Growth and Development of Horticultural Crops (1+1)

Programmed Cell Death (PCD)

• As mentioned earlier, senescence is controlled by plants own genetic programme and the death of the plant or plant part consequent to senescence is called as programmed call death (PCD). The distinct set of morphological and biochemical changes accompanying the PCD have also been called as apoptosis (from a Greek Word meaning ‘falling off’).
• PCD plays an important role in normal vegetative and reproductive development in plant and also in defense against pathogens:
1. Senescence is one form of PCD. The nutrients and other substrates from senescing cells and tissues are remobilized and reallocated to other parts of the plant that survive.
2. The protoplasts of developing tracheary elements (xylem vessels and tracheids) die and disappear at maturity to make elements functionally efficient as conduit for water transport.
3. In aquatic plants, aerenchyma is normally formed in different parts of the plant such as roots and stems which enclose large air spaces that are created via PCD.
4. In the development of unisexual flowers, primordial for both male and female flowers are present in the earlier stages, but only one of these two completes its development while the other aborts via PCD.
5. In ovules, the megaspore mother cell divides meiotically to form four megaspores of which only one remains functional to form the female gametophyte while the rest degenerate. This is predetermined and genetically controlled and is an example of programmed cell death.
6. On being infected by a pathogen, the host plant cells die rapidly around the infection site forming a necrotic lesion. This deprives the pathogen of the nutrient supply and prevents its spread in the host plant. This is also a form of PCD that is beneficial to plant.