**4. Plant PCD in response to biotic and abiotic stress**

Plant PCD occurs mainly during the time of plant-pathogen interaction as well as in response to different abiotic stresses [73]. There are several indentified major causal biotic and abiotic factors that induce plant PCD are presented in **Figure 5**.

Plant-pathogen interaction mediated hypersensitivity response (HR) is a mode of broad-spectrum resistance in plants. Whether the pathogen is compatible or incompatible is determined by the interaction between resistance genes in the host and the avirulence gene in the pathogen as explained in Flor's hypothesis [74]. Hypersensitive cell death is the localized rapid death of cells at the point of infection, and it serves not only to restrict the growth but also stop the invasion of pathogens to other cells/tissue of the plant by undergoing suicide of the infected cells. The complex signalling of this hypersensitive reactions involves cascade of events such as protein phosphorylation, reactive oxygen species (ROS) production, and modification of ion fluxes [75].

HR is known as a defense mechanism against biotrophic plant pathogens or microbes, which depend on nutrients for survival on the host. However, it is observed that HR might be beneficial in the early stages for local adaptation against the infection but not in the later stages [76]. HR is observed in most plant species and it can be influenced by a wide range of plant pathogens such as oomycetes, viruses, fungi, and even insects [77]. All HR-induced PCD have one feature in common which distinguishes it from vacuolar-type, that is, this type of PCD is not initiated by vacuolar swelling and release of lytic enzymes inside plant cell (i.e. non-destructive). However, this type of response may include the release of vacuolar content by fusion with the plasma membrane [78]. Even it has been proved that VPE is essential for a virus-induced HR that involves PCD [11]. Thus it leaves no doubt that vacuolar rupture might take place during HR. Besides that, necrotic cell death is mainly triggered by abiotic stress in plants. The cell death via AL-PCD or necrosis is dependent upon the severity of the stress that the higher heat stress

*Programmed Cell Death (PCD) in Plant: Molecular Mechanism, Regulation, and Cellular… DOI: http://dx.doi.org/10.5772/intechopen.97940*

**Figure 5.**

*Diagrammatic presentation of various biotic and abiotic factor-induced PCD in the plant.*

leads to necrosis, while moderate heat stress leads to AL-PCD [57]. Previous study also suggests some difference between AL-PCD and necrosis. These include (1) shrinking of protoplast is absent in necrosis while in AL-PCD protoplast shrinks from the cell wall; (2) unlike apoptosis, necrosis is an energy-independent process which proceeds through sudden permeabilization of the plasma membrane and plasma membrane leakage. Additionally, a array of researcher revealed that PCD can induced by a range of abiotic stress tolerance. For more details description see **Table 2**.
