Leaf Vascular Pattern Formation.

Abstract

INTRODUCTIONThe 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. VASCULAR PAlTERN IN MATURE LEAVES DicotsThe relatively small simple leaves of Arabidopsis illustrate many of the characteristic features of leaf venation found in

SHORTROOT-Mediated Increase in Stomatal Density Has No Impact on Photosynthetic Efficiency

“…In this regard, photosynthetic function in plant leaves requires that chloroplast-containing cells in the middle leaf layers are interspersed with veins (to supply water and to redistribute metabolites) and are overlaid with stomatal pores through which carbon dioxide can enter the leaf. In grass leaves, cellular arrangements are defined by parallel longitudinal veins that extend from the base of the leaf sheath to the tip of the leaf blade, with short transverse veins interconnecting the longitudinal network (Sharman, 1942;Esau, 1943;Nelson and Dengler, 1997). This vascular framework underpins linear files of stomata in the epidermis, with each vein being flanked by one to three rows of stomata on both the medial and lateral sides (Stebbins and Shah, 1960).…”

Indole Acetic Acid Distribution Coincides with Vascular Differentiation Pattern during Arabidopsis Leaf Ontogeny

“…The directionality of auxin flow is attributed to polar distribution of the efflux carrier molecules in the plant cell membrane (Galweiler et al, 1998). Two models, canalization of auxin flow and reaction-diffusion prepattern, have been proposed to explain the pattern of vascular differentiation (Nelson and Dengler, 1997). The canalization of signal flow hypothesis is based on a positive feedback mechanism: a proposed gradual restriction of IAA flow from a field to specialized files of cells, resulting in provascular, and later vascular, differentiation (Sachs, 1981).…”

“…The 3°veins form bridges between 2°v eins, whereas quaternary veins extend from 3°veins and end blindly in areoles (Mattsson et al, 1999). The hierarchical differentiation of 1°, 2°, 3°, and higher order veins provides an excellent system to study the mechanism of vascular differentiation and pattern formation (Nelson and Dengler, 1997).Vascular differentiation is related to auxin flux (Aloni, 1995). Auxin transport appears to be mediated by specific cellular influx and efflux proteins (Lomax et al, 1995; Estelle, 1998).…”

“…The 3°veins form bridges between 2°v eins, whereas quaternary veins extend from 3°veins and end blindly in areoles (Mattsson et al, 1999). The hierarchical differentiation of 1°, 2°, 3°, and higher order veins provides an excellent system to study the mechanism of vascular differentiation and pattern formation (Nelson and Dengler, 1997).…”

The Procambium Specification Gene Oshox1 Promotes Polar Auxin Transport Capacity and Reduces Its Sensitivity toward Inhibition

“…We have previously developed assays with which different stages of procambial cell fate commitment in rice (Oryza sativa) could be distinguished (Scarpella et al, 2000). However, despite the detailed studies available on different aspects of vascular development (for review, see Sachs, 1981Sachs, , 2000Fukuda, 1996Fukuda, , 1997Nelson and Dengler, 1997;Berleth et al, 2000;Aloni, 2001;Dengler, 2001;Dengler and Kang, 2001;Kuriyama and Fukuda, 2001;Ye, 2002), the molecular mechanisms underlying procambial cell fate commitment remain elusive (e.g. Savidge, 2001).…”