Abstract. Although studies in plant and animal cell culture systems indicate farnesylation is required for normal cell cycle progression, how this lipid modi®ca- tion of select proteins translates into whole-organism developmental decisions involving cell proliferation or dierentiation is largely unknown. The era1 mutant of the higher plant Arabidopsis thaliana (L.) Heynh. oers a unique opportunity to understand the role farnesylation may play in regulating various processes during the development of a multicellular organism. Loss of farnesylation aects many aspects of Arabidopsis growth and development. In particular, apical and axillary meristem development is altered and these phenotypes are contingent on the growth conditions. Key words: Arabidopsis (farnesylation, mutant) ± Farnesylation ± Lipid modi®cation ± Meristem development ± Mutant (Arabidopsis, farnesylation) ± Photoperiod Introduction Protein prenylation involves the attachment of a farnesyl (15-carbon) or geranylgeranyl (20-carbon) isoprenoid to a select group of molecules thereby facilitating protein interaction with membrane lipids and/or other proteins (Zhang and Casey 1996). The farnesyltransferase (FTase) and geranylgeranyltransferases (GGTase I and II) that carry out these respective reactions have been identi®ed in animals, plants and lower eukaryotes and so these mechanisms of lipid modi®cation of proteins have been evolutionarily conserved (Casey and Seabra 1996; Rodriguez-Concepcion et al. 1999). Although both farnesylation and geranylgeranylation are biochemically related, the target proteins of these enzymes are usually very dierent. Recently, farnesylation has garnered much attention because in animal cells mitogenic RAS needs to be farnesylated to be fully functional (Casey et al. 1989; Der and Cox 1991). The potential to alleviate oncogenic transformation by inhibiting FTase activity has focused a great deal of research in the molecular characterization of FTases from both mammalian and fungal systems. Consequently, there is now a clear sense of the kinetics and the substrate speci®cities of this enzyme. The farnesyl lipid is covalently attached to a cysteine located in a carboxyl-terminal `CAAX box' motif, where the C is the cysteine, A is usually an aliphatic amino acid and X can be any amino acid (Moores et al. 1991; Reiss et al. 1991). Apart from the RAS superfamily of small GTP-binding proteins, there is a growing roster of proteins that are involved in signal transduction and are farnesylated in fungi and animals (Davey et al. 1998; Inglese et al. 1992). Although the biochemical characterization of farn- esylation is advanced, the full extent of this modi®cation as a regulatory mechanism in higher eukaryotes is still unclear. Studies in yeast and cultured plant and animal cells using FTase inhibitors suggests that proteins associated with cell division and growth are commonly aected (Miquel et al. 1997). In plants, the peak activ- ities of FTases and the growth sensitivity of tobacco cell cultures to FTase inhibitors are coincident, and FTase peak activities precede the onset of mitosis (Randall et al. 1993). Furthermore, FTase inhibitors are eective at blocking cell cycle progression before the onset of G1 and S phase but not at G2. Together, these studies suggest plant farnesylation is essential for normal cell cycle progression (Qian et al. 1996). The diversity of farnesylated proteins in fungi, plants and animals, however, has made it dicult to determine how speci®c this type of modi®cation is in regulating cell cycle events. Furthermore, extrapolations from cell culture to whole organisms, particularly with the use of inhibitors is always problematic due to the inherent diculties of *Present address: The University of Hong Kong, Department of Botany, Pokfulam Road, Hong Kong Abbreviations: ABA = abscisic acid; CL = continuous light; FTase = farnesyltransferase; SD = short day Correspondence to: P. McCourt; E-mail: mccourt@botany.utoronto.ca; Fax: +1-416-978-5878 Planta (2000) 211: 182±190 Farnesylation is involved in meristem organization in Arabidopsis Dario Bonetta, Peter Bayliss, Susanna Sun*, Tammy Sage, Peter McCourt Department of Botany, University of Toronto, Toronto, Ontario, Canada M5S 3B2 Received: 25 October 1999 / Accepted: 22 December 1999