Final Activity Report Summary - FLOWER P450S (Functional analysis of orphan cytochrome P450 genes in A. thaliana: P450s and flower development) The IBMP paper in Science (September 25 2009) describes birth of a new metabolic pathway, Metabolic plasticity, which largely relies on the creation of new genes, is an essential feature of plant adaptation and speciation and has led to evolution of large gene families. A typical example is provided by the diversification of cytochrome P450 enzymes in plants. The IBMP team «Cytochromes P450» with the support of the Marie Curie IIF programme just described the evolutionary process that led to the birth of a new metabolic pathway need for pollen development in Arabidopsis thaliana. A retroposition, neofunctionalization and duplication sequence via selective and local amino acid replacement led to the evolution of a novel phenolic pathway in Brassicaceae. This pathway involves a cascade of 6 successive hydroxylations by two partially redundant cytochromes P450 leading to the formation of N1,N5-di(hydroxyferuloyl)-N10-sinapoyl spermidine, a major pollen constituent and so far overlooked player in phenylpropanoid metabolism. This example shows how positive Darwinian selection can favour structured clusters of non-synonymous substitutions needed for transition to new function.