Cell wall degradation and modification during programmed cell death in lace plant, Aponogeton madagascariensis (Aponogetonaceae)

An unusual form of leaf morphogenesis occurs in the aquatic, lace plant, Aponogeton madagascariensis (Aponogetonaceae). Early in development, discrete patches of cells undergo programmed cell death (PCD) and form perforations during leaf expansion. In addition to the protoplasts, walls of the dying cells are degraded during PCD. The cuticle of the perforation site is eroded first, followed by dissolution of cell wall matrix components, so that walls appear as loose fibrillar networks as perforations form. Gel diffusion assays of wall-degrading enzyme activity indicated that pectinases are active throughout leaf development, while cellulase activity was restricted to early stages of perforation formation. Alcian blue staining showed that degrading walls remain rich in pectin, and immunolocalization of pectin epitopes indicated that the proportions of esterified and de-esterifed pectins do not change significantly. Walls of perforation border cells are modified by suberin deposition late in development, and reactive oxygen species, thought to have a role in polymerization of phenolic suberin monomers, are present at the same stage. This timing suggests that suberization may limit the spread of PCD and provide an apoplastic barrier against microbial invasion but does not initiate PCD.