Transverse vein differentiation associated with gas space formation - Fate of the middle cell layer in leaf sheath development of rice

In monocotyledons, the leaf vascular network consists of a hierarchical sequence of vertical vascular bundles and numerous transverse veins that interconnect adjacent vertical veins. In the leaf sheath of these species, especially grasses, lysigenous gas cavities (gas spaces) are developed into intervascular spaces and provide a gas conducting system to non-aerial Darts under flooded conditions. The spatial relationship between gas space formation and transverse vein differentiation was investigated using the leaf sheath of rice (Oryza sativa L.). Histochemical observation showed that patterns of differentiation of the transverse vein are distinct from those of vertical vascular bundles. On the other hand, gas spaces are formed through the processes of cell death (collapse). Both events are initiated at a specific cell position in the middle layers of the leaf sheath, from which the vascular system of the leaf is derived; this indicates that differentiation of transverse veins is associated with gas space formation. The cell-to-cell movement of fluorescein isothiocyanate-conjugated dextran injected into middle layer cells coincided with the area where cell collapse occurred, indicating a close relationship between the middle and adaxial cell layers, but not abaxial cell layers. A uniform cell number between each transverse vein in the leaf sheath suggested the involvement of spatial regulation in transverse vein formation regardless of clonal history at the later stage of leaf vein canalization. (C) 2000 Annals of Botany Company.