Parthenocarpy in Plants: Mechanisms of Seedless Fruit Development & Hormonal Control

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Table of content

Introduction to Parthenocarpy

Definition & Biological Significance
Evolutionary Advantage of Seedless Fruit Set

Types of Parthenocarpy

Genetic / Natural Parthenocarpy
Environmental / Steno-parthenocarpy
Chemically Induced Parthenocarpy

Hormonal Control & Physiological Mechanisms

Role of Auxins in Ovary Expansion
Gibberellins and Fruit Development
Interplay of Cytokinins and Abscisic Acid (ABA)

Comparative Analysis: Parthenocarpy vs. Apomixis

Key Differences in Tissue Origin & Seed Formation (Summary Table)
Biological Context (Vegetative Reproduction vs. Seedless Development

Agricultural & Commercial Applications

Commercial Benefits in Crops (Grapes, Banana, Watermelon, Citrus)
Synthetic Growth Regulators in Modern Farming

Pre-University Research-Level Problem Sets
Advanced Analytical Case Studies (Global Medical/Research Entry Standards)
Knowledge with Understanding (Direct & Recall Questions

Parthenocarpy - Definitions

Parthenocarpy has a great effect on parts of plants, especially on the ovule. After the fertilisation, the ovary is developed into fruit. In some plants , fruits are developed without the process of fertilization.
The fruits that are developed without fertilization are called parthenocarpic fruit and the process is called parthenocarpy. Parthenocarpy was defined by Winkler and term was given by Noll.
In Banana, Citrus, Grapes and in some species of apple and pear, Parthenocarpic fruits are developed. In addition to this Pineapple, cucumber, watermelon, orange, grapefruit, , fig are some examples of Parthenocarpy.
Parthenocarpic fruits may be produced due to - absence of fertilization, failure of fertilization and zygotic sterility.

Biological significance

Parthenocarpic fruits are of great significance in horticulture. These fruits have more edible parts than normal fruit but Parthenocarpic fruits are seedless.
Parthenocarpy is induced by the plant growth hormones auxin and gibberellic acid. This hormone causes the maturation of the ovaries without the process of fertilization and stimulates the process of formation of fruits.
Plant hormones gibberellin, auxin and cytokinin can often stimulate the development of parthenocarpic fruit. This is called artificial parthenocarpy.
Plant hormones are frequently used to produce parthenocarpic fruit on a commercial scale within a short time. Bananas are a good example of parthenocarpy.
These fruits of bananas are sterile and developed without the function of ovaries and do not produce seeds.
Pineapples and figs are also examples of parthenocarpy which occur naturally.

Comparative Cross Section of Seedless Commercial Triploid Banana vs Seed-Filled

Advantages of parthenocarpy-

It provides seedless fruits with good quality with in less price. This improves crop yield without using organic pesticides.
Plant growth regulators are natural and they produced fruits produced at large scale.
Parthenocarpy provides protection to plants from insects and pests.
Parthenocarpy reduces the dependency of pollinators because there is no requirement of pollinating insects for the formation of fruits.
Parthenocarpy also protects the plants from being attacked by pesticides.

Types of Parthenocarpy

Different types of Parthenocarpy, on the basis of various mechanism , are as follows:

1. Genetic / Natural Parthenocarpy

This type occurs naturally due to genetic factors or mutations within the plant, without requiring any external human intervention or special environmental stimuli. It is broadly categorized into two sub-types:

A. Obligate Parthenocarpy:

It occurs in plants that are completely incapable of undergoing normal sexual reproduction or seed formation.
Examples: Commercial cultivars of Banana (Musa) and Pineapple (Ananas comosus).

B. Facultative Parthenocarpy:

It occurs when pollination or fertilization fails due to external factors, but the plant can still produce fruits normally if pollination takes place.
Examples: Tomato, Fig, and certain varieties of grapes.

Transverse section of parthenocarpic seedless tomato showing solid locular pulp tissue"

Causes / Mechanisms of Genetic Parthenocarpy

Hormonal Imbalance: High concentrations of natural growth hormones such as auxins, gibberellins, and cytokinins in the ovary tissue prior to pollination.
Polyploidy or Chromosomal Aberrations: Triploid plants (3n) often exhibit genetic parthenocarpy because irregular chromosome pairing during meiosis prevents viable seed formation.

2. Environmental parthenocarpy

It is triggered by specific abiotic or climatic factors that disrupt normal pollination, pollen tube growth, or fertilization while leaving ovary development intact.
It occurs due to Unusual low or high temperatures during flowering (e.g., low night temperatures induce seedless fruit set in tomatoes and capsicum).
Short daylight hours or low illumination levels or Photoperiod can alter endogenous hormone levels, prompting seedless fruit development.
Cold damage can kill pollen grains or damage ovules, yet ovary growth continues.

3.Stenospermocarpy

In this Parthenocarpy , Pollination and fertilization do take place, and seed formation begins. However, the embryo or endosperm aborts early during fruit maturation, leaving small, soft, unnoticeable seed traces (appearing seedless)

Stenospermocarpy

Examples: Seedless Watermelon (Citrullus lanatus) and Thompson Seedless Grapes (Vitis vinifera).

Important Point :

Stenospermocarpy is Often Grouped with Environmental/Natural Parthenocarpy. Stenospermocarpy is technically distinct from true parthenocarpy.

4. Chemically Induced (Induced / Artificial) Parthenocarpy

This occurs when exogenous plant growth regulators (PGRs) or chemical sprays are applied artificially to un pollinated pistils/flowers to stimulate ovary growth without fertilization.

Common Chemical Agents Used:

Auxins: Application of Indole-3-acetic acid (IAA), Indole-3-butyric acid (IBA), or Naphthaleneacetic acid (NAA). Used on: Tomatoes, Cucumbers, and Eggplants.
Gibberellins (GA₃): Highly effective in stimulating fruit cell enlargement and division in seedless crops. Used on: Grapes, Apples, and Pears.
Cytokinins: Promotes cell division in fruit wall development, often used in combination with auxins or gibberellins.

Type	Trigger / Mechanism	Seed Status	Example Crops
Genetic / Natural	Internal genes, high endogenous PGRs, or polyploidy	Completely seedless	Cultivated Banana, Pineapple
Environmental	Abiotic stress (low temp, frost, light hours)	Completely seedless	Greenhouse Tomato, Pepper
Stenospermocarpy	Fertilization occurs followed by embryo abortion	Aborted seed traces	Thompson Seedless Grapes, Seedless Watermelon
Chemically Induced	Spraying exogenous auxins, gibberellins, or cytokinins	Completely seedless	Seedless Tomato, Cucumbers

Hormonal Control & Physiological Mechanisms

The Hormonal Control & Physiological Mechanisms involved in fruit development, focusing on the specific roles and interplay of key plant hormones:

1. Role of Auxins in Ovary Expansion

Auxin (primarily Indole-3-acetic acid / IAA) plays a primary role in initiating and driving early fruit growth right after fertilization.

Triggering Fruit Set:

Prior to fertilization, the ovary remains in a state of developmental arrest.
Pollination and fertilization trigger a localized surge in auxin levels synthesized by developing seeds and adjacent maternal tissues, releasing the ovary from dormancy and initiating fruit set.

Stimulation of Cell Division & Elongation:

Auxin increases cell wall extensibility by activating plasma membrane H+ and ATPase pumps (acid growth hypothesis).
This lowers apoplastic pH, activating expansion proteins to loosen cell walls.
It enhances water uptake by regulating aquaporins and solute accumulation, driving cell enlargement in the pericarp.

Prevention of Early Abscission:

High auxin concentrations in the pedicel/fruit stalk repress the expression of cell wall-degrading enzymes (e.g., polygalacturonase, cellulase) in the abscission zone, preventing premature fruit drop.

Parthenocarpy (Seedless Fruit Induction):

Exogenous application of auxin onto unpollinated ovaries can induce fruit growth without fertilization, producing parthenocarpic fruits (e.g., in tomatoes, cucumbers).

2. Gibberellins (GAs) and Fruit Development

Gibberellins act in parallel and synergy with auxins to regulate fruit size, shape, and growth dynamics.

Cell Division and Expansion:

While auxins predominantly drive cell expansion, gibberellins strongly promote both pericarp cell division (mitosis) and cell elongation.
GAs derepress growth pathways by signaling the degradation of DELLA proteins (growth-repressing transcription factors) via the ubiquitin-proteasome pathway.

Coordination with Seed Maturation:

In seeded fruits, the young seeds are rich sources of bioactive GAs (such as GA1 and GA3). These GAs signal the surrounding ovary walls to expand proportionally to the number of developing seeds.

Commercial Application for Fruit Quality:

Like auxins, GAs can induce seedless fruit set in many species (e.g., grapes, pears).
Application of GA3 in table grapes increases berry size and loosens clusters to allow better air circulation and growth space.

💡Read more to understand about the Gibberellins (GAs): Plant Hormone Functions & Signaling Pathway (AP Biology Guide)

3. Interplay of Cytokinins and Abscisic Acid (ABA)

The balance between cytokinins and ABA represents a key developmental switch between growth promotion and maturation/stress response during fruit development.

A. Cytokinins: Driving Early Mitotic Growth

During the initial phase of fruit set (Phase I), cytokinins (e.g., zeatin, isopentenyladenine) peak in concentration to promote active mitotic division in pericarp tissues, establishing the total cell number of the mature fruit.
Cytokinins increase sink strength by upregulating cell wall invertases and sugar transporters, directing photosynthate transport from source leaves to the rapidly growing fruit sink.

B. Abscisic Acid (ABA): Regulating Maturation, Ripening & Dormancy

As fruit expansion ceases, cytokinin levels drop and ABA levels typically rise (especially in non-climacteric fruits like strawberries, grapes, and citrus).
ABA enhances sugar accumulation by promoting sucrose transporter expression and regulates cell wall softening enzymes during the ripening phase.
Under water stress or extreme environmental conditions, elevated ABA promotes premature fruit drop by inducing abscission layer formation in conjunction with ethylene.

C. Dynamic Interplay & Antagonism

Early fruit development is characterized by high Cytokinin + Auxin + GA levels (pro-growth) and low ABA levels.
As the fruit transitions toward maturity and ripening, the ratio flips: cytokinin and auxin signaling decrease while ABA and Ethylene signaling increase.
Cytokinins promote active cell division and maintain tissue juvenility, counteracting ABA’s actions toward growth arrest and senescence. Conversely, elevated ABA inhibits cytokinin-induced cell cycle progression as the fruit enters the ripening phase.

💡Read more to understand about the Abscisic Acid (ABA) Signaling Pathway: Plant Stress Responses & Mechanisms | AP Biology Unit 4

Comparative analysis of Parthenocarpy and Apomixis :

Comparative analysis of Parthenocarpy and Apomixis, structured to clearly distinguish their biological mechanisms, tissue origins, seed development outcomes, and evolutionary significance.

1. Key Differences in Tissue Origin & Seed Formation (Summary Table)

Feature / Metric	Parthenocarpy	Apomixis
Primary Definition	Development of a fruit without prior fertilization of the ovule.	Asexual reproduction through seeds without fertilization or meiosis.
Fruit Formation	Yes (typically results in seedless fruit).	Yes/No (fruit develops, but it encloses a seed).
Seed Formation	No (seedless/stenospermocarpic; seeds are absent or aborted).	Yes (produces viable seeds that are genetically identical to the parent).
Tissue Origin of Embryo	None (No embryo is formed).	Maternal tissues (e.g., unreduced egg cell, nucellus, or integuments).
Meiosis Status	Normal meiosis may occur in ovules/pollen, but fertilization fails or is absent.	Meiosis is bypassed (Apospory/Diplospory) or mitigated; non-recombinant diploid gametes/somatic cells form the embryo.
Hormonal Trigger	High concentrations of Auxins and Gibberellins in the ovary wall/carpel.	Regulated by maternal gene networks and specific epigenetic triggers bypassing double fertilization.
Propagation Type	Vegetative/Fruit development mechanism.	Seed-based asexual reproduction (Agamospermy).
Economic Application	Production of high-value seedless commercial fruits (e.g., banana, pineapple, seedless watermelon).	Fixation and perpetuation of hybrid vigor (heterosis) across successive seed generations.

2. Biological Context: Vegetative Reproduction vs. Seedless Development :

While both parthenocarpy and apomixis represent deviations from typical sexual reproduction, they serve entirely different biological functions and ecological strategies.
The core distinction lies in what the plant is trying to achieve: expanding the ovary to create a fruit (parthenocarpy) versus cloning its genetic material into a viable, dispersible seed (apomixis).

Seedless Fruit Development) Mechanism:

Parthenocarpy refers specifically to the development of the ovary into a fruit without the double fertilization process.
It can occur naturally (via mutation or environmental stress) or artificially via exogenous application of plant growth regulators (auxins, gibberellins).

Evolutionary Perspective:

Seedless fruits generally represent an evolutionary dead end in natural ecosystems because they do not produce viable offspring via seed dispersion.
However, they may attract frugivores that aid in vegetative propagation or survive under human-assisted selection.

Types of Parthenocarpy :

Vegetative Parthenocarpy: Fruit develops without any pollination stimulus (e.g., banana, cucumber).
Stimulative Parthenocarpy: Requires pollination or chemical/hormonal stimulation to trigger ovary growth, but fertilization does not take place or the embryo aborts early (Stenospermocarpy, e.g., seedless grapes).

vegetative or Asexual Seed Reproduction Mechanism:

Apomixis (specifically Agamospermy) mimics sexual seed development but bypasses both meiosis (apomeiosis) and egg fertilization (parthenogenesis of the egg cell or adventitious embryony).

Clonal Seed Production:

The resulting seed contains an embryo that is a genetic clone of the maternal parent plant.

Types of Apomixis :

Gametophytic Apomixis: The embryo sac arises without meiosis either from an unreduced megaspore (Diplospory) or from a somatic cell of the nucellus (Apospory).
Sporophytic Apomixis (Adventitious Embryony): Embryos develop directly from somatic diploid cells of the nucellus or integuments alongside or replacing the sexual embryo sac (e.g., Citrus species).
Evolutionary & Agronomic Value: Apomixis allows plants to rapidly clone well-adapted genotypes through durable seeds, making it a critical focus in agricultural biotechnology for locking in hybrid vigor (heterosis) without segregation in subsequent generations.

Agricultural & Commercial Applications

Parthenocarpy and hormonal manipulation have revolutionized commercial horticulture.
By decoupling fruit set from pollination and fertilization, agricultural producers can overcome climatic constraints, enhance post-harvest quality, and meet consumer demand for seedless produce.

Grapes (Vitis vinifera):

Stenospermocarpic Advantage: Commercial cultivars like 'Thompson Seedless' undergo fertilization, but embryo abortion is induced or enhanced through exogenous GA3 (Gibberellic Acid) sprays.
Berry Enlargement & Cluster Loosening: GA3 application increases individual berry volume by up to 200% and elongates pedicels to prevent cluster overcrowding, reducing fungal pathogen risk (e.g., Botrytis cinerea).

Banana (Musa spp.):

Obligate Triploid Parthenocarpy: Commercial bananas (such as 'Cavendish') are triploid (3n=33). Meiotic irregularities prevent viable seed formation, while high endogenous auxin levels in the ovary wall drive autonomous fruit development.
Yield & Uniformity: Ensures edible pulp filling without hard, inedible seeds, providing a uniform crop suitable for global supply chains.

Watermelon (Citrullus lanatus):

Induced Triploid Sterility: Seedless watermelons are produced by crossing a tetraploid (4n) female parent with a diploid (2n) male parent to yield triploid (3n) seeds.
Enhanced Shelf Life & Texture: Without seed development drawing sugars, triploid watermelons maintain a higher soluble solid concentration (brix level) and firmer flesh texture for extended transport.

Citrus (Citrus spp.):

Self-Incompatibility & Isolation: Varieties like 'Satsuma' mandarin and 'Navel' orange produce parthenocarpic fruit due to high endogenous PGRs combined with ovule sterility or self-incompatibility.
Processing Efficiency: Seedless fruits drastically reduce processing machinery downtime and seed-oil bitterness in juice extraction industries.

📝Pre-University Research-Level Problem Sets

Context 1 : A plant physiologist treated unpollinated tomato (Solanum lycopersicum) ovaries with varying concentrations of exogenous Indole-3-Acetic Acid (IAA) and Gibberellic Acid GA3). The table below records the average pericarp volume (cm3) after 14 days:

Treatment Group	IAA Concentration (μM)	GA₃ Concentration (μM)	Mean Pericarp Volume (cm³)	Seed Status
Control (Unpollinated)	0.0	0.0	0.2	No growth / Abscised
Treatment A	10.0	0.0	3.4	Completely Seedless
Treatment B	0.0	10.0	2.8	Completely Seedless
Treatment C	5.0	5.0	6.1	Completely Seedless
Natural Pollinated (Control)	N/A	N/A	5.8	Normal Seeded

Question:1 Explain the physiological mechanism behind the results observed in Treatment C compared to Treatments A and B. What specific hormonal interaction is demonstrated?

Answer : Synergism. Auxin and Gibberellins act synergistically to drive pericarp expansion. IAA activates proton pumps H+ and ATPase causing acid growth and cell wall loosening, while GA 3 derepress DELLA proteins to promote cell division and elongation. Combined at lower concentrations (5.0 micro M each), they yield a higher growth volume (6.1 Cm3) than either hormone applied individually at higher concentrations (10.0 micro M).

Question : 2 Predict the impact on fruit set if an auxin transport inhibitor (such as NPA / 1-N-naphthylphthalamic acid) is applied to the pedicel during Treatment A. Justify your answer using the concept of polar auxin transport.

Answer : Fruit growth will be severely reduced or the ovary will undergo abscission. NPA blocks PIN-FORMED (PIN) efflux carriers responsible for basipetal/polar auxin transport. This prevents IAA from accumulating in the pericarp and abscission zone, leading to premature ethylene sensitivity and fruit drop.

Context 2 : Cultivated triploid bananas (3n=33) are obligate vegetative parthenocarpic fruits, whereas 'Thompson Seedless' grapes (2n=38) exhibit stenospermocarpy.

Question 1 : Compare the cytological mechanisms leading to seedlessness in 3n bananas versus stenospermocarpic grapes.

Answer : In 3n bananas, seedlessness is driven by meiotic failure during megasporogenesis (unequal chromosome segregation during Anaphase I leading to inviable gametes). In stenospermocarpic grapes, meiosis and double fertilization occur normally, but embryo/endosperm development halts due to targeted embryo abortion.

Question 2 : In stenospermocarpic grapes, fertilization occurs normally. At what embryonic stage does abortion typically occur, and which maternal or embryonic hormone deficiency primarily triggers this arrest?

Answer : Embryo abortion typically occurs at the globular stage, driven by a drop in endogenous gibberellin (GA) and auxin levels supplied by the developing endosperm.

Context 3 : A breeder aims to produce seedless triploid watermelons (3n).

The breeder treats diploid (2n=22) seedlings with colchicine to induce tetraploidy (4n).

The tetraploid plant is used as the seed parent (female), and a diploid plant is used as the pollen parent (male).

Question 1 : State the chromosome number of:

(i) The endosperm cell of the resulting triploid seed before germination.

(ii) The pericarp tissue of the seedless watermelon fruit.

Answer (i) : Endosperm ploidy = 2n (maternal tetraploid egg/polar nuclei )

2n+2n=4n) + 1n (pollen)= 5n = 55 chromosomes.

(ii) Pericarp tissue is maternal diploid/tetraploid tissue belonging to the female parent, so it contains 4n = 44 chromosomes.

Question 2 : Why is the tetraploid plant selected as the female parent rather than the diploid plant in commercial seed production? Describe the physiological anomaly observed in the fruit if the cross is reversed (2n female x 4n male).

Answer 3(b): Using a 4n female parent ensures a larger ovary wall and proper maternal-to-endosperm gene dosage ratio, yielding normal-sized seedless fruits. The reverse cross (2n female x 4n male). results in a high maternal-to-pollen ratio mismatch in the endosperm, producing small, misshapen fruits with thick, hard rinds.

📝Advanced Analytical Case Studies (Global Medical/Research Entry Standards)

Case study: A viticulturist crosses a high-yield seeded table grape cultivar (Vitis vinifera cv. 'Emperor', 2n=38) with a high-sugar stenospermocarpic variety (V. vinifera cv. 'Thompson Seedless', 2n=38) to introduce seedlessness into the high-yield strain.

When 'Thompson Seedless' is used as the maternal parent (♀), fertilization occurs, but ovules stop growing at 20 days post-anthesis (DPA), leading to seed traces rather than viable seeds. To salvage the progeny, the breeder employs in vitro Ovule / Embryo Rescue techniques at 14 DPA.

Pollination (DPA 0) ---> Fertilization ---> Globular Embryo (DPA 10-14) --- >Embryo Abortion at DPA 20]

Question 1 : Why does embryo abortion occur in stenospermocarpic grapes despite successful double fertilization? Specify the biochemical failure of the endosperm.

Answer : Embryo abortion in stenospermocarpic grapes is caused by the premature breakdown or degeneration of the endosperm tissue. Because the endosperm fails to provide essential nutrient mobilization and growth signals (primarily endogenous gibberellins and cytokinins), the developing zygotic embryo starves and arrests at the globular stage.

Question 2 : Which combination of plant growth regulators (PGRs) must be added to the basal Murashige and Skoog (MS) media at 14 DPA to ensure successful development of the isolated globular embryo?

Answer : The media must be supplemented with a low concentration of Auxin (e.g., IAA or NAA) to maintain tissue polarity, a balanced level of Cytokinin (e.g., Benzyladenine / 6-BA) to drive mitotic cell division in the pro-embryo, and a high sucrose concentration (3-5%) as an osmotic stabilizer and carbon source to replace the missing endosperm.

Question 3 : If seedlessness in grapes is primarily controlled by the dominant SdI (Seed Development Inhibitor) locus, predict the phenotypic ratio of seeded to seedless offspring in a cross between a heterozygous stenospermocarpic parent (SdI/sdi) and a true-breeding seeded parent (sdi/sdi).

Answer : Cross: SdI / sdi (Stenospermocarpic) × sdi / sdi (Seeded).

Genotypic Ratio: 1 SdI/sdi : 1 sdi/sdi.

Phenotypic Ratio: 1 Seedless (Stenospermocarpic) : 1 Seeded (50% seedless yield).

📝Knowledge with Understanding (Direct & Recall Questions

Question 1 : Define the following terms precisely in the context of plant developmental biology:

(a) Parthenocarpy

(b) Stenospermocarpy

(d) Agamospermy

Answer : (a) Parthenocarpy: The development of a fruit from an ovary without prior fertilization of the ovules, resulting in seedless fruit.

(b) Stenospermocarpy: A form of seedlessness where pollination and fertilization occur normally, but the embryo aborts during early development, leaving small, soft seed traces.

(c) Acid Growth Hypothesis: The mechanism by which auxin activates plasma membrane H+-ATPase pumps, lowering apoplastic pH to activate expansins, which loosen cell wall microfibrils for cell elongation.

(d) Agamospermy: A form of apomixis where viable seeds are produced asexually directly from maternal tissues without meiosis or fertilization

Question 2: Describe the sequence of hormonal events that occurs in a carpel immediately following successful double fertilization. Contrast this with the hormonal status of an unpollinated carpel that undergoes senescence.

Answer : Fertilized Carpel: Double fertilization triggers a sharp increase in endogenous auxins (IAA) and gibberellins (GAs) synthesized by the developing seed and endosperm. This hormonal burst overrides growth inhibitors, stimulates cell division and cell wall expansion in the pericarp, and suppresses abscission layer formation.

Unpollinated Carpel: In the absence of fertilization, auxin and GA levels remain extremely low. Levels of abscisic acid (ABA) and ethylene increase in the pedicel, triggering cell wall hydrolases in the abscission zone and leading to fruit drop (senescence)

Question 3: Explain why triploid (3n) plant species, such as commercial Musa acuminata (banana), fail to form viable seeds during sexual reproduction. Specify the key event during meiosis where this disruption occurs.

Answer : Triploid (3n) plants possess three homologous sets of chromosomes. During Anaphase I of meiosis in megasporogenesis, the three set sets cannot pair evenly into bivalents. This leads to random, unequal segregation of homologous chromosomes, producing aneuploid, inviable gametes that prevent embryo sac development and functional seed formation.

Question 4: Identify the specific synthetic plant hormone class used commercially for each of the following agricultural objectives:

(a) Increasing individual berry volume and cluster length in table grapes.

(b) Inducing fruit set in greenhouse tomato crops under low ambient temperature conditions.

Answer : (a) Gibberellins: Gibberellic Acid.

(b) Synthetic Auxins: 2,4-D (2,4-Dichlorophenoxyacetic acid) or NAA (Naphthaleneacetic acid).

(c) Auxins: NAA or IBA (Indole-3-butyric acid) applied to pedicels to maintain apical dominance and delay abscission layer activity.

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