Introduction The leaves of plants show tremendous variation in size and shape, despite all leaf primordia initially developing as simple protrusions from the flanks of the shoot apical meristem (Tsukaya 2006). Aside from variation among species, some species show abrupt changes in leaf form in response to surrounding environmental conditions. This phenomenon is termed heterophylly, and it is generally defined as the variation in leaf form in a single plant in response to environmental conditions (Zotz et al. 2011). Heterophylly allows us to study not only the environmental plasticity of plant development but also leaf development and regulation of leaf form. However, there is limited information on the mechanisms that regulate heterophylly, probably due to the lack of an adequate model, despite numerous morphological studies that have been conducted on this subject (Arber 1920, Bell 2008, Fassett 1957). A thorough understanding of leaf development is required to understand heterophylly. Many recent studies have contributed toward improving our knowledge of leaf development. In addition to understanding the structure and development of leaves, many genes involved in leaf morphogenesis have been isolated, and their networks have been studied in several model species (Blein et al. 2009, Ichihashi et al. 2011, Moon and Hake 2011, Szakonyi et al. 2010, Tsukaya 2006). Moreover, research on the morphological diversification of leaves during the course of evolution is also in progress. In particular, molecular mechanisms, such as those responsible for the difference between simple and compound leaves and the difference in the complexity of leaf form among related species, have been examined in several model plants and their close relatives (Hay and Tsiantis 2006, Kimura et al. 2008, Piazza et al. 2010). Furthermore, the links between phytohormones and leaf development have been examined in detail (Bilsborough et al. 2011, Braybrook and Kuhlemeier 2010, Koenig et al. 2009, Shani et al. 2010, Umehara et al. 2008). Indeed, phytohormones, particularly abscisic acid (ABA) and ethylene, are critical for heterophyllous transformation in various species (Anderson 1978, Hsu et al. 2001, Kane and Albert 1987, Kuwabara et al. 2003). Thus, it appears that the foundation has been laid toward elucidating the mechanisms of heterophylly To facilitate our investigation on heterophylly, we selected Neobeckia aquatica (Eaton) Greene as the model system. N. aquatica is an aquatic member of Brassicaceae and is closely related to the genera Rorippa and Cardamine (Les 1994). N. aquatica exhibits distinct leaf morphology, depending on the environmental conditions; it exhibits simple leaves in terrestrial environments and Plant Morphology vol. 24 pp. 57-63