Lecture-17 Circadian Rhythm, Physiology of Senescence and Ageing, programmed cell death

Circadian Rhythm:-
> A circadian rhythm, or circadian cycle, is a natural oscillation that repeats roughly every 24 hours. 
> Circadian rhythms can refer to any process that originates within an organism (i.e., endogenous) and responds to the environment (is entrained by the environment). 
> Circadian rhythms have been widely observed in animals, plants, fungi and cyanobacteria and there is evidence that they evolved independently in each of these kingdoms of life.
> Its primary function is to rhythmically co-ordinate biological processes so they occur at the correct time to maximise the fitness of an individual.
> The term circadian comes from the Latin circa, meaning "approximately", and dies, meaning "day". 
Circadian Rhythm in Plants:-
- Plant circadian rhythms tell the plant what season it is and when to flower for the best chance of attracting pollinators. 
- Behaviors showing rhythms include leaf movement (Nyctinasty), growth, germination, stomatal/gas exchange, enzyme activity, photosynthetic activity, and fragrance emission, among others.
- Circadian rhythms occur as a plant entrains to synchronize with the light cycle of its surrounding environment. 
- These rhythms are endogenously generated, self-sustaining and are relatively constant over a range of ambient temperatures. I
-mportant features include two interacting transcription-translation feedback loops: proteins containing 
i. PAS domains, which facilitate protein-protein interactions.
ii. Several photoreceptors, that fine-tune the clock to different light conditions. 
-  A better understanding of plant circadian rhythms has applications in agriculture, such as helping farmers stagger crop harvests to extend crop availability and securing against massive losses due to weather.
- Light is the signal by which plants synchronize their internal clocks to their environment and is sensed by a wide variety of photoreceptors. 
- Red and blue light are absorbed through several phytochromes and cryptochromes. 
- Phytochrome A, phyA, is light labile and allows germination and de-etiolation when light is scarce.
- Phytochromes B–E are more stable with phyB, the main phytochrome in seedlings grown in the light. 
- The cryptochrome (cry) gene is also a light-sensitive component of the circadian clock and is thought to be involved both as a photoreceptor and as part of the clock's endogenous pacemaker mechanism. 
- Cryptochromes 1–2 (involved in blue–UVA) help to maintain the period length in the clock through a whole range of light conditions.

Physiology of Senescence and Ageing:- The condition or process of deterioration with age is called senescence. It is a process by which a cell ages and permanently stops dividing but does not die. It is found naturally in desiduous plants. Gibberellin hormone reduces senescence in plants while ABA hormone increases senescence in plants.
Causes of senescence:-
> Process of leaf senescence comes along with early loss of chlorophyll, enzymes and RNA.
> As a result of a faster break down or slower synthesis, there is a decrease in the cellular constituents.
> Competition between reproductive and vegetative structures for nutrients.
> Long-night and short-day conditions induce leaf senescence and flowering.
> Senescence factor is secreted in fruits of soybean which passes to leaves causing senescence.
> The process of senescence is also governed hormonally.
> Loss of integrity in food storage cells of seeds and degradation of food reserves.
Physiological changes occurring during senescence:-
> Chlorophyll degradation.
> Decrease in starch content.
> Decrease of proteins and RNA.
> Process of photosynthesis stops.
> Enzyme DNase degrades DNA molecules.
> Decrease in growth promoting hormone such as cytokinin.
> Functioning of vacuoles as lysosomes and digestion of cellular matter.
> Reddening of leaves as a result of accumulation of Anthocyanin pigments in leaves.
> Increase in the content of deteriorative hormones such as abscisic acid and ethylene.
Difference between Abscission and Senescence:-

Programmed Cell Death (Apoptosis):- If cells are no longer needed, they commit suicide by activating an intracellular death program. This process is therefore called programmed cell death, although it is more commonly called apoptosis (from a Greek word meaning “falling off,” as leaves from a tree).
Apoptosis Pathways:- The process of apoptosis undergoes two pathways:
i. Extrinsic Pathway:- This pathway triggers apoptosis in response to external stimuli, like, ligand binding at death receptors on the cell surface. These receptors are members of the Tumor Necrosis Factor gene family. The receptor binding initiates caspase activation.
ii. Intrinsic Pathway:- This pathway triggers apoptosis in response to internal stimuli such as biochemical stress, DNA damage and lack of growth factors. This pathway is modulated by two groups of molecules- Bax, and Bcl-2. These groups of molecules determine whether a cell will survive or undergo apoptosis in response to the stimuli.
Significance of Apoptosis:- Apoptosis is significant for the following reasons:
i. It helps to maintain homeostasis in the multicellular organisms.
ii. Proper size of the body is maintained by apoptosis.
iii. Apoptosis maintains the constancy of cell number in an organism.
iv. The unwanted cells are eliminated from the body by apoptosis.
v. The dangerous T-lymphocytes are eliminated by apoptosis.
vi. Programmed cell death is crucial for cell development.
Apoptosis in plants:-
> Apoptosis is an integral part of plant ontogenesis; it is controlled by cellular oxidative status, phytohormones, and DNA methylation. 
> In wheat plants apoptosis appears at early stages of development in coleoptile and initial leaf of 5-6 days old seedlings. 
> Distinct ultrastructural features of apoptosis observed are: 
i. Compaction and vacuolization of cytoplasm in the apoptotic cell, 
ii. Specific fragmentation of cytoplasm and appearance in the vacuole of unique single-membrane vesicles containing active organelles, 
iii. Cessation of nuclear DNA synthesis, 
iv. condensation of chromatin in the nucleus, 
v. internucleosomal fragmentation of nuclear DNA, and 
vi. intensive synthesis of mitochondrial DNA in vacuolar vesicles.
> Peroxides, abscisic acid, ethylene releaser ethrel, and DNA methylation inhibitor 5-azacytidine induce and stimulate apoptosis. 
> Modulation of the reactive oxygen species (ROS) level in seedling by antioxidants and peroxides results in tissue-specific changes in the target date for the appearance and the intensity of apoptosis. 
> Antioxidant butylated hydroxytoluene (BHT) reduces the amount of ROS and prevents apoptosis in etiolated seedlings, prolongs coleoptile life span, and prevents the appearance of all apoptotic features mentioned. 
> In roots of etiolated wheat seedlings, BHT induces differentiation of plastids with the formation of chloro(chromo)plasts. 
> Therefore, ROS controlled by BHT seems to regulate mitosis, trigger apoptosis, and control plastid differentiation and the organization of various cellular structures formed by endocytoplasmic reticulum.
> PCD in pollen prevents inbreeding:- 
- During pollination, plants enforce self-incompatibility (SI) as an important means to prevent self-fertilization. 
- Research on the corn poppy (Papaver rhoeas) has revealed that proteinsin the pistil on which the pollen lands, interact with pollen and trigger PCD in incompatible (i.e., self) pollen. 
- The researchers, Steven G. Thomas and Veronica E. Franklin- Tong, also found that the response involves rapid inhibition of pollen-tube growth, followed by PCD.