Mechanisms of programmed cell death

Programmed cell death

Programmed cell-death (PCD) is death of a cell in any form, mediated by an intracellular program. In contrast to necrosis, which is a form of cell-death that results from acute tissue injury and provokes an inflammatory response, PCD is carried out in a regulated process which generally confers advantage during an organism's life-cycle. PCD serves fundamental functions during both plant and metazoa (multicellular animals) tissue development.

·     Apoptosis or Type I cell-death

·     Autophagic or Type II cell-death ( cytoplasmic: characterized by the formation of large vacuoles which eat away organelles in a specific sequence prior to the nucleus being destroyed.)

Besides these two types of PCD, other pathways have been discovered. Called "non-apoptotic programmed cell-death" (or "caspase-independent programmed cell-death" or "necrosis-like programmed cell-death") these alternative routes to death are as efficient as apoptosis and can function as either backup mechanisms or the main type of PCD. Plant cells undergo particular processes of PCD which are similar to autophagic cell death. However, some common features of PCD are highly conserved in both plants and metazoa.

History

The concept of "programmed cell-death" was used by Lockshin & Williams in 1964 in relation to insect tissue development, around eight years before "apoptosis" was coined. Since then, PCD has become the more general of these two terms.  PCD has been the subject of increasing attention and research efforts. This trend has been highlighted with the award of the 2002 Nobel Prize in Physiology or Medicine to Sydney Brenner (United Kingdom), H. Robert Horvitz (US) and John E. Sulston (UK).

The cell cycle, or cell-division cycle, is the series of events that take place in a eukaryotic cell leading to its replication. In "APL regulates vascular tissue identity in Arabidopsis", Bonke and colleagues state that one of the two long-distance transport systems in vascular plants, xylem, consists of several cell-types "the differentiation of which involves deposition of elaborate cell-wall thickenings and programmed cell-death." The authors emphasize that the products of plant PCD play an important structural role.

Basic morphological and biochemical features of PCD have been conserved in both plant and animal kingdoms. It should be noted, however, that specific types of plant cells carry out unique cell-death programs. These have common features with animal apoptosis for instance, nuclear DNA degradation but they also have their own peculiarities, such as nuclear degradation being triggered by the collapse of the vacuole in tracheids elements of the xylem.

PCD in pollen prevents inbreeding

During pollination, plants enforce self-incompatibility (SI) as an important means to prevent self-fertilization. Research on the corn poppy (Papaver rhoeas) has revealed that proteins in the pistil on which the pollen lands, interact with pollen and trigger PCD in incompatible (i.e. self) pollen. The researchers, Steven G. Thomas and Veronica E. Franklin-Tong, also found that the response involves rapid inhibition of pollen-tube growth, followed by PCD.