered cortex of these species gives us the opportunity of studying the developmental basis of the formation of a multi-layered cortex. T. halophila is one of the few examples of plants that form a double endodermis. This project will try to uncover the molecular mechanism underlying this almost unique ground tissue organization. Our hypothesis is that similar key regulatory genes will regulate the formation of complex and simple ground tissue systems in this family and that changes in their patterns of expression are responsible for the differences that arise between species. To check this hypothesis we will analyse the expression, function and molecular differences of the orthologous genes of three ground tissue A. thaliana transcription factors, SCHIZORIZA, SHORTROOT and SCARECROW. To identify new genetic components of this process we will generate mutagenized populations and search for mutants with defective ground tissue phenotypes and will compare transcriptional profiles between species. Finally, the analysis of the correlation between specific habitats and ground tissue characteristics in different species of the family will allow us to assess the adaptive value of this developmental trait in the Brassicaceae.