Sex Plant Reprod (2003) 15:311–320 DOI 10.1007/s00497-003-0167-7 ORIGINAL ARTICLE Vladimir Brukhin · Michel Hernould · Nathalie Gonzalez · Christian Chevalier · Armand Mouras Flower development schedule in tomato Lycopersicon esculentum cv. sweet cherry Received: 3 September 2002 / Accepted: 3 February 2003 / Published online: 15 March 2003  Springer-Verlag 2003 Abstract The ontogeny of tomato (Lycopersicon escu- lentum cv. sweet cherry) flowers was subdivided into 20 stages using a series of landmark events. Stamen primor- dia emergence and carpel initiation occur at stage 4; archesporial and parietal tissue differentiate at stage 6 and meiosis in anthers begins at stage 9. Subepidermal meristematic ovule primordia are formed on the placenta at stage 9; megasporogenesis begins at stage 11–12 and embryo sac differentiation and ovule curvature take place at stage 14, once the pollen is maturing. We established a correlation between the characteristic cellular events in carpels and stamens and morphological markers of the perianth. The model of tomato flower development schedule was then used to analyse the spatial, temporal and tissue-specific expression of gene(s) involved in the regulation of floral organ development. As an example, the expression pattern of ORFX, a gene controlling cell size in tomato fruits, shows that expression starts very early during the ontogeny of reproductive organs. Keywords Lycopersicon esculentum · Flower development · Microsporogenesis · Megasporogenesis · Histological analysis Introduction Research in plant developmental biology has progressed very rapidly in the past decade due to the application of molecular biology techniques, the isolation and charac- terisation of various developmental mutants, and the cloning of genes specifically involved in different stages of plant development. It has been established that floral organ development is controlled by the differential expression of tissue- and organ-specific genes (Meyer- owitz 1998; Theissen et al. 2000). Such genes may be involved in organ/tissue differentiation by triggering the regulatory cascade of key genes controlling the identity of floral organs, i.e. the “ABC” model of floral development (Coen and Meyerowitz 1991; Weigel and Meyerowitz 1994; Ng and Yanofsky 2000). However, a prerequisite to understanding the mechanisms of flower development is its dissection into distinct stages. Initially, this requires morphological and cytological definition of developmen- tal stages at high resolution from initiation of floral meristem to floral maturity, upon which a subsequent systematic genetic and molecular analysis may be based. Chronological ranking of these stages and linking them to the size of the flower meristem is also required in order to study temporal changes in flower development. Such a morphological description has been carried out for widely used model plants e.g. Nicotiana tabacum (Koltunow et al. 1990), Arabidopsis thaliana (Smyth et al. 1990; Schneitz et al. 1995), and Silene latifolia (Farbos et al. 1997). However, despite many years of intensive genetic selection, we still lack information about many aspects of tomato (Lycopersicon esculentum) flower development, i.e. how it is coordinated with embryonic or seed development, how development of gynoecium and an- droecium are correlated with each other and with other flower structures and, finally, what the tomato flower development schedule is. Previous analyses of very early stages of tomato flower development, performed by light- and scanning electron microscopy (SEM), showed that flower organs in the four whorls are produced in a serial sequence (sepals, petals, V. Brukhin ( ) ) · M. Hernould · N. Gonzalez · C. Chevalier · A. Mouras UMR 0619 Physiologie Biotechnologie VØgØtales, INRA-UniversitØs Bordeaux 1 et 2; Institut Biologie VØgØtale MolØculaire, INRA-CR Bordeaux, BP 81, 33883 Villenave d’Ornon Cedex, France e-mail: vbrukhin@botinst.unizh.ch Tel.: +41-1-6348265 Fax: +41-1-6348204 Present address: V. Brukhin, Institute of Plant Biology, University of Zurich, Zollikerstrasse 107, 8008 Zurich, Switzerland e-mail: vbrukhin@botinst.unizh.ch Tel.: +41-1-6348265 Fax: +41-1-6348204