The role of the margin in leaf development has been debated over a number of years. To investigate the molecular basis of events in the margin, we performed an enhancer trap screen to identify genes specifically expressed in this tissue. Analysis of one of these lines revealed abnormal differentiation in the margin, accompanied by an abnormal leaf size and shape. Further analysis revealed that this phenotype was due to insertion of the trap into DWF4, which encodes a key enzyme in brassinolide biosynthesis. Transcripts for this gene accumulated in a specific and dynamic pattern in the epidermis of young leaf primordia. Targeted expression of DWF4 to a subset of these cells (the leaf margin) in a dwf4 mutant background led to both restoration of differentiation of a specific group of leaf cells (margin cells) and restoration of wild-type leaf shape (but not leaf size). Ablation of these cells led to abrogation of leaf development and the formation of small round leaves. These data support the hypothesis that events in the margin play an essential role in leaf morphogenesis, and implicate brassinolide in the margin as a key mediator in the control of leaf shape, separable from a general function of this growth factor in the control of organ size.

To study whether sucrose synthase (SuSy, EC 2.4.1.13) plays a role in phloem loading/unloading, we have directly assessed the distribution of SuSy transcripts and protein in young maize (Zea mays L.) leaves. Our data show that at the RNA and protein levels there is no expression of SuSy in phloem of sink tissue and that SuSy expression is speci®cally excluded from this tissue. These data, in conjunction with others in the literature, indicate that the localisation of SuSy to phloem may be limited to source tissue, whereas a nonvascular expression is linked with sink tissue activity.Keywords Sucrose synthase á Phloem á Sink/source á Zea (sucrose synthase) á Leaf á RNA localisation Abbreviations SuSy: sucrose synthase á GUS: b-glucuronidase Planta (2001) 214: 326±329

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