Don Kaplan - his legacy: Influencing teaching and research
Dengler, Nancy G. .
Alternate modes of leaf morphogenesis.
One of Don Kaplan's most significant contributions to plant morphology has been to provide vivid and convincing examples that illustrate the fundamental principles of plant construction. For instance, a differential elaboration of the same leaf components through localized directional growth can yield different mature leaf morphologies. In contrast, apparently similar mature forms can develop through distinctly different mechanistic pathways. In this paper we compare the morphogenetic and histogenetic aspects of leaf development in two species of Aponogeton, A. ulvaceus and A. madagascariensis. In both species, simple, strap-shaped leaves are initiated in a sub-opposite pattern and leaf primordia are convolute. Tissue patterning, including formation of parallel longitudinal veins and regularly-spaced intercostal veins, is complete before leaves unfurl. In A. madagascariensis, the initially simple leaf shape is remodeled through the formation of perforations that are positioned equidistantly between the longitudinal and transverse veins. Discrete subpopulations of cells undergo programmed cell death in a synchronous manner. Tonoplast rupture appears to be the first cytological marker of cell death, as indicated by alterations in cytoplasmic streaming, loss of anthocyanin color, and ultrastructural appearance. Nuclei become TUNEL-positive and genomic DNA is fragmented soon afterward, while shrinkage and degradation of organelles occur late in the cell death process. Enzymatic degradation of cell walls results in a region of mechanical weakness analogous to an abscission zone, so that the zone of dying cells is ripped apart during leaf expansion. Mesophyll cells left exposed on the perforation rim transdifferentiate as epidermal cells. Although the mode of leaf development in A. madagascariensis appears unique in many respects, it displays several common processes of plant development, including the spatial patterning of tissues with respect to the vascular system and the use of an abscission-like process to spearate previously continuous tissues.
1 - University of Toronto, Department of Botany, 25 Willcocks St., Toronto, Ontario, M5S 3B2, Canada
programmed cell death.
Presentation Type: Symposium
Location: Ballroom 2 (Cliff Lodge)
Date: Tuesday, August 3rd, 2004
Time: 8:40 AM