PROGRAMMED CELL DEATH IN PLANTS

PRESENTED BY:SHAILJA GUPTAL-2008-BS-149M

• What is PCD?• PCD – programmed cell

death• It is death of a cell in any

form, mediated by an intracellular program.

• It is an active process which occurs during development and in response to enviormental cues.

• (PCD) is more commonly known as apoptosis.

• The term apoptosis comes from plant kingdom from old Greek apoptosis that originally means the loss of petals or leaves.

• If cells are no more needed, they die by activating intracellular death program

history

• The concept of "programmed cell-death" was used by Lockshin & Williams in 1964 in relation to insect tissue development, around eight years before "apoptosis" was coined. Since then, PCD has become the more general of these two terms.

• PCD has been the subject of increasing attention and research efforts. This trend has been highlighted with the award of the 2002 Nobel Prize in Physiology or Medicine to Sydney Brenner (United Kingdom), H. Robert Horvitz (US) and John E. Sulston (UK).

• image of an embryo with apoptotic cells:-

• Three cells indicated by arrows underwent programmed cell death in a bean/comma stage embryo and exhibit a refractile, raised-button-like appearance.

cell death is accomplished by two processes:-

1. Necrosis,oncosis,traumatic cell death,accidental cell death,necrotic cell death, toxicological cell death – cell death due to acute exogenous factors such as stress, disease, toxin, trauma etc.

2. programmed cell death, physiological cell death, apotopsis, cell suicide, necrobiosis, shrinkage necrosis, autoschizis – cell death due to endogenous factors.

• Stress can cause type of cell death termed as necrosis whereas sub lethal stress can induce programmed cell death.

Two main types of PCD are there:-

• Apoptosis or type I programmed cell death is characterized by condensation of cytoplasm and preservation of organelles, essentially without autophagic degradation.

• Autophagic cell death or type II programmed cell death exhibits extensive autophagic degradation of Golgi apparatus, polyribosomes and endoplasmatic reticulum, which precedes nuclear destruction.

Differentiating between necrosis and PCD

PCD and Necrosis

• Cells that are damaged and unable to function correctly undergo PCD. This removes potentially harmful cells and prevents them from multiplying and spreading. However, not all damaged cells undergo PCD; cells that are damaged by stresses at overwhelming levels undergo necrosis instead .

• Necrosis is a nonphysiological process involving cell swelling, lysis, and the inflammatory leakage of cell contents. Moreover, whereas PCD occurs during development and is regulated by a complex mechanism, necrosis is not normally associated with development, does not require the activity of proteases and nucleases dedicated to controlled cell disassembly, does not operate through gene-dependent signal transduction pathways, and does not require Ca2+ or changes in protein phosphotylation.

apoptosis necrosis

Pattern of death Single cells Groups of neighbouring cells

Cell size shrinkage swelling

Plasma membrane FragmentationPreserved continuityBlebbed Increased membrane permeability

smoothing

mitochondria Contents released into cytoplasmCytochrome c; Apaf1Structure relatively preservedcontracted

Swelling disordered structure

apoptosis necrosis

Organelle shape contracted swelling

nuclei Clumps and fragmented Membrane disruption

DNA degradation Internucleosomal cleavageFree 3‛ endsLaddering on electrophoresisDNA appears in cytoplasm

Diffuse and random

Cell degradation Phagocytosis No inflammation

Macrophage invasioninflammation

Roles of PCD in plants

In development• In monocot seeds aleurone

cells form a secretory tissue that releases hydrolases to digest the endosperm and nourish the embryo.

• Aleurone cells are unnecessary for postembryonic development and die as soon as germination is complete.

• physiological evidence suggest that their death involves PCD.

Contd….• in the aleurone, secretory

processes and cell death are stimulated by gibberellin (GA), whereas abscisic acid (ABA) blocks the effects of GA and retards seed germination and cell death

• Moreover, an elevation in cytosolic Ca2+ occurs in aleurone cells treated with GA , suggesting that a signal transduction pathway controls secretion and cell death.

In root cap cells• A cap of cells protects the root

apical meristem during seed germination and seedling growth.

• Cell death occurs in root caps when roots are grown in water, showing that cell death is a normal part of development and not a consequence of abrasion during soil penetration.

• PCD is an integral part of root cap development

In TE cells

• Vascular plants transport water in columns of specialized dead cells termed TEs.

• Differentiation of TEs involves cell elongation, the deposition of cell wall components, including lignin, and autolysis .

• Autolysis begins as the cytoplasm and nuclei become lobed, condensed,and shrunken and ends as the cytoplasm breaks into small packets . cell death in TEs requires protein synthesis.

During germination• Apoptosis occurs during the

germination of plants and it is also formed in the seed storage tissues.

• Endosperm supplies nutrients to the embryo for development and germination and undergoes PCD.

• This process generally associated with lytic enzyme activities, for instance -amylase is secreted from aleurone layer which surrounds the endosperm

• PCD occurs in a gibberellic acid dependent

During reproduction

• Unpollinated flowers are fully thrown away. • Ovaries with fertilized egg cells in ovules on the

same plant are retained forming fruits while the other parts; petals,sepals or tepals fall off.

• Stigmas and pistils may also be eliminated. • In apomictic species, the fruits develop without

fertilization, which means that the ovaries with ovules are retained forming fruit, but the otherflower parts are eliminated.

• PCd is involved in the formation of female gametes in seed plants.

• Single meiotic division gives four haploid megaspore cells, three of them undergo apoptosis, remaining one have two additional mitotic division and bring to egg and associated cells of the embryo (Bell, 1996).

• Apoptosis is also involved in the formation of male sexual organs.

• Tapetum layer is surrounding the pollen during maturation undergoes apoptosis .

• The growth of the pollen tube through the pistil is associated by selective cell death.

• Therefore pistil cells along the growth way of the pollen tube undergo apoptosis while the rest of the tissue stays intact .

• Two synergid cells are present at the entry to the egg sack, one of them undergoes apoptosis for arriving pollen tube to enter and release sperm cells.

During embryogenesis• Cell death within the embryo does

occur as part of its normal development and includes the death of scutellar cells surrounding the developing radicle,death of suspensor and death of nucellus from which the egg cell originates.

• The death of these cells is essential for the final development of the embryo.

• The transient endosperm undergoes a cell death that is followed by its re absorption during embryogenesis that is thought to facilitate embryo growth, whereas in other species in which the endosperm is persistent, it survives as a part of the mature seed.

During somatic embryogenesis

• Cultured cells of some plant species can be induced to develop into somatic embryos.

• In embryogenic suspensioncultures,totipotent cells divide asymmetrically into cell pairs,one member of which stops synthesizing DMA and dies,whereas the other member goes on to establish an embryo.

PCD during vegetative period• cells die at early stages of leaf

development, resulting in the formation of holes in the mature leaf .

• Sclerenchyma cells are dead because thick cell walls perform the mechanical function.

• Cork is constituted of characteristic cells with thick suberinised layer of the cell wall.

• Suberin combined with lack of intercellular spaces, protects internal tissues against dessication.

• The protoplast is no longer needed, therefore it is eliminated. The continuous growth of the stem is also result with the cell death.

• Cell division in the cambium layer causes cell death in the cork layer, that is replaced with the ruptured epidermis and also in parenchyma cells at the stem pith.

Role in xylogenesis• Tracheal elements

(vessels/tracheids) are composed of a series of hollow dead cells.

• After the formation of secondary walls tracheal elements lose their cellular contents to become empty dead cells. living tracheary elements demonstrated that rapid

• nuclear degradation is triggered by vacuolar rupture. Nucleoids in chloroplasts are also degraded rapidly after vacuole rupture.

• Secondary wall lignification is initiated before the vacuole rupture.

• xylem (the waterconducting cells) and phloem (the nutrient-conducting cells) undergo autolysis as they differentiate and mature.

• xylogenesis requires RNA and protein synthesis and thus satisfies the criterion of being an active process.

• the cell death is entirely cell autonomous since the dying cells can influence their neighbors to differentiate.

Contd…

• Relative to the cytoplasm of living cells , the cytoplasm in the dead cells is condensed and shrunken.

• In some dead cells, the cytoplasm is broken into small, membrane-sealed packets, which suggests that the cells have undergone a form of PCD similar to apoptosis.

Role in senescence

• Senescence is the final phase of plant vegetative and reproductive development, preceding the widespread death of cells and organs.

• Senescence involves the active turnover and recapture of cellular material for use in other organs

• Membrane integrity and cellular compartmentalization are maintained until late into the senescence process, suggesting that there is little or no leakage of cellular contents .

• Senescence, which can be induced by ethylene, requires nuclear functions and involves an increase in the generation of 02-and Hz02.

Role of hexokinase in PCd

• PCD is a major factor in the life of a cell, glucose metabolism (and hexokinase activity) also plays a key role in PCD.

• The first step in glucose metabolism, ATP-dependent phosphorylation to yield glucose-6-phosphate, is catalyzed by hexokinase.

• Plants and animals contain a number of isozymes of hexokinase, which differ in subcellular localization and in their catalytic and regulatory properties, and selective expression of the various hexokinases is thought to be a major factor in the regulation of glucose metabolism (Wilson, 2003).

• Thus, hexokinase is well-positioned to be a major player in controlling life and death processes within the cell.

Interactions with enviornment

• Hypoxia• Cell death can occur in the cortex of the root

and stem base response to waterlogging and hypoxia.

• The aerated tissue so formed is termed aerenchyma, and the internal air spaces generated by cell death facilitate more efficient transfer of O2 from aerial organs to waterlogged stem bases and roots.

Contd….

• Ethylene, which is generated in response to O2 deprivation, is the trigger for a signal transduction pathway that leads to aerenchyma formation.

• The existence of a signal transduction pathway suggests that cell death during hypoxia is PCD.

• Aerenchyma formation also involves the cell condensation and shrinkage and DNA fragmentation that are characteristic of PCD.

Plant – pathogen interactions

• Hypersensitive cell death occurs at the site of attempted attack by an avirulent pathogen, and the HR leads to the formation of a dry lesion that is clearly delimited from surrounding healthy tissue.

• Attacks by virulent pathogens,which do not trigger the HR, lead instead to disease.

• The bundle sheath cells surrounding veins are more susceptible to death-inducing signals than are mesophyll cells .

• The localized deaths of bundle sheath cells may prevent pathogens from gaining entry to the vascular system and spreading systemically

HYPER SENSITIVE CELL DEATH

• 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 processthat is similar physiologically to PCD.

• The HR cell death itself is under genetic control.

SIGNALS FOR APOPTOSIS

• decrease in mitochondrial transmembrane potential • exposure of phosphatidylserine in the plasma• Membrane.• These events are followed by the activation of

proteases,phospholipases and phosphatases. • Calcium role is also important. • The activation of nucleases leads to cleavage of nuclear

DNA .• Internucleosomal DNA cleavage results in the formation

of small fragments