Petra M. Donnelly

Programmed cell death and leaf morphogenesis in Monstera obliqua (Araceae).

The unusual perforations in the leaf blades of Monstera obliqua (Araceae) arise through programmed cell death early in leaf development. At each perforation site, a discrete subpopulation of cells undergoes programmed cell death simultaneously, while neighboring protoderm and ground meristem cells are unaffected. Nuclei of cells within the perforation site become terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL)-positive, indicating that DNA cleavage is an early event. Gel electrophoresis indicates that DNA cleavage is random and does not result in bands that represent multiples of internucleosomal units. Ultrastructural analysis of cells at the same stage reveals misshapen, densely stained nuclei with condensed chromatin, disrupted vacuoles, and condensed cytoplasm. Cell walls within the perforation site remain intact, although a small disk of dying tissue becomes detached from neighboring healthy tissues as the leaf expands and stretches the minute perforation. Exposed ground meristem cells at the rim of the perforation differentiate as epidermal cells. The cell biology of perforation formation in Monstera resembles that in the aquatic plant Aponogeton madagascariensis (Aponogetonaceae; Gunawardena et al. 2004), but the absence of cell wall degradation and the simultaneous execution of programmed cell death throughout the perforation site reflect the convergent evolution of this distinct mode of leaf morphogenesis in these distantly related plants.

Comparison of shoot vascular organization in isophyllous (Pilea cadieri) and anisophyllous (Pellionia daveauana) species of the Urticaceae

Primary vascular organizations in isophyllous shoots of Pilea cadieri and in anisophyllous shoots of Pellionia daveauana were compared using computer-assisted reconstruction of serial cross sections of mature stems and shoot tips. Stems of P. cadieri are characterized by a closed vascular system with six sympodia and trilacunar three-trace nodes. In contrast, shoots of P. daveauana have an intermediate system with four sympodia and trilacunar four-trace nodes on the ventral side of the stem and unilacunar one-trace nodes on the dorsal side of the stem. This highly modified vascular architecture reflects the extreme anisophylly and dorsiventral symmetry of shoots. In P. daveauana, the dorsal leaf trace differs from the ventral leaf traces in phloem cross-sectional area and in tracheary element number and diameter; in P. cadieri, all traces are similar in size. Leaf-trace procambium develops simultaneously into both leaves of a pair in P. cadieri, but in P. daveauana the median trace of the ventral leaf differentiates one plastochron before the dorsal leaf median trace. These features indicate a strong developmental interaction between expansion of the leaf primordium and differentiation of procambial tissue.