Preventing leaf identity theft with hormones Shelley Lumba and Peter McCourt Genetic analysis of plant development has begun to demonstrate the importance of hormone synthesis and transport in regulating morphogenesis. In the case of leaf development, for example, auxin pooling determines where a primordium will emerge and leads to the activation of transcription factors, which determine leaf identities by modulating abscisic acid (ABA) and gibberellic acid (GA) concentrations. Signal transduction studies suggest that negative regulation of transcription factors through protein turnover is commonly used as a mechanism of hormone action. Together, these findings suggest that auxin might degrade a repressor that allows the activation of genes that modulate ABA/GA ratios in emerging leaves. With our increased understanding of the molecular basis of hormone signaling, it is becoming possible to overlay important regulators onto signaling modules that determine morphological outputs. Addresses Department of Botany, University of Toronto, Toronto, Ontario M5S 3B2, Canada Corresponding author: McCourt, Peter (mccourt@botany.utoronto.ca) Current Opinion in Plant Biology 2005, 8:501–505 This review comes from a themed issue on Cell signalling and gene regulation Edited by George Coupland and Salome Prat Monguio Available online 27th July 2005 1369-5266/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. DOI 10.1016/j.pbi.2005.07.015 Introduction Approximately sixty years ago, CP Snow used the famous phrase ‘the two cultures’ to convey regret at the rift that had grown between literary intellectuals and scientists with respect to appreciating each other contributions [1]. To some extent, a similar void has existed between plant developmental geneticists and plant hormone biologists. The former usually isolated mutants that often identified transcription factors that are involved in morphogenesis, whereas the latter attempted to understand how a small group of organic molecules signal to influence plant growth and physiology. Historically, each group cared little about the other’s work. Morphological mutants were rarely studied with respect to hormone status and devel- opmentally important transcription factors were seldom monitored during hormone manipulation experiments. How many of the developmental processes that have been identified by mutational analysis of morphogenesis actually manifest their phenotypes because of modulation of hormone levels, transport or sensitivity? Fortunately, unlike the still-existing gulf between prose and science alluded to by Snow, developmental geneticists and hor- mone biologists are beginning to listen to each other, and this is reflected in the growing number of papers showing that morphological regulators can control hormone bio- synthesis and transport. In this opinion, we use studies of the role of hormones in the determination of leaf iden- tities as an example of how recent molecular information on hormone signal transduction can be mapped onto our present understanding of foliar organ development. Although this is only one example, we believe that the lessons learned here apply to other morphogenetic mod- els of plant development. Making your point with hormones In the past two years, two seminal papers have been published on the role of auxin in the initiation and patterning of lateral organ formation in Arabidopsis [2,3]. In one case, Benkova et al.[2] used the auxin- inducible reporter to mirror the dynamic flux of auxin localization in both aerial and underground-formed organs. Although an output reporter gene cannot provide a direct measure of auxin levels, the gene expression patterns suggest that auxin movement results in gradients of auxin maxima that mark the formation of nascent primordia. The authors go on to show a correlation between auxin maxima and the dynamic rearrangement of the auxin efflux carrier PIN-FORMED1 (PIN1), pro- viding a mechanism for the regulation of auxin pooling with respect to primordia development. The second study focuses on the localization of the PIN1 protein in both the vegetative and floral apices of Arabi- dopsis and on the role of auxin in regulating phyllotaxy [3]. In addition to visualizing PIN1 localization at the cellular level, Reinhardt et al.[3] took advantage of an ingenious technique that localizes the application of exogenous auxin to the apices of various mutants, such as pin1, thereby illustrating the direct role of auxin in lateral organ initiation. Despite being defective in auxin transport, pin1 mutants can form leaves but these leaves are abnormally shaped and positioned [4,5]. Treatment of pin1 plants with auxin, however, can rescue the leaf-shape pheno- type. In addition, the restoration of organ formation is dependent on both the site and concentration of auxin application. The results from both groups provide strong evidence that polar auxin transport is required to deter- mine the initiation sites of lateral organ primordia. One obvious conclusion from these experiments is that any www.sciencedirect.com Current Opinion in Plant Biology 2005, 8:501–505