Programmed cell death is necessary to prevent from effect of spurious infection in plants.

2.  The number of cells in a body of organism is tightly regulated. Its not only the
control over cell division but also on rate of cell death.
 Controlled cell death is needed for normal development and good health
throughout life along with normal development of cells and cell cycle maturation.

3.  APOPTOSIS: Genetically programmed
process of deliberate sucide by an
unwanted cell in a multicellular
organism.
 AUTOPHAGY : Plays a housekeeping
role in removing misfold proteins,
clearing damaged organelles as well as
eliminating intracellular pathogens.
 NECROSIS : Non-programmed cell
death, death of a cell caused by external
factors such as trauma or infection.
 NECROPTOSIS : Programmed form
of necrosis.
 ONCOSIS: Series of cellular reactions
following injury that ultimately leads to
cell death.
CELL DEATH
NECROSIS NECROPTOSIS ONCOSIS
IMMUNOGENIC
CELL DEATH
AUTOPHAGY APOPSTOSIS

4.  Often occurs when the structure they form is no longer needed.
 To eliminate abnormal cells, cells that non-functional or potentially dangerous
to the animal.
 Cell death can prevent viral replication because they need a cell alive to
reproduce.
 Failure of cells to die or cells dying when they shouldn’t can lead to many
diseases.

5.  Cell death occur in many ways.
1.Infection
2.Poisioning
3.Lack of oxygen
 An uncontrolled death messy. In this cells swells up and its contents leak away.
This may damae surrounding cells too.
 On the other hand, there is another tidier way to go, i.e., Programmed cell death
or apopstosis, in which cell often choose to kill themselves.

6.  During viral infections.
 During DNA damage.
 To make way for new cells.
 Ex: In the womb, our fingers and toes are connected to one another by a sort of
webbing. Apopstosis is what causes that webbing to disappear, leaving us with 10
separate digits.
 As our brains develop, the body creates millions more cells than it needs, the ones
that don’t form synaptic connections undergo apopstosis so that remaining cells
function well.
 Programmed cell death is also necessary to prevent from efeect of spurious
infection in plants.

7.  During viral infections.
 During DNA damage.
 To make way for new cells.
 Ex: In the womb, our fingers and toes are connected to one another by a sort of
webbing. Apopstosis is what causes that webbing to disappear, leaving us with 10
separate digits.
 As our brains develop, the body creates millions more cells than it needs, the ones
that don’t form synaptic connections undergo apopstosis so that remaining cells
function well.
 Programmed cell death is also necessary to prevent from efeect of spurious
infection in plants.

8.  Mediated by an intacellular program.
 It includes mainly-Apopstosis and Autophagy.
 Non-apoptopic programmed cell death - can function as either backup
mechanisms or the main type of PCD.
 It is not only confined to animals, in plants also it occurs during development and
in the senescence of flowers and leaves.
 PCD also occurs in unicellular organisms including yeasts and bacteria.

9.  It is characterized by compact cytoplasm, vacuoles in the cytoplasm membrane, nuclear
chromatin condensation, DNA fragmentation and the formation of apoptotic bodies.
 At the periphery of the nuclear membrane, chromatin condensation starts, forming a ring
like structure.
 Later on, the nucleus progressively condenses and breaks up (karyorrhexis).
 The cell detaches from the surrounding tissue and its outlines become convoluted and form
extensions.
 The apoptotic bodies are rapidly phagocytosed into neighbouring cells, including
macrophages and parenchymal cells. Apoptotic bodies can be recognised inside these cells,
but eventually they become degraded.

10.  Variety of stimuli.
 Withdrawl of essential growth factors.
 Gamma irradiation.
 Treatment with glucocorticoids.
 Activation of certain receptors.
 In immune system, it is initiated where Tc lymphocytes attack target cell.

11.  Apoptosis is a tightly regulated and efficient cell death program involving
multiple factors.
 Caspases take major and a central role in apoptotic mechanism. The term caspases
is derived from cysteine-dependent aspartate-specific proteases. Caspases are
central to the mechanism of apoptosis as they are both the initiators and
executioners.
 Contain key cysteine residue at catalytic site.
 Cleave protein at site just C-terminal to aspartate residue.
INITIATORS EFFECTOR/EXECUTIONERS
CASPASES 2, 8, 9 and 10 CASPASES 3, 6 and 7

12.  INTRINSIC PATHWAY.
 EXTRINSIC PATHWAY.
 PERFORIN AND GRANZYME PATHWAY.
Active
CaspaseInactive
procaspase

13.  Cell activate their apoptopic programs from inside the cell in response to injury or
other stresses like DNA damage or lack of oxygen, nutrients or free radicals,
growth factors deprivation.
 Mitochondria dependent pathway.
 Regulated by BCl-2 family of proteins (B cell lymphoma-2 gene)
 Approximately 15 members of Bcl-2 family present.
 Human Bcl-2 protein can suppress apoptosis when expressed in C.elegans.

14. BCl-2 family
proteins
Pro-apoptopic
Promote apoptosis
BH123 proteins
(Bax, Bak)
BH 3-only proteins
(Bid, Bim, Bik, Bad,
PUMA, NOXA)
Anti-apoptopic
Inhibit apoptosis
(Bcl-2, Bcl-XL, Mcl-1, Bcl-1)

15.  BH123 proteins become activate and induces release of Cyt c and other
intermembrane proteins from mitochondria into the cytosol.
 Bax, Bak- main BH123 proteins atleast 1 of them is required for the intrinsic
pathway of apoptosis to operate.
 Mutant mouse cells that lack both the proteins are resistant to all pro-apoptopic
signal.
 BH3 only proteins-promote apoptosis mainly by inhibiting anti-apoptopic Bcl2
proteins.

16.  Apoptopic signals (cell stress, free radicle
damage, etc.,)
 Pro-apoptopic BH-3 proteins in the cytosol
gets activated.
 BH123 proteins relocate to the surface of
mitochondria.
 Interaction of pro and anti apoptopic
proteins- disruption of anti apoptopic, Bcl2
proteins.
 Leads to formation of PT (Permeability
transition pore)
 Activated BH123 proteins promote release
of AIF (Apoptosis Inducing Factor),
DIALBO & Cyt c into the cytosol.
 Activation of Apaf 1 (Apoptotic protease
activating factor-1).

17. • Apaf 1 oligomerize into a
wheel-like heptamer complex
called Apoptosome.
• Recruitment & activation of
Procaspase 9 through CARD
domain.
• Activation of Caspase 3
• Execution pathway.

18.  Extracellular signal proteins binds to cell surface death receptors that trigger
the extrinsic pathway.
 Death receptors are transmembrane proteins having extracellular ligand
binding domain, transmembrane domain and an intracellular death domain
which activated the apoptopic program.
 Ligands and death receptors are homotrimers.
 Receptor belongs to TNF (Tumor Necrosis Factor) receptor family. TNF
receptor (binds to TNFα ligand) and Fas death receptor (binds to Fas ligand).

19.  Fas ligand on cells binds to Fas receptor on
target cell.
 Death domains then recruits intracellular
adaptor proteins-
 FADD (Fas ligand associated death domain)
 TRADD ( TNFα receptor associated death
domain)
 It then recruits pro caspase 8 or 10, forming
DISC (Death Inducing Signalling Complex)
 Activation or cleavage of procaspase 8 or 10.
 Formation of Caspase 8 or 10
 Caspase 3 activation
 Execution pathway.

20.  Another pathway of apoptosis.
 It involves interaction between Cytotoxic T cells/ CD8+ and infected cell (viral /
cancerous)
 It is a type of Intrinsic pathway as.
It involves cellular components.
Doesn’t involve ligand receptor concept.
Independent of Receptor mediated pathway.
 Perforin is a pore forming protein and also known as cytoplasmic granule toxins.
 Granzyme is a family of structurally related serine protease stored within the cytotoxic
granules of cytotoxic lymphocytes.

23.  Anti-cancer drugs and radiation work by triggering apoptosis in diseased cells.
 If apoptosis is stopped, uncontrolled cell division happen leading to cancer.
 Many diseases and disorders are linked with the life and death of the cells,
increase apoptosis is a characteristic of AIDS, Alzhemer’s and Parkinson’s
disease. While deceases apoptosis can signal cancer.
 Autoimmune disease will occur.
 Improper cell cycle.

24.  It is commonly known that animal pathogens often target and suppress
programmed cell death (PCD) pathway components to manipulate their hosts.
 In contrast, plant pathogens often trigger PCD. In cases in which plant PCD
accompanies disease resistance, an event called the hypersensitive response.
 In plants without genetic disease resistance, these secreted molecules serve as
virulence factors that act through largely unknown mechanisms. A number of
fungal pathogens secrete PCD-promoting molecules that are critical virulence
factor.
 HR cell death is an active process in which the accumulation of 02
-and H202 leads
to an elevation in cytosolic Ca2+ and triggers a protein kinase-mediated cell death
process that is similar physiologically to PCD.

25.  Perhaps the most well-known cell death response in plants is the hypersensitive
response (HR) associated with a phenomenon termed the resistance response (RR).
 The RR involves the co-ordinate activation of many defences that limit pathogen
growth.
 HR is considered to be only the cell death component of the RR.
 An RR is triggered when the host has a dominant R gene that corresponds to a
dominant avr gene in the pathogen. Pathogen
AVR avr
Host
R-
Incompatible
(Resistant)
Compatible
(Susceptible)
rr
Compatible
(Susceptible)
Compatible
(Susceptible)

26.  The view that the HR is an active process of the host and may be a form of
programmed cell death (PCD) was supported by early observations that host cells
must be metabolically active and, in some cases, the HR requires active host
protein synthesis for its induction by fungi and bacteria.
 The morphology of cells undergoing the HR at late stages suggest that it is a form
of PCD with some apoptotic features.
 In particular, apoptotic like bodies with avirulent Pseudomonas syringae
infections were observed (Levine et al., 1996).
 It was found early changes in mitochondrial morphology (swelling and cristae
disorganization) in avirulent P. syringae-infected lettuce, similar to what occurs in
animal cells undergoing apoptosis (Wakabaashi and Karbowski, 2001).

27.  Later stages of the infection were accompanied by membrane dysfunction (loss of
ability to be plasmolysed) and progressive vacuolization of the cytoplasm.
 Membrane damage was proposed to be the critical event for cell death.
Apoptosis-related chromatin condensation and endo nucleolytic cleavage were
not reported.
 However, there was a gap in the time course in which these events may have
occurred. Thus, the HR in this system has a subset of apoptotic features.
 Further looking at a range of host–pathogen interactions using detailed time
courses infers that apoptosis-related events such as mitochondrial swelling,
chromatin condensation and endonucleolytic cleavage occur before general
organelle dysfunction. This would be expected if the HR occurs by an apoptotic-
like mechanism.

28.  Although there is clear evidence that PCD occurs during plant development and
environmental responses, the signals that trigger PCD in plants are unknown.
 However, O2
- and H202 accumulate in senescing leaves (Pastori and del Rio,
1997). Thus, it is possible that ROS are a general trigger for PCD in plants.
 Expression of a single gene is thought to be sufficient to trigger PCD in the
megaspores of the fern Marsilea (Bell, 1996a), and it is possible that the product
of such a gene could generate enough H2O2 to trigger PCD in several different
kinds of plant cells (Demura and Fukuda, 1994).

29. REPRODUCTIVE STAGE
 Gametophyte development.
 Megaspore cell death
 Cell death of the nucellar tissue
 Antipodal cell death
 Tapetum cell death
 Cell death in sex determination
 Self-incompatibility-induced cell
death.
VEGETATIVE STAGE
 Xylogenesis
 Lateral Root Cap Differentiation
 Aerenchyma Formation
 Leaf Morphogenesis
 Organ Abscission and Dehiscence
 Leaf Senescence
 Floral Organ Senescence

30.  Autophagic PCD is characterized by cytoplasm vacuolization followed by the destruction of cell
components by an intensive activation of lysosomal machineries. Autophagic features have been
also found in different kinds of plant PCD.
 In plants, the vacuole is responsible for lytic activity which leads to the removal of organelles and
cell corpses.
 In plants, different types of PCD play crucial roles in vegetative and reproductive development
(dPCD), as well as in the reaction to environmental stresses (ePCD).
 Forms of ePCD result in the sacrifice of cells in response to abiotic stresses, e.g., temperature or
irradiation or biotic aggressors like pathogens (Wu et al. 2014).
 Various dPCD events can be distinguished on the basis of their developmental context (Beers
1997).

31.  dPCD is generally autolytic, as happens in xylem tracheary formation, where
cytoplasm clearance is required for vessel functionality.
 In fact, some research studies shown that autolytic events and PCD are actuated
during tracheary element differentiation as separate processes with autolysis only
occurring when PCD has terminated.
 However, dPCD can also include autophagic features,as occurs during
embryogenesis and germination processes.
 ePCD is triggered by external insults which include pathogen attack as well as
abiotic stress, such as heat stress, salinity, drought, and flooding.
 In such cases, plants can activate a hypersensitive response (HR), which is a plant
cell death activated against biotrophic pathogens at the site of attack

32.  Plants activate PCD in senescent tissues, such as leaves, petals, sepals, in order to
recycle nutrients before eliminating the tissues that are no longer required.
 Differentiation-induced dPCD occurs as an inherent final differentiation step of
particular cell types, e.g., xylem, anther tapetum, or root cap cells. Some cell
types, however, can initiate dPCD in a facultative fashion.
 Finally, age-induced dPCD occurs in all cell types of organs or even entire
organisms as the end point of plant senescence.
 dPCD and ePCD have traditionally been studied independently, and recent
comparative analyses detected only limited similarity in their transcriptional
signatures.

33.  On the other hand, plants can fail to activate defense responses against abiotic
stress and as consequence activate PCD.
 This happens when the stress-dependent metabolic impairment overwhelms the
plant’s ability to restore the physiological background and/or counteract the
derived oxidative stress.
 Abiotic stresses, such as drought, flooding, heat stress, and heavy metals in the
soil, generally affect plant metabolism by restraining photosynthesis, reducing
protein synthesis and thus protein turnover, and increasing electron flow in the
respiratory chain into the mitochondria.

34.  The impairment of all these pathways increases the production of reactive oxygen
and nitrogen species (ROS and RNS) in the cell. When the plant fails to
counteract the metabolic imbalance leading to the overproduced reactive species,
it activates PCD.
 Several plant hormones are also involved in the induction of both dPCD and
ePCD: salicylic acid is required in HR cell death, ethylene in aerenchyma
formation and tissue senescence, gibberellin in endosperm/aleurone development.


36.  Autophagy is a self-degradative process that is important for balancing
sources of energy at critical times in development and in response to nutrient
stress.
 In addition to elimination of intracellular aggregates and damaged organelles,
autophagy promotes cellular senescence and cell surface antigen presentation,
protects against genome instability and prevents necrosis, giving it a key role in
preventing diseases.
 Greek meaning “Eating of cell”.
 Coined by Christian Duve.
 Self-degradative process..
 Plays a house keeping role in removing misfolded or aggregated proteins, clearing
damaged organelles and eliminating intracellular pathogens.
 Its deregulation has been linked to non-apoptopic cell death

37. APOPTOSIS
 Pre programmed cell death.
 Balances no. of cells in a multicellular
organism.
 Caused by oxidative stress, extrinsic or
intrinsic signals etc.,.
 Lysosomes are not involved.
 Do not allow cell to survive.
 Excessive apoptosis leads to atrophy.
AUTOPHAGY
 Self-degenerative process of its own
components.
 Balances the energy sources in the cell.
 Caused by cellular stress like starvation,
hypoxia etc.,
 Lysosomes are involved.
 Allows the cell to survive stress.
 Excessive autophagy leads to cell death.

38.  Will ‘Autophagic cell death’ applies to Programmed Cell Death (PCD) in plants..?
 Possible role of autophagy as an executioner of programmed cell death (PCD) in
plants.
 Andrew Love et al.(2004) Suggested that autophagy is responsible for the bulk of
cellular dismantling and remobilization of various compounds, during senescence,
which is a type of developmental PCD
 Developmental PCD in plants: The tonoplast becomes highly permeable (or
ruptures); this results in the release of a massive amount of hydrolases, which
rapidly degrade whatever is by then left of the cytoplasm, and in some cases also of
the cell wall.
 The action of hydrolytic enzymes, resulting in self-degradation, we proposed, can be
regarded as a type of autophagy. The cell is after all ‘eating’ itself,

40.  Micro-autophagy is the uptake of materials at the surface of a lytic vacuole. It
occurs by the invagination or protrusion of the vacuolar membrane. The vesicles
produced have only a single membrane.
 Cacas and Diamond claimed that autophagic structures should possess a double
membrane. This is not correct, as micro-autophagy is also autophagy and does not
involve a vesicle with a double membrane.
 They claim that autophagy has not been shown to be an executioner of plant
PCD, therefore, cannot be based on the argument that no double-membrane
structures have been observed during plant PCD.
 Macro-autophagy starts further away from a lytic vacuole. In animal cells a
portion of the cytoplasm, which can include various organelles, is surrounded by a
growing double-membrane structure. Upon closure of the double membrane, the
structure with its enclosed cytoplasmic material is an autophagosome.

41.  In animals the outer membrane of the autophagosome subsequently fuses with a
lysosome, a small vesicle containing hydrolases. This fusion results in degradation of
the material inside the outer membrane of the autophagosome.
 Upon fusion with a lysosome, the autophagosome is called an autolysosome. Yeasts
also have autophagosomes, which fuse with a large lytic vacuole.
 Macro-autophagy carried out by autophagosomes and autolysosomes is ubiquitous in
animal cells. ‘Autophagic cell death’ in animals has been defined by the increase in
the number of autophagosomes, autolysosomes and small lytic vacuoles produced by
autolysosomes (together termed autophagic vacuolization) in the cytoplasm,
independent of the process being a cause of death or only associated with death.
 If macro-autophagy is defined as in animals (by the presence of autophagosomes and
autolysosomes) it has not yet been demonstrated in plant cells.