3.5.2 Programmed cell death
In animals, One route by which cells die is the highly controlled process of apoptosis. This involves a regulatory cascade that eventually leads to an ordered dismantling of the major cell components and, finally, the engulfment by phagocytosis of the dead cell, usually by a macrophage. This last phase obviously cannot happen in plants, and there has been much discussion about whether plants exhibit a cell death mechanism equivalent to apoptosis.
What is clear is that there are several ways in which a plant cell may die; furthermore, as in animals, programmed cell death (PCD) often occurs as part of normal development. This form of PCD has many similarities to apoptosis. Proteins are degraded by cysteine proteases which, although they have only very limited sequence similarity to the caspases involved in apoptosis, clearly have the same function. Chromatin is degraded in an ordered manner, producing a characteristic ‘ladder’ of DNA fragments whose sizes correspond to multiples of the amount of DNA associated with nucleosomes. Organelle breakdown starts with deformation of membranes or envelopes, and the whole process moves in a regulated sequence to the final stage of autophagy (‘self-eating’), in which the cell contents are completely broken down.
Whether or not this can be called apoptosis (as some plant scientists do call it) is a matter for discussion. The importance for this chapter is that, whatever term we use to describe it, this form of PCD is an important feature of the control of gene expression during plant development.