Abstract The definition of the patterns of cell division and expansion in plant development is of fundamental importance in understanding the mechanics of morpho- genesis. By studying cell division and expansion pat- terns, we have assembled a developmental map of Petu- nia hybrida petals. Cycling cells were labelled with in situ markers of the cell cycle, whereas cell expansion was followed by assessing cell size in representative re- gions of developing petals. The outlined cell division and expansion patterns were related to organ asymmetry. Initially, cell divisions are uniformly distributed through- out the petal and decline gradually, starting from the bas- al part, to form a striking gradient of acropetal polarity. Cell areas, in contrast, increased first in the basal portion and then gradually towards the petal tip. This growth strategy highlighted a cell size control model based on cell-cycle departure time. The dorso-ventral asymmetry can be explained in terms of differential regulation of cell expansion. Cells of the abaxial epidermis enlarged earlier to a higher final extent than those of the adaxial epidermis. Epidermal appendage differentiation contrib- uted to the remaining asymmetry. On the whole our study provides a sound basis for mutant analyses and to investigate the impact of specific (environmental) factors on petal growth. Keywords Cell division · Histone H4 · Mitotic cyclin · Petal development · Petunia hybrida Introduction Development in plants is achieved mainly during post- embryogenesis, according to programs that coordinate diverse processes, ranging from cell division and en- largement to differentiation. Shoots and their attendant structures such as leaves and flowers differentiate from a shoot apical meristem (SAM), which is composed of three clonally distinct layers of cells. As organ formation requires the coordinated proliferation of cells in all three layers, the rate and pattern of cell division in each of the layers must be controlled in order to establish the size and shape of these organs. Controls, ensuring that cell division and expansion in the different layers, are mutu- ally coordinated are indispensable for this process (Mey- erowitz 1997). The existence of structural similarities between organs, such as the proximodistal and dorsoven- tral asymmetries, suggests that at least some of these de- velopmental programs are shared between different or- gans. The role of cell division and expansion on the mor- phogenesis of vegetative organs has long been a subject of study. Patterns of cell cycling related to morphogene- sis of the leaf blade, tissue proliferation and differentia- tion of specific cell types occur along a prominent longi- tudinal gradient. In Arabidopsis leaves, for example, di- viding cells are initially spread throughout the leaf, but gradually become restricted to proximal portions of the blade and to the petiole, forming a typical basipetal gra- dient (Donnelly et al. 1999). Although the frequency of cell cycling varies for each specific layer, this gradient is L. Reale · C. Moretti · B. Romano · F. Ferranti Dipartimento di Biologia Vegetale e Biotecnologie Agroambientali, Università degli Studi di Perugia, Borgo XX Giugno 74, 06121 Perugia, Italy A. Porceddu ( ✉ ) Istituto per il Miglioramento Genetico delle Piante da Orto e da Fiore (CNR), via Università 133 Portici (NA), 06100 Perugia, Italy L. Lanfaloni Dipartimento di Biologia Cellulare e Molecolare, Università degli Studi di Perugia, Via Elce di Sotto, 06121 Perugia, Italy S. Zenoni · M. Pezzotti Dipartimento Scientifico e Tecnologico, Università degli Studi di Verona, Strada Le Grazie 7, 37134 Verona, Italy Present address: A. Porceddu, Istituto per il Miglioramento Genetico delle Piante Foraggere (CNR), via Madonna Alta 130, 06100 Perugia, Italy e-mail: A.Porceddu@irmgpf.pg.cnr.it Tel.: +39-75-5014865, Fax: +39-75-5014869 Sex Plant Reprod (2002) 15:123–132 DOI 10.1007/s00497-002-0150-8 ORIGINAL ARTICLE Lara Reale · Andrea Porceddu · Luisa Lanfaloni Chiaraluce Moretti · Sara Zenoni · Mario Pezzotti Bruno Romano · Francesco Ferranti Patterns of cell division and expansion in developing petals of Petunia hybrida Received: 17 May 2002 / Accepted: 19 July 2002 / Published online: 11 September 2002 © Springer-Verlag 2002