Cell Lineage of Vein Formation in Variegated Leaves of the C4Grass Stenotaphrum secundatum

Abstract

Clonal analysis of variegated leaves of the C4grass, Stenotaphrum secundatum, indicates that invasions among meristematic layers occur during the organogenetic stage of leaf development, resulting in long, broad white and green stripes. These layer invasions cease prior to the second phase of leaf development when delimitation of leaf regions occurs. Vein precursors mostly arise during the second phase, so that procambial strand formation is superimposed on the lineage makeup of earlier-formed tissue. Anatomical evidence indicates that procambium arises through formative divisions within ground tissue of leaf primordia and that each strand is derived from a variable number (one–four) of ground meristem precursors. If a developing vein straddles the boundary between previously-formed green and white sectors, then the mature vein is half green and half white, reflecting its mixed cell lineage. In Stenotaphrum, 24.8% of the sectors observed were bounded by such ‘half veins’. The temporal relationship of layer invasion and tissue system delimitation in this species supports the view that positional signals are more important than lineage history in the determination of tissue type. However, analysis of planes of cell division in developing veins indicates, that, once formed, procambial strands are discrete lineage units that extend longitudinally by proliferative divisions. Thus, lineage restrictions may play an important role in the third stage of leaf development, differentiation of tissues and cells, which also includes the maintenance of cell identity.