The asymmetric leaves2 (as2) mutant of Arabidopsis thaliana generated leaf lobes and leaflet-like structures from the petioles of leaves in a bilaterally asymmetric manner. Both the delayed formation of the primary vein and the asymmetric formation of secondary veins

The left-right symmetry of most living things has been of general interest not only to biologists but also to researchers in other fields. In contrast to animals that have developed bodies with left-right symmetry, which is believed to be tightly linked to the capacity for mobility, plants are basically immovable and their overall body shapes exhibit conic symmetry (Gardner, 1990). It is, nonetheless, generally accepted that leaves of many angiosperms exhibit obvious but approximate left-right symmetry with the rachis as the axis (Hickey, 1973; Hickey, 1979; Sinha, 1999), even though exceptions exist, such as Begonia spp. (Lieu and Sattler, 1976), Tropaeolum (Whaley and Whaley, 1942) and many species of Urticaceae (Dengler, 1999). Regardless of the complexity of leaf shape (e.g., a simple leaf or a compound leaf), the two sides of each leaf are nearly mirror images of one another (Ogura, 1962; Hickey, 1973; Hickey, 1979). Leaves develop from a shoot apical meristem (SAM) along three-dimensional axes (the proximodistal, transverse and adaxial-abaxial axes; Steeves and Sussex, 1989; Waites et al., 1998), as well as with left-right symmetry. Various mutants with altered leaf morphology that is related to abnormal expansion along these axes, altered adaxial and abaxial identity, and the altered overall shape of leaves have been isolated, and some of the genes responsible for the mutant phenotypes have been cloned and characterized (Hake et al., 1989; Conway and Poethig, 1997; Höfer et al., 1997; Waites et al., 1998; Kim et al., 1998; Berná et al., 1999; SerranoCartagena et al., 1999; Timmermans et al., 1999; Tsiantis et al., 1999; Sawa et al., 1999; Siegfried et al., 1999). However, our understanding of the way in which the nearly mirror-image 1771 Development 128, 1771-1783 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 DEV0348