Membrane degradation and phospholipase D activity during developmental cell death in Iris flowers

Iris x hollandica (cv. Blue Magic) flowers show the first symptoms of senescence within 4 days after the onset of flower opening. Wilting of the tepals starts at the distal edges, rapidly proceeds to the base, and is followed by total discolouration of the tepal. Senescence of Iris flowers is not induced or promoted by ethylene, the hormone often involved in senescence processes in plants. Whether other signals trigger cell death in Iris flowers is still unknown. The senescence process of Iris tepals is characterized by specific changes on the cellular level prior to the visible symptoms. Mesophyll cells show the presence of organelle désintégration, except for mitochondria, and their subsequent autophagic lysis in vacuoles. The vacuoles, in turn, deteriorate when they have emptied most of the cytoplasm and digested its constituents. The total protein and phospholipid content of the tepals decreases dramatically. At a later stage the plasma membrane loses its integrity, resulting in leakage of cellular contents and loss of turgor, which is accompanied by tepal wilting. The decrease in protein levels seems mainly the result of increased protein degradation by protease activity. Membrane degradation and decreased phospholipid contents could be caused by a concerted action of enzymes with lipase activity. Phospholipase D (PLD) may initiate phospholipid degradation in plants, as it has high abundance and because of the lipid degradation products found. Since PLD activity can be regulated on a transcriptional and post-translational level, an in vivo assay for monitoring PLD activity was developed. An early increase in PLD activity was observed, which correlated well with a decrease in phospholipid content. The results, therefore, strongly indicate that PLD is involved in membrane degradation associated with tepal senescence.