SENESCENCE AND PROGRAMMED CELL DEATH IN PLANTS

SENESCENCE

•  Every autumn plants changes their colour, after that the loss of leaves can be seen  from deciduous trees. The reason behind this is , because of change in  day length and cooling temperature which triggers developmental process that lead to leaf senescence and death.
• Senescence and necrosis are two different processes,  although both of them  lead to death of the cells.
• Necrosis is death brought out by physical damage, poison or any external injury. In contrast,  Senescence is a normal ,energy dependent developmental process, that is controlled by plant's own genetic program.
• Senescence of plant organs is generally associated with abscission, a process where specific cells in petiole differentiate to form an abscission layer,allowing the senescent organ to separate from the plant. Ethylene have a role in controlling abscission.
• Cytokinins and a Ethylene can act as signaling agents that regulate plant senescence.

IMPORTANCE OF SENESCENCE IN PLANTS

• Leaves are genetically programmed to die and their senescence can be initiated by environmental factors.  New leaves are initiated from the shoot apical meristem, older leaves often are shaded and loose the ability to function efficiently in photosynthesis.
• Senescence helps in recovering of some valuable resources that the plant had invested in leaf formation.
• Senescence cause the production of   many hydrolytic enzymes , that breakdown many  proteins ,carbohydrates and nucleic acids in the cell  ,are then transported back into the plant via the phloem,  where they will be reused for synthetic processes.
• After senescence, many minerals\nutrients  are also transported back to the plant for use from the dead cells .

VARIOUS TYPES OF SENESCENCE IN PLANTS

• Senescence occurs in various of organs of plant and is induced by  many different environmental factors.
• Many annual plants, including major crop plants such as wheat, maize and soyabeans ,get yellow and die following fruit production even under optimal growing conditions.
• Senescence of an entire plant after a single reproductive cycle is called Monocarpic senescence.
• Other types of senescence include the following-

1. Senescence of aerial shoot in herbaceous perennials
2. Seasonal leaf senescence (as in deciduous trees)
3. Sequential leaf senescence (in which the leaves die when they reach a certain stage)
4. Senescence is storage cotyledons & floer organs
5. Senescence (ripening)of fleshy fruits ;senescence of dry fruits
6. Senescence of specialized cell types (e.g. trichomes ,tracheids &vessels elements)etc.

• The triggers for the various types of senescence are different and can be internal, as in monocarpic senescence, or external such as day length and temperature in autumnal leaf senescence of deciduous trees.
• Regardless of initial stimulus, the different senescence patterns may share common integral programmes in which a regulatory senescence gene initiates a cascade of secondary gene expression that eventually brings about senescence and death.

CYTOLOGICAL AND BIOCHEMICAL EVENTS IN SENESCENCE

• On the cytological level some  organs are destroyed while others remain active.
•  The chloroplast is the first organ to deteriorate during the onset of leaf senescence, with destruction of thylakoid protein component and stromal enzymes. Whereas , the nucleus remains structurally and functionally intact until the late stages of senescence.
• Senescence tissues carryout  processes that requires the synthesis of various kind of hydrolytic enzymes, such as proteases, nucleases, lipases and chlorophyll degrading enzymes.
• These senescence specific enzymes are synthesised after  the activation of specific genes.
• In leaf  most mRNA levels decline significantly during senescence phase, but the abundance of certain specific mRNA transcripts increases.
• Genes whose expression decreases during the senescence phase are called Senescence Down-regulated Genes (SDGs).SDGs includes genes that encode proteins involved in photosynthesis. However senescence involves much more than the simple switching off to photosynthetic genes.
• Genes whose expression is increased during senescence are called Senescence Associated Genes (SAGs).SAGs include genes that encode hydrolytic enzymes such as proteases, ribonucleases and lipases, as well as enzymes involved in ethylene synthesis.

PROGRAMMED CELL DEATH IS SPECIAL TYPE OF SENESCENCE

• Senescence can occur at the level of whole plant (as in monocarpic senescence);at the organ level (as in leaf senescence) ;and at the cellular level (as in tracheary elements differentiation).
• The process whereby individual cells activate an intrinsic senescence program is called Programmed Cell Death (PCD).
• PCD can be initiated by specific signals, such as errors in DNA replication during divisions and involves the expression of a characteristic set of genes. These gene expression  results in cell death.
• In animals PCD is studied extensively in comparison to plants because of less available knowledge about PCD in plants.
• In animals,PCD is followed by changes in morphology and biochemical processes  in the body called Apoptosis. During apoptosis cell nucleus condenses and nuclear DNA fragments in a specific pattern caused by DNA degradation.
• Some plants cells, particularly in senescing tissues, exhibit similar cytological changes.
• One of the important functions of PCD in plants is protecting against pathogens. When a pathogenic organism, infects a plant, signals from the pathogen cause the plant cells at the site of infection to quickly accumulate high concentration of toxic phenolic compounds and die. The dead cells form a circular island of cell death called necrotic lesions.
• The necrotic lesions isolates and prevents the infection from spreading to surrounding healthy tissues, by surrounding the pathogen with toxic and nutritionally depleting environment. This rapid, localized cell death due to pathogen attack is called Hypersensitive response.
• It is studied  Arabidopsis mutants that shows to have mimic the effect of infection and trigger the entire cascade of events leading to the formation of necrotic lesions, even when there is no  pathogen , its existence has shown that the hypersensitive response in plants  is a genetically programmed thing not a simple lesions.