2. Programmed cell death
• Physiological cell death process involved in selective elimination of unwanted
tissues (Ellis et al., 1991)
• Programmed cell death (PCD) has been defined as a sequence of (potentially
interruptible) events that lead to the controlled and organized destruction of
the cell. (Lockshin and Zakeri, 2004) .
• The term apoptosis –A Greek word originally means that ‘’fall of the leaf or leaf
falling’’ . (John et al., 1972)
• Synonyms = Chromatolysis, Karyolysis, Karyohexis, Shrinkage necrosis,
Programmed cell death, Cell suicide, Self destruction and Apoptosis
3. Types of PCD
Autolytic PCD
• Rapid cytoplasm clearance after
tonoplast rupture & replaced by
vacuolar volume
• Developmental PCD
• Similar to autophagy in animal
• For example, PCD that occurs during
the formation of the male and female
zygotes, in embryonic structures, and
during development of roots and
shoots
• Mild abiotic stress, such as lack of
oxygen and drought
Non – Autolytic PCD
• Death occurs prior to tonoplast
rupture or tonoplast rupture does not
occur or tonoplast rupture is not
followed by complete clearance of
the cytoplasm
• Hypersensitive response (HR)-related
PCD, Necrotrophic PCD.
• Endosperm in cereal seeds is an
example (no tonoplast rupture)
(Vandroon et al., 2011) 3
4. What is the purpose of this PCD ?
• Developmental PCD
Essential for successful development & growth
of complex multicellular organisms.
Regulates the rate of cell division.
Shaping of cells, tissues & organs.
• Defensive PCD
Control of cell populations & defense against
invading microbes.
5. Steps in apoptosis:
1. The decision to activate the pathway;
2. The actual "suicide" of the cell;
3. Engulfment of the cell remains by specialized immune
cells called phagocytes;
4. Degradation of engulfed cell.
The actual steps in cell death require:
1. condensing of the cell nucleus and breaking it into
pieces
2. condensing and fragmenting of cytoplasm into
membrane bound apoptotic bodies;
3. breaking chromosomes into fragments containing
multiple number of nucleosomes (a nucleosome
ladder)
7. Regulation of PCD in Plants
• Plant Proteases
– Metacaspases: A family of cysteine proteases in plants
that are most similar to animal caspases.
– Metacaspases divided into Type 1 Metacaspases (AtMC
1, AtMC 2) , Type 2 Metacaspases (AtMC 4, AtMC 9)
• Subtilisin-like serine proteases.
• VPE family of protease : vacuolar processing
enzyme (VPE) family of cysteine proteases.
8. Localization of Protease in Plant Cell
• Proteolytic enzymes that are involved in PCD
are localized in different compartments of
plant cells:
– The cytoplasm (metacaspases),
– The vacuoles (VPE),
– The intercellular fluid (phytaspases)
9. Role of Vacuoles in PCD
• During viral infection, the tonoplast is lysed with the
release of lytic enzymes of vacuoles into the cytosol.
• The vacuole tonoplast fuse with the plasmalemma to
release vacuolar contents extracellularly, or it disintegrate
to release its contents to the cytosol.
• Induction of rapid cell death pathways may be an effective
way of cleansing cytoplasm from viral growth.
• Sudden release of vacuolar contents into the cytoplasm
causes rapid cell death as was also noted for the HR
response to TMV in tobacco.
10. Developmental PCD in plants
Reproductive phase
• Cell death in female germline
– Non functional pollen grain
– Nucellus
– Antipodal and synergid cells
• Cell death in male germline
– Tapetum
– Pollen self incompatibility
– Transmitting tract
– Pollen tube burst
Vegetative phase
• Seed Dev. & Germination
– Embryonic suspensor
– Endosperm
– Seed coat
• Vegetative Development
– Xylem trachery vessel
– Leaf morphogenesis
– Lateral and adventitious root
differentiation
– Aerenchyma cells
• Senescence
• Leaf senescence
(Vanhautegem et al., 2015)
10
11. Cell death associated with non functional
megaspore
• The selection of the functional megaspore is
position-dependent and varies between species
• In Arabidopsis (Arabidopsis thaliana), the
arabinogalactan protein AGP18 is expressed
during meiosis in the abaxial pole of the ovule
• Callose deposition
• The deletion of the nonfunctional megaspores is
important for optimal seed development
(Vanhautegem et al., 2015)6/10/2020 11
12. Tapetal cell death
• PMC – 4 pollen grains
• Each pollen grain mature to produce vegetative
cells and 2 sperm cells with the support of
tapetum layer
• Tapetum differentiation & PCD onset via
GA-regulated myeloblastosis (GAMYB)
transcription factors in rice and their homologues
MYB33 and MYB65 in Arabidopsis
(Kurusu et al., 2017)
12
13. Prominent calcium
ion (Ca2+) signature
Drop in cytoplasmic
pH in pollen tube
Downstream of
putative PCD
signals
Alterations of the
cytoskeleton
Caspase like protease
activity, MAPK signaling -
nuclear DNA fragmentation
and PCD execution
Peaks of ROS and nitric
oxide (NO) levels
Incompatible pollen pistil
interaction
Rapid inhibition of pollen-tube growth,
followed by PCD - self-incompatibility
induction
Arabidopsis
Poppy
Incompatible reaction
6/10/2020 13
Steven G. Thomas and
Veronica E.
FranklinTong
14. • The suspensor fixes the embryo proper within the seed and contributes to the
setup of polarity of the early embryo
• Transfers nutrients and signals to the embryo
• Only required in the first stages of embryogenesis, and subsequently
degenerates (In tobacco Cathepsin H-like protease NtCP14 – Promotes PCD)
Unequal division
Embryonic suspensor PCD
(Vanhautegem et al., 2015)
6/10/2020 14
15. Lace plant
Aponogeton madagascariensis which
is known as the Madagascar Lace
Plant , decorative aquatic plant -
skeleton-like appearance
• PCD process starts in young leaves between
leaf veins
• Alterations in the actin cytoskeleton
• Chloroplast accumulation around the
nucleus
• Organelles are taken up into the vacuole in
membrane bound vesicles - autophagy
– Loss of mitochondrial membrane
potential,
– DNA fragmentation,
– Activation of caspase-like proteases
– Before vacuolar collapse, plasma
membrane rupture and cell wall
degradation
• Perforation PCD occurs between the leaf
veins, creating the fenestrate pattern of
mature lace plant leaves
16. Senescence
• Regulated by diverse endogenous
and environmental factors
• ROS signalling - promote
senescence and age-induced PCD
via the induction of senescence
associated genes (SAGs)
• Metacaspases (MCs) - upregulated
in senescent leaves and flowers
ORESARA1 (ORE1) - Senescence
Young leaves = ORE1 - suppressed by miR164
Ageing leaves = ORE1 degraded via EIN2
EIN2 activates EIN3, directly activates ORE1 or
represses miR164
“Plant senescence is a genetically encoded program that ultimately
leads to the death of plant organs or the entire plant”
(Vanhautegem et al., 2015)
6/10/2020 16
19. REFERENCES
• Kabbage M. et. al. (2017): The Life and Death of a
Plant Cell. Annual Review of Plant Biology. 2017.
68:375–404.
• Martin B. et. al. (2013): Centrality of Host Cell
Death in Plant-Microbe Interactions. Annual
Review of Phytopathology. 2013. 51:543–70
• John M. et. al. (2003) Plant disease resistance
genes: recent insights and Potential applications.
Trends in Biotechnology Vol. 21 No.4 April 2013.
178-183.
• Agrios G N; Plant Pathology. Elsevier Academic
Press, Burlington, MA, 2005.
