There are many root words that start with ab-, abs-, a-, and apo-. The meaning of these prefixes in Latin, from where these root words are originated, is ‘away, from.’ Some of the words starting with ab-, ad- and abs- are abscission, adaxial, ablation, and aphasia.
Abscission is the process of cutting off and is used to describe the natural detachment of plant parts from the plant. Adaxial is used to denote the position of a plant part near the axis. Ablation means the removal of the biological functionality of a structure. And finally, aphasia is the impairment of language, affecting the production or comprehension of speech and the ability to read or write. Each of these terms has been discussed in detail in this article.
What is Abscission?
Abscission is derived from the Latin word “ab” meaning "away" and "scindere" meaning "to cut'. It is the act or process of cutting off. It is the natural detachment of plant parts like ripening and falling of fruits. It is a physiologically determined program of cell separation occurring at the abscission zones (AZs) at specific positions of the plant body and occurs in a controlled manner. Various phytohormones and environmental factors control abscission.
Abscission zone has certain characteristics:
They have dense cytoplasm.
They have smaller intercellular spaces, with possible differences in the makeup of the cell
They respond differently to plant hormones such as ethylene and auxins.
Mechanism of Abscission
The turgor theory suggested that the solute concentration at the separation zone cells increased as a result of starch degradation. This increased turgor pressure caused them to round up and tear the wall along the side of the middle lamella.
During abscission, the actual cell separation is brought about by the production of hydrolytic enzymes that break down the pectin in the middle lamella.
In addition to undergoing cell separation, the cells that comprise the abscission zone also exhibit other modifications. Many of these are related to protecting the fracture surface from invasion by opportunistic pathogens. These events include the synthesis of new cell wall material and pathogenesis resistance proteins
Abscission of leaves
The detachment of the older (rather senescent) leaves or leaf fall is a common phenomenon in plants and is called as abscission of leaves. Abscission is quite distinctive in deciduous trees and shrubs of temperate regions in autumn when all the leaves of such plants fall at about the same time giving the plants a naked appearance, the new leaves developing in the subsequent spring.
Leaf abscission takes place at the base of the petiole which is internally marked by a distinct zone of few layers of thin-walled cells arranged transversally across the petiolar base. This zone is called the abscission zone or abscission layer. The cells of the abscission layer separate from each other due to the dissolution of the middle lamella and also the primary cellulose walls under the influence of the increased activity of the enzymes pectinase and cellulose.
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Figure 1- Leaf Abscission
At this stage, the petiole remains attached to the stem only by vascular elements and very soon due to its own weight and pressure of the wind is detached from the stem. The broken vascular elements are soon plugged with tyloses or gums.
Mature-fruit abscission (MFA) in fleshy-fruit is a genetically controlled process, While global transcriptome analyses have been used to investigate immature fruit abscission in fleshy fruit, no global gene expression changes specific to MFA have been described. Experiments and analysis of gene expression from these AZ’s suggest that membrane microdomains involving sterols/sphingolipids and remorins (a family of plant-specific proteins) together with signalling proteins are potentially involved in MFA.
Reactive oxygen species (ROS) are produced in response to many environmental stresses, such as UV, chilling, salt and pathogen attack. These stresses also accompany leaf abscission in some plants, however, the relationship between these stresses and abscission is poorly understood. A variety of ROS, including H2O2, superoxide, singlet oxygen, and the hydroxyl radical, are generated during stresses such as UV, chilling, high light, salt and pathogen attack.
Continuous production of hydrogen peroxide (H2O2) is involved in leaf abscission signalling. Continuous H2O2 production is required to induce expression of the cell wall-degrading enzyme, cellulase and functions downstream of ethylene in abscission signalling. Furthermore, enhanced production of H2O2 occurs at the execution phase of abscission, suggesting that H2O2 also plays a role in the cell-wall degradation process. These data suggest that H2O2 has several roles in leaf abscission signalling.
Phytohormonal Control of Abscission
ABA, IAA, and GAs interact to influence both the development and abscission of fruitlets (Smith, 1969). So, appropriate changes in auxin levels could either amplify or counteract abscission by accelerating the influence of moderate levels of ABA (Addicott, 1970). Auxin, a plant hormone, and ethylene have been implicated as prominent regulators of abscission signalling. The two compounds work in a synergistic fashion. As the auxin levels decrease, the flux of auxin to the abscission zone is reduced. Exhaustion of auxin makes the abscission zone sensitive to ethylene. Application of GA to the stem apex increases the rate of abscission of de-bladed petioles. The effect on abscission is accompanied by an increase in the level of endogenous auxin in the stem.
Last few words on Abscission
Thus abscission occurs as a natural process in plants. Leaf, fruit abscission is also an adaptive advantage for the plant. Dead, damaged or infected leaves and fruits drop to the ground rather than shading healthy leaves or spreading disease. The plant can minimize water loss in the winter when the water in the plant is often frozen or in summer when water is scarce.
What does Adaxial Mean?
Adaxial means located on the side nearest to the axis of an organ or organism. The upper surface of a lateral organ such as a leaf or petal is known as it’s adaxial side.
Characteristics of the adaxial leaf surface
The characteristics of an adaxial leaf surface are:
Adaxial surface show slightly wavy cell walls.
The ratio of stomata on the adaxial and abaxial (side opposite to adaxial) leaf surfaces is similar in all growth conditions.
The photosynthesis and transpiration rates are always higher on the abaxial surface than the adaxial surface.
The CO2-response curve of photosynthesis on the adaxial surface is specifically determined by growth CO2 abundance and tracks transpiration. Conversely, photosynthesis on the abaxial surface is largely independent of CO2 concentration and rather independent of stomatal function.
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What is Ablation?
In biology, ablation is the removal of a biological functionality or a structure – either an organ; or at the cellular level. Example: Gene ablation, organ ablation. Gene ablation is another term for genetic slicing.
Types of Ablation
Catheter ablation – It is a type of organ ablation. Also known as a cardiac ablation or radiofrequency ablation, this procedure guides a tube into your heart to destroy small areas of tissue that may be causing abnormal heartbeat. Not everyone with a heart arrhythmia needs a catheter ablation. It’s usually recommended for people with arrhythmias that can’t be controlled by medication or with certain types of arrhythmia from the heart’s upper chambers i.e. the atria. Catheter ablation is also sometimes recommended for people with arrhythmia that begins in the lower chambers of the heart i.e. the ventricles.
Laser ablation tomography (LAT)- It is a novel method that allows for rapid, three-dimensional quantitative and qualitative analysis of root anatomy, providing new opportunities to investigate interactions between roots and edaphic organisms.
Gene ablation- This process involves modifying DNA in order to change or disrupt the production of a specific gene. Gene ablation involves the homologous genetic recombination of a gene in such a way that the gene has been made “null”. This may lead to call ablation. Thus cell ablation is often synonymous to gene ablation. Genetic ablation is a genetic engineering technique used to suppress selectively the growth of a specified cell line or cell type in an animal rather than suppress the activity of an individual gene.
What is Aphasia?
Aphasia is an impairment of language, affecting the production or comprehension of speech and the ability to read or write. Aphasia is always due to injury to the brain-most commonly from a stroke, particularly in older individuals. In India, More than 1 million cases per year are reported.
The three most common types of aphasia are:
Common causes of aphasia include Stroke, traumatic brain injury, brain tumours, brain surgery, brain infections, progressive neurological diseases (e.g., dementia).
1. What is the difference between Abscission and Senescence?
The main difference between abscission and senescence is that abscission is the separation of a plant part or organ from the plant body whereas senescence is the age-related deterioration of plant organs.
Abscission is a controlled process while senescence literally means "the process of growing old."
2. Name the plant hormones that control Abscission?
The phytohormone Auxin functions as a brake — if high Auxin concentrations reach the AZ cells, abscission is not activated. Researchers originally believed abscisic acid to be the hormone that stimulates abscission (for which the hormone was named), it was later proven that it does not play a primary role. Ethylene also acts as a prominent regulator of abscission signalling. There is a delay of leaf abscission in response to indoleacetic acid (IAA), and the typical acceleration of abscission occurs in response to gibberellic acid (GA).
3. What is mixed Aphasia?
Mixed Transcortical Aphasia is a type of aphasia in which repetition is the primary language ability that is present. It is an uncommon type of aphasia. Mixed transcortical aphasia has also been called isolation aphasia.
4. Give two tests for Aphasia?
Common Aphasia Screening and Evaluation Tests are:
-Mississippi Aphasia Screening Test (MAST): a brief screening tool that can be administered verbally and done in 5-15 minutes.
-Western Aphasia Battery-Revised (WAB-R): a complete assessment of language skills related to aphasia in all modalities.