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

In monocotyledons, the leaf vascular network consists of a hierarchical seq uence of vertical vascular bundles and numerous transverse veins that inter connect adjacent vertical veins. In the leaf sheath of these species, espec ially grasses, lysigenous gas cavities (gas spaces) are developed into inte rvascular spaces and provide a gas conducting system to non-aerial Darts un der flooded conditions. The spatial relationship between gas space formatio n and transverse vein differentiation was investigated using the leaf sheat h of rice (Oryza sativa L.). Histochemical observation showed that patterns of differentiation of the transverse vein are distinct from those of verti cal 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 vasc ular system of the leaf is derived; this indicates that differentiation of transverse veins is associated with gas space formation. The cell-to-cell m ovement of fluorescein isothiocyanate-conjugated dextran injected into midd le layer cells coincided with the area where cell collapse occurred, indica ting a close relationship between the middle and adaxial cell layers, but n ot abaxial cell layers. A uniform cell number between each transverse vein in the leaf sheath suggested the involvement of spatial regulation in trans verse vein formation regardless of clonal history at the later stage of lea f vein canalization. (C) 2000 Annals of Botany Company.