Leaf Vascular Pattern Formation.

The pattern and ontogeny of leaf venation appear to guide or limit many aspects of leaf cell differentiation and function. Photosynthetic, supportive, stomatal, and other specialized cell types differentiate in positions showing a spatial relationship to the vascular system. These spatial relationships are of obvious importance to leaf function, which relies on venation for the servicing of cells engaged in photosynthesis, gas exchange, and other leaf processes. Although the need for coordinated organization of cell types around the vascular system is clear, the means by which this is achieved during development is not well understood. In the few systems in which it has been possible to follow the ontogeny of the venation along with the differentiation and function of surrounding cell types (e.g., in C4 grasses), observations suggest that the developing vascular system may have a role in providing positional landmarks that guide the differentiation of other cell types. Another possible explanation is that an underlying pattern guides the differentiation of both venation and surrounding cells. Whether the process of vascularization creates or reveals a pattern, studies to date are largely descriptive, and little is understood of the underlying mechanisms. These mechanisms must be highly regulated, as evidenced by the successful use of species-specific leaf vascular pattern as a taxonomic characteristic (e.g., Klucking, 1992) and by the predictable effect of certain mutations. In this review, we summarize the vascular patterns and their ontogenies in dicots and monocots, referring extensively but not exclusively to Arabidopsis and maize as examples. We also discuss a variety of models that seek to explain vascular pattern formation, and we provide a summary of molecular and genetic investigations of the process.