Programmed cell death remodels lace plant leaf shape during development

Programmed cell death (PCD) functions in the developmental remodeling of leaf shape in higher plants, a process analogous to digit formation in the vertebrate limb. In this study, we provide a cytological characterization of the time course of events as PCD remodels young expanding leaves of the lace plant. Tonoplast rupture is the first PCD event in this system, indicated by alterations in cytoplasmic streaming, loss of anthocyanin color, and ultrastructural appearance. Nuclei become terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling positive soon afterward but do not become morphologically altered until late stages of PCD. Genomic DNA is fragmented, but not into internucleosomal units. Other cytoplasmic changes, such as shrinkage and degradation of organelles, occur later. This form of PCD resembles tracheary element differentiation in cytological execution but requires unique developmental regulation so that discrete panels of tissue located equidistantly between veins undergo PCD while surrounding cells do not.