**2. Central role of developmental PCD (dPCD) in plant**

Inductions of morphogenetic changes are an integral part of the development of an organism. PCD is one of the major fundamental cellular processes that plays crucial role in morphogenetic changes in plant systems [13]. In plants, the occurrence of cell death during development is termed developmental PCD (dPCD). PCD can occur throughout the life cycle at any cell type, tissue, and organ part of the plant (**Table 1**).


#### **Table 1.**

*List of important plant parts/sites of PCD in a vascular plant suggested developmental PCD (dPCD) involved cellular differentiation of specific cell types.*

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

Among the two other types of cell death, apoptosis is the most understood type of PCD [5]. In plants, the process of true apoptosis is absent but has a similar kind of programmed cell death termed apoptotic like programmed cell death (AL-PCD). AL-PCD is morphologically and biochemically similar to apoptosis, but due to the structural and functional differences of plant cells, there are some changes in the execution process. The general similarities between apoptosis and AL-PCD include (1) cell shrinkage (2) chromatin condensation, (3) mitochondrial permeabilization and depolarization, (4) cytochrome c release, (5) vacuole leakage and fusion with plasmalemma. In the case of mitochondrial permeabilization, the release of cytochrome *c* leads to the formation of a caspase-like protein termed metacaspase. This in turn results in the release of nuclease and several protease enzymes which finally leads to DNA fragmentation and protein degradation [28]. The fundamental phenomenon of PCD in plants occurred across the whole life cycle depicted in **Figure 1**.

Developmental PCD (dPCD) is triggered by vascular cell death [30]. Vascular cell death is mainly associated with developmental stages includes morphogenesis and senescence. The existence of vascular system in the plant cell is the key difference from an animal cell. Vacuolar cell death in plants resembles autophagy in animals. A large portion of the plant cell is occupied by vacuoles and the presence of these lytic vacuoles plays a major role in plant cell death. This type of cell death is known as vacuolar cell death [30]. The mechanism involved in this vacuolar cell death involves

#### **Figure 1.**

*Central role of PCD across the plant life cycle. Pre-zygotic and post-zygotic developmental changes such as tapetum layer degeneration, daughter cell degeneration, antipodal-synergids cell degeneration, pollen tube degeneration, aleurone layer, central endosperm, suspensor cell death are controlled by PCD. The process of xylogenesis is also influenced by PCD [29]. During the differentiation of tracheary elements, vacuole swelling and rupture are coordinated with the thickening and restructuring of the cell wall. The final collapse of the vacuole immediately precedes nuclear DNA fragmentation, which occurs at the late stages of the cell-death process before the final autolysis of the cell. In mature sporophyte senescence, abscission and dehiscence are regulated by PCD. PCD is also observed during* in vitro *somatic embryogenesis [25]. Symplastic cell-to-cell trafficking connection between the somatic embryo and mother explant is broken due to PCD. Degeneration of suspensor cell of somatic embryo mediated by PCD.*

the increase in the size of the vacuoles by fusing small vacuoles and results in the formation of larger vacuoles. Autolytic PCD occurs due to tonoplast rupture and the clearance of cytoplasm. It is not clear the exact reason for the rupture of tonoplast but the clearance of cytoplasm is due to the flow of hydrolases enzyme that degrades the cytoplasm [31, 32] which is released to aid differentiation of tracheary elements [30, 33]. The dPCD involved to develop various organs like integuments [34], megasoprogenesis [35], pollen tube development [36], leaf senescence [37].

Plant cells contain two types of vacuoles in different types of tissues, lytic vacuole (LV) and protein storage vacuole (PSV) [38]. PSV may contain many kinds of proteins especially defense and stress-related, and the pH value found close to neutral [39–41]. Unlike the PSV, LV helps in ion and water homeostasis of the cell [42]. It is reported that PSV of cereal aleurone transformed from storage compartments to lytic organelles and fusion of several PSV, acidification takes place in the vacuolar lumen [43].

Studies also suggest that a particular cysteine protease called the 'vacuolar processing enzyme' (VPE) functions as a key regulator or the executioner of plant vacuolar PCD during development and also during stress [44]. Located in the vacuole, VPE ruptures the lytic vacuolar membrane. VPE is a cysteine protease and involved in cleaves the peptide bond at the C-terminal side of asparagine and aspartic acid [45]. The up-regulation of VPE genes was associated with various types of cell death under stressed conditions. This is essential in processing seed storage proteins and for mediating the susceptible response of toxin-induced cell death [46]. The collapse of the vacuolar membrane allows the release of hydrolytic content and causes the destruction of other organelles [47]. For example degradation of nuclear and chloroplast DNA, without condensation of DNA, can be completed immediately within 20 minutes of vacuole rupture, but chlorophyll degradation takes more time. Even the plasma membrane can be completely degraded as observed in endosperm tissue. However lignified tissues are exempted from this degradation. Furthermore, PCD in tracheary cells and fiber cells is delayed so that more lignification can take place to allow those cells to become more rigid for structural support [47].

Arabidopsis genome codes for at least 4 VPE homologous αVPE, βVPE, γVPE, δVPE. All of which are located in the vacuole [48]. The last one (δVPE) is only found in dicots [49]. Among these βVPE is specifically located to seeds, whereas αVPE and γVPE are specific to vegetative organs [50] and the δVPE is expressed in seed coat development at the early stage of seed formation [51]. However, the number of VPE genes in various plant species differs, for example four genes have been described in Arabidopsis [52], eight in the case of barley [53], 5 in rice [54], and 14 in tomato [55]. In the recent advancement of the genomic and transcriptomic data, the activities of VPE genes along with their expression pattern will become clearer. In an experiment, it has been proved that when all 4 VPE genes are mutated in Arabidopsis (VPE null mutant) and detectable activities of caspase-1 or VPE in the fungal toxin fumonisinB1 (FB1)-treated leaves whereas wild-type leaves had the caspase-1 and VPE activities [46]. It demonstrated that in planta VPE is solely responsible for caspase-1 activity [52].
