Programmed Cell Death as a Response to High Light, UV and Drought Stress in Plants - Abiotic Stress: Plant Responses and Applications in Agriculture - page 209

cells undergoing PCD to healthy, not-affected cells and triggers stress tolerance and acclima‐
tion to adverse conditions [6,15].
2. Hallmarks and the regulation of programmed cell death
While the cascade of events and molecules regulating PCD have been already well described in
animal cells, mechanisms underlying plant PCD remain still inexplicable. Therefore, numer‐
ous studies in plants rely on the comparison of PCD mechanisms to animals. Apoptosis (well-
studied form of animal PCD) features in cell shrinkage, chromatin condensation, cleavage of
DNA (called DNA laddering) and nuclear fragmentation. The mechanism of PCD depends on
a family of cysteine proteases called caspases that cleave their target proteins after aspartic acid
residues. Caspases are synthesized in the cell as inactive precursors (procaspases). Once acti‐
vated, caspases cleave and activate other procaspases which results in a self-amplifying cas‐
cade. They can also cleave other proteins such as nuclear lamins or proteins that hold DNA-
degrading enzymes in inactive form, releasing DNases to cut DNA. The destructive protease
cascade is irreversible, therefore caspase activity needs to be tightly controlled. Procaspase ac‐
tivation is induced by the release of electron carrier protein - cytochrome c from mitochondria
to the cytosol. The family of Bcl-2 proteins regulates the activation of programmed cell death.
Some members of this family (e.g. Bcl-2) block the release of cytochrome c, inhibiting apopto‐
sis. Others (e.g. Bax and Bak) act as PCD inducers, promoting cytochrome c leakage. IAP (in‐
hibitor of apoptosis) proteins are another family involved in apoptosis regulation as they bind
to some procaspases, preventing their activation or to caspases, inhibiting their activity. Pro‐
teins that block IAPs are released together with cytochrome c which increases the efficiency of
cell death process [16].
Many hallmarks of plant PCD seem to be similar to animals such as cytoplasm shrinkage,
chromatin condensation and DNA cleavage, mitochondrial swelling, disruption of organ‐
elles and plasma membrane collapse [17]. The major difference in executing PCD between
animals and plants lies in the process of removing the cell content after its death. While in
animal cells, removal action is undertaken by other cells to avoid the activation of inflamma‐
tory response, in plants there is a leakage of the cell content into the apoplast and remains
are not engulfed by surrounding cells [10]. Moreover, plants exhibit some distinctive fea‐
tures of PCD that result from the presence of chloroplasts and the significance of vacuoles
[18,19]. Plant vacuoles represent important storage organelles that are the repository of hy‐
drolytic enzymes such as proteases, lipases and nucleases. Vacuoles are therefore postulated
to play a role in the turnover of organelles and cytoplasm during autophagy as a part of
clean-up system for dying cells. The component of this system is a caspase-like protease -
the vacuolar processing enzyme (VPE) which plays a crucial role in such PCD pathways as
senescence, lateral root formation and hypersensitive response. Upon receiving pro-apoptot‐
ic signals, VPE activates hydrolases that execute the degradation of vacuolar membrane re‐
sulting in the release of hydrolytic enzymes and subsequent degradation of cell content [19].