Development of a C4 leaf can be viewed as an add-on to the default program followed by C3 leaves (Dengler and Nelson 1999, Dengler and Taylor 2000). The C4 program is active at three levels: first, to modify tissue pattern within leaves (spacing of veins); second, to modify cell patterns within tissues (organization of BSC and MC); and, third, to direct differentiation of specialized cells, BSC and MC. In the early stages of leaf development, dermal- and ground-tissue precursors are first established. Provascular strands then appear; those for the major veins occur first and then minor veins develop between the major veins as the leaf elongates and becomes broader (Nelson and Dengler 1997). The C4 program adds an additional set of veins to give closer spacing of adjacent veins. This is the first obvious modification to basic C3 developmental programs.
MC develop from the ground meristem located between and around provascular strands and there are no obvious differences between C3 and C4 programs at early stages. BSC develop from provascular strands and are distinguishable in C4 leaves at early stages by their size and chloroplast numbers. More complex patterns of BSC development are found in NAD malic enzyme (NAD-ME)- and phospho<?/w/pyruvate carboxykinase (PCK)-type C4 plants (Dengler and Nelson 1999). Differentiation of BSC follows vein development, occurring first around the major veins. Although BSC and MC tend to develop from different cell lineages, cell position plays a major role in the final determination of cell fate. This point was demonstrated by clonal analysis in maize leaves in which cell lineages were marked by mutant sectors (Langdale et al 1989).
The final step in the C4 program involves the structural and functional differentiation of BSC and MC. This differentiation is the result of cell-specific expression of nuclear and organellar genes. In some plants, such as maize, full differentiation is light-dependent. Light is required to promote high gene expression and to suppress expression in the inappropriate cell type (Langdale and Nelson 1991).
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