What is Senescence? – Senescent Cells And Senescence In Plants

Senescence is the process by which cells stop dividing and enter a state of permanent growth arrest without undergoing cell death. Senescence cells get old and weaken, causing a decline in tissue function and decay. Senescence in plants is a programmed death of leaves as part of developmental cycles. In animals, senescent cells contribute to aging and age-related diseases.

Senescence can be induced by unrepaired DNA damage or other cellular stresses. It plays a role in normal development, maintains tissue homeostasis, and limits tumor progression. In this article, we will read about what is senescence, its causes, and effects, senescent cells, the different types of senescence in plants, theories of aging, factors affecting senescence, and its importance.

Senescence Definition

Senescence is a biological process that involves cells aging and permanently stopping cell division. 

Senescence Meaning

Senescence is derived from the Latin word ‘senescere‘, which means “to grow weak, become exhausted, and to be in a decline”. It generally refers to the process of growing old and is associated with decay and mortality or decreased fertility with age. It can occur at the cellular level or affect the entire organism. Senescence causes a gradual functional decline across organ systems, leading to progressive deterioration and tissue dysfunction. The process is influenced by genetic and environmental factors.

In humans, senescence starts in their 20s, at the peak of their physical strength, and continues for the rest of their life. senescence in plants is the final stage of development. Some common examples of senescence that most people experience as they age include: wrinkles, worsening eyesight, and worsening hearing. During this stage, the plant reclaims the cellular building blocks that have been deposited in the leaves and other parts of the plant during growth.

Senescent Cells

Senescent cells are cells that have stopped dividing but remain alive and release chemicals that can cause inflammation. They are also known as “zombie cells” because they refuse to die easily. Senescent cells have a flattened and enlarged appearance. They have molecular markers like lipofuscin granules, DNA scars, and altered gene expression. Senescent cells undergo phenotypic changes, including:

• Chromatin changes
• Secretome changes
• Tumor-suppressor activation

Over time, large numbers of senescent cells can build up in the body’s tissues. The accumulation of these cells can impair intercellular communication and compromise tissue function. This can lead to inflammation, cell death, and loss of cardiomyocytes. Senescent cells secrete a variety of extracellular modulators, including: Cytokines, Chemokines, Proteases, Growth factors, Bioactive lipids.

The secretion of these modulators is known as the senescence-associated secretory phenotype (SASP). The SASP can affect the surrounding cells and contribute to cancer and the aging process. Senescent cells can alter the local tissue environment and contribute to chronic inflammation and cancer. Senolytics are drugs that can eliminate senescent cells.

Causes of Senescence

Senescence can be caused by a number of factors, including:

• DNA Damage: Accumulation of DNA damage over time can trigger cellular senescence. It leads to a halt in cell division and altered cellular function.
• Telomere Shortening: As cells divide, the protective caps on the ends of chromosomes, called telomeres, gradually shorten and once they reach a critical length, cells enter senescence.
• Oxidative Stress: Increased levels of reactive oxygen species (ROS) from metabolic processes can damage cellular components and contributes to senescence.
• Mitochondrial Dysfunction: Mitochondrial function that are responsible for energy production decreases which disrupts cellular homeostasis and lead to senescence.
• Genetic Factors: Certain genetic factors and regulatory pathways can predispose cells to enter a senescent state.
• Inflammatory Signals: Chronic inflammation, either due to external factors or the senescence-associated secretory phenotype (SASP), can induce senescence in neighboring cells.
• Environmental Stressors: Exposure to environmental stressors such as toxins, radiation, and pollutants can accelerate the onset of senescence.

Types of Senescence

Senescence, or cell aging, can occur in different types of cells, including melanocytes, endothelial cells, epithelial cells, glial cells, and stem cells. Various type of senescence are:

Senescence in Plants

Leaf senescence means the partial or complete death of the leaf. Senescence is the final stage of plant development and is a highly regulated program that recycles nutrients, mostly nitrogen, to the plant’s growing and storage organs. The senescence hormones regulates the process in plants. Some signs of senescence in plant include:

• The loss of chlorophyll, which causes leaves to turn yellow.
• Stoppage of the process of photosynthesis.
• Decrease in growth promoting hormone such as cytokinin and decrease in starch content.

Different type of senescence patterns observed in plants are:

• Overall Senescence: The entire plant is affected and undergoes senescence, leading to its death. Commonly seen in annual flowers and crops that complete their life cycle within a single growing season.
• Top Senescence: Affects only the above-ground parts of the plant. The underground parts, such as roots, may remain viable. Trees and shrubs that shed their leaves in autumn.
• Deciduous Senescence: It involves the senescence of all the leaves of the plant, in response to seasonal changes. It is less severe and occurs in deciduous trees, which lose their leaves in autumn as a part of the natural senescence process.
• Progressive Senescence: Innvolves the gradual progression of leaf senescence from the base upwards as the plant grows. Seen in some perennial plants where older leaves at the base of the plant senesce first, followed by the upper leaves as the plant continues to grow.

Replicative Senescence

This type of senescence occurs when telomeres shorten. It’s a physiological process that happens during aging. Replicative senescence involves an irreversible halt to cell proliferation and changes to cell function.

Stress-induced Senescence

Stress-induces senescence also known as stress-induced premature senescence (SIPS), regulates cell metabolism and function. It’s caused by oxidative stress to prevent cell growth. It occurs independently of a change in telomere length.

Developmental Senescence

Developmental senescence is a temporary, programmed cellular senescence that occurs during mammal’s embryonic development. It’s mediated by pathways or pluripotency genes. The role of developmental programmed senescence is to promote tissue remodeling.

Theories of Aging

Aging theories explains the cumulative damage, be it from oxidative stress, telomere shortening, or mitochondrial dysfunction that contributes to the gradual decline in cellular function and overall aging. Theories of aging are:

Oxidative Stress Theory

The oxidative stress theory of aging suggests that age-related reductions in physiological functions are caused by a slow accumulation of oxidative damage caused by reactive oxygen species (ROS) to macromolecules. This damage increases with age and is associated with life expectancy. ROS are byproducts of cellular metabolism, and if not properly balanced by antioxidants, they can damage cellular components like DNA, proteins, and lipids.

Telomere Theory

The theory suggests that telomeres shorten each time a cell divides, eventually disappearing. Once telomeres are gone, the cell can no longer copy its DNA and ages, becoming unable to replicate. Regular physical activity is believed to positively affect telomere length, prolonging human lifespan.

Mitochondrial Theory

The mitochondrial theory of aging is a theory that suggests that damage to mitochondria and mitochondrial DNA (mtDNA) leads to aging in humans and animals. It is a variant of the free radical theory of aging. Mitochondria are the main producers of ROS in the cell. According to the theory, mitochondria accumulate ROS-induced damage and become dysfunctional with age. Over time, cell function declines, causing aging and death.

Effects of Senescence

Senescence can cause a number of effects, including:

• Loss of Cell Proliferation:Senescent cells cease to divide that limits the body’s ability to replace damaged or dying cells.
• Altered Cellular Function: Senescent cells undergo changes in gene expression and functionality that impact their normal roles or can disrupt tissue and organ functions.
• Inflammation: Senescent cells can release pro-inflammatory molecules as part of the senescence-associated secretory phenotype (SASP) that contributes to chronic inflammation and promote age-related diseases.
• Tissue Dysfunction: Accumulation of senescent cells in tissues can contribute to impaired tissue function that affects organs and systems throughout the body.
• Contribution to Aging: The presence of senescent cells is associated with the aging process, and their accumulation over time is linked to age-related diseases and degenerative conditions.
• Cancer Prevention: While senescence can limit the proliferation of damaged cells and prevent the development of cancer.
• Impact on Surrounding Cells: The SASP released by senescent cells can influence neighboring cells, either promoting inflammation or affecting the behavior of nearby healthy cells.

Factors that Influence the Senescence in Plants

Factors that influence the senescence process in plants are:

• Hormonal and Growth Regulators: The hormones affecting are:
  • Cytokinins: These hormones can delay senescence by promoting cell division and maintaining tissue integrity.
  • Ethylene: Often associated with promoting senescence, ethylene is a plant hormone that influences fruit ripening and leaf abscission.
• Nutrients: Nutrient deficiencies or imbalances, such as nitrogen deficiency, can accelerate senescence. Adequate nutrients are crucial for sustaining cellular functions and delaying aging.
• Water Stress: Insufficient water availability can accelerate senescence, leading to wilting, leaf yellowing, and premature aging. Water stress affects nutrient transport and overall plant health.
• Light: Changes in day length (photoperiod) can influence the timing of senescence. Short day length, associated with seasonal changes, often triggers senescence in deciduous plants.
• Temperature: Both excessively high and low temperatures can impact the rate of senescence. Extreme heat can lead to dehydration and accelerate aging, while frost can cause cellular damage and premature senescence.

Importance of Senescence

The following points highlight the importance of senescence :

• Senescence protects plants from disease by causing only one part of the plant to die so that the rest of the plant can live. It also allows plants to go into a state of dormancy until favorable growing conditions return.
• Cellular senescence plays a crucial role in wound healing. Senescent hepatocytes induce fibrosis, which limits tissue injury.
• Senescence plays a key role in tumor suppression by enhancing immune response and decreasing malignant cells proliferation.
• Senescence is an adaptive mechanism that helps plants respond to environmental stresses.
• It allows plants to reabsorb and recycle nutrients from aging tissues, ensuring efficient resource utilization.

Conclusion – Senescence

Senescence definition states that it is a biological process that involves cells aging and permanently stopping cell division. It can either occur at the cellular level or senescence of the whole organism can take place. Senescent cells are those cells that have stopped dividing but remain alive and release chemicals that can cause inflammation. Senescence benefits a plant by protecting it from disease, causing only one part of the plant (such as a tree branch) to die so that the rest of the plant can live. It also benefits plants by allowing it to go into a state of dormancy until favorable growing conditions return.

FAQs – Senescence – Senescent Cells And Senescence In Plants

What is the Difference Between Aging and Senescence?

Aging is a biological process that involves a decline in physiological functions and an increased risk of mortality. Senescence is a stable state of cell cycle arrest and the secretion of inflammatory molecules.

What are the Two Types of Senescence?

There are two main types of senescence: Acute senescence: This is a normal biological process that is beneficial during tissue repair, embryonic development, and wound healing. Chronic senescence: This is a persistent type of senescence.

What is the Difference Between Quiescence and Senescence?

Quiescence is a reversible and dormant state where cells temporarily cease division, while senescence is an irreversible and permanent cell cycle arrest associated with aging or damage.

What are the Different Types of Senescent Cells?

There are two types of senescence : the irreversible senescence which is mediated by INK4a/Rb and p53 pathways and the reversible senescent phenotype which is mediated by p53.

Do All Cells Undergo Senescence?

No, not all cells undergo senescence. Cellular senescence is a stress response that can be triggered in all types of somatic cells by different stimuli. However, postmitotic cells, which do not replicate, do not undergo cellular senescence.

What is Senescence Also Known as?

Senescence is also known as biological aging. It’s a gradual deterioration of the functional characteristics of living organisms.

What are the Causes of Senescence?

Senescence can be triggered by a number of factors, including oxidative stress, telomere damage or shortening, DNA damage, mitochondrial dysfunction, chromatin disruption, inflammation, epigenetic dysregulation, oncogene activation.

What is the Significance of the Senescence?

The significance of senescence lies in its role in regulating aging processes, maintaining ecological balance, and facilitating nutrient recycling in ecosystems.

What are the Signs of Senescence?

The signs of senescence include physical changes such as wrinkles, reduced mobility, decreased sensory perception, and declining reproductive function. Senescent cells become flattened and enlarged morphologically.