Pollen-pistil interaction is a specified, complex process determining fertilization efficiency in flowering plants. This process involves coordinated physical, biochemical, and molecular events between the pollen grain and pistil.
This interaction is meant for species specificity, and it favors double fertilization, which is one of the characteristic features of angiosperms. This is important for the survival of plant species and has consequences for agriculture and ecosystem balance.
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Importance of pollen pistil interaction
- It facilitates successful sexual reproduction in plants.
- It ensures genetic diversity and prevents inbreeding.
- It impacts seed and fruit formation, thus influencing agricultural activity.
- It gives a better understanding of evolutionary changes in flowering plants.
Pollen Structure
A pollen grain consists of an outer layer exine, inner layer intine, vegetative cell, and generative cell.
Exine – It is the outermost layer and is highly resistant to environmental disturbances. It is made up of sporopollenin and consists of two sub-layers, sexine and nexine which are separated from each other by columella. This columella prevents the collapsing of the pollen wall during the loss of water. The exine of pollen is preserved in fossils.
Intine – The inner layer is, however the intine of the pollen wall which is located underneath the exine. The intine contains cellulose and pectin; hence, it is softer and more flexible as compared to the exine.
Intine has a less complex ornamentation in comparison with the exine and, therefore is relatively smooth and uniform. The intine is thus critical for pollen grain survival during germination. When the pollen comes to a compatible stigma, the intine contributes to the formation of the pollen tube, which allows for the transfer of male gametes to the ovule. It also contains enzymes that help in breaking down the surface of the stigma, allowing penetration and growth of the pollen tube.
Internally, pollen grain consists of vegetative and generative cells.
Stages in pollen pistil interaction
Pollen capture and adhesion– The pollen must adhere effectively to the stigmatic surface. A wide variety of stigmatic surfaces and the presence or absence of exudates can affect pollen adhesion.
In the case of wet stigma, in a genus like Lilium, the pollen capture is less specific and the stigmatic surface secretes exudates whereas in the case of dry stigma, in a genus like Brassica, the stigmatic surface does not secrete exudates and is highly pollen-specific.
Wet stigma–
The pollen adhesion is a passive process with low specificity in the case of wet stigma, so any pollen can get trapped due to surface tension in the stigmatic surface. The secretions vary between different taxonomic families.
In the case of the family Solanaceae, the stigmatic secretion is mainly lipidic, whereas in Liliaceae, the stigmatic secretions are aqueous and rich in carbohydrates. The release of pollen secretions in this case is often mediated by peroxidase and esters and is facilitated by insect activities.
Dry stigma–
In the case of a dry stigmatic surface, pollen adhesion is a highly specific and active process. Here the adhesion takes place with the help of a pollen coat that forms the foot on the stigmatic surface.
S locus glycoprotein SLG and S locus-related protein SLR of stigma and Pollen Coat Proteins PCP of pollen interact with each other to facilitate Pollen-stigma signal transduction, germination initiation, and hydrolysis of stigma wall.
Pollen hydration– The hydration of compatible pollen, which is a prerequisite for pollen activation and germination, is highly regulated.
In plants with wet stigma, the exudates released by the secretory zone of the stigma promote hydration and germination of pollen. It has been demonstrated that lipids in wet stigma exudates are necessary for the penetration of pollen tubes into the stigma.
In plants with dry stigma, the pollen coat performs the function of exudates in wet stigma. Like wet stigma in dry stigma, lipids in the pollen coat are needed for the germination of the pollen.
Hydration of pollen leads to physiological activation of pollen cytoplasm and subsequent germination of pollen. The pollinated stigmatic papillae also show subsequent rises in the level of calcium ions.
Pollen germination– Hydration of the intine layer that bursts the exine and this is the first event that has been associated with germination of the growth process of pollen tubes. The surface of the stigma releases Boron and Calcium, critical nutrients absent in pollen that become essential for germination.
Vegetative cell along with exine forms the tube in the pistil. The whole cytoplasmic contents are transferred into the tip of the tube creating a vacuole at the remaining portion of the tube. Callose plugs are formed at periodical intervals within the tubes to limit the cytoplasm at the tip of the tube.
The pollen tube is three-layered with an outer pectin stratum and an inner pecto-cellulosic stratum. The pollen tube grows into the intercellular gaps of the style. The next course of the tube is determined by the different characteristic features of the style.
Entry of pollen tube into ovule- Following a successful passage through the style into the ovary, the pollen tube approaches the ovule. The pollen tube may gain entry into the ovule through one of the three pathways depending on the type of plant species-
Porogamy: Here, the pollen tube penetrates the ovule by entering the micropyle, which is the smallest opening in the ovule. This is the most common method among angiosperms.
Chalazogamy: The pollen tube enters through the chalaza, which is the region opposite the micropyle.
Mesogamy: the pollen tube enters through the integuments or the middle region of the ovule.
In most plants, the main method of entry is through porogamy. The pollen tube, once it has approached the vicinity of the ovule, is directed by the chemical signals originating from the synergid cells, which lie next to the egg cell in the embryo sac.
These guide the tube exactly towards the micropyle. As soon as it reaches the micropyle, the pollen tube breaks into the nucellus tissue and penetrates the embryo sac.
Entry of pollen into embryo sac– The pollen tube entry into the embryo sac is highly coordinated, the most crucial step involved in the process of flowering plant fertilization. While it has been able to pierce through the micropyle to reach into the ovule in the case of most plants, further growth inside the nucellus leads toward the embryo sac. A key role here is taken up by the chemical signalings such as LURE peptides and many attractant molecules secreted from the synergid cells of the embryo sac.
The pollen tube moves along the direction of one of the two synergid cells, usually the closer to the micropyle. The synergid cell relaxes its cell wall to let the entry of the pollen tube. When the pollen tube has entered the synergid cell, it ruptures, releasing its contents into the embryo sac. The contents that are released include two male gametes (sperm cells) and the cytoplasmic material that is associated with them.
Fertilization- The egg cell fuses with one of the two male gametes to become the zygote that develops to give the embryo. The other of the two male gametes fuses with the central cell containing containing two polar nuclei, forming the triploid endosperm, which will nourish the embryo. A synergid cell facilitates the entry of the pollen tube bursting and delivering the gametes; once it has performed its role in fertilization, the synergid cell degrades.
The mechanism known as double fertilization occurs only in flowering plants or angiosperms and involves effective coordination of embryogenesis with endosperm development to ensure that resources for seed development are efficiently used.
Strategies to ensure successful fertilization
Plants have developed several strategies to ensure successful fertilization which are as follows-
1. Chemical Signalling: The pistil releases some chemicals that attract and direct compatible pollen tubes. Attractant peptide from LURE present in the ovule guides the pollen tube to direct navigation.
2. Self-Incompatibility: The majority of plants depend on genetic controls to prevent acceptance of self-pollen or incompatible pollen.
3. Pollen Tube Guidance: Cells within the embryo sac such as synergids, release attractants that point the pollen tube toward the ovule.
4. Floral Adaptations: Specific features such as flower color, shape, and nectar production attract specific pollinators and increase the effectiveness of pollination.
5. Selective Pollen Tube Growth: Only the most viable and compatible pollen tubes are permitted to reach the ovules to optimize fertilization success.
Role of pollen tube in fertilization
Pollen tube growth is essential for the delivery of male gametes to the ovule.
Once a compatible pollen grain has landed upon the stigma, it germinates into an elongated tube to begin growing through the style into the ovary.
Such a tube serves like a channel, carrying along within it two sperm cells, toward the embryo sac; the growth is sharply and accurately guided by chemical signals released by the cells within the ovule.
The pollen tube ruptures upon reaching the embryo sac, and from there, the sperm cells are released for double fertilization. One fuses with the egg cell, forming a zygote, while the other unites with the central cell to produce the triploid endosperm.
Successful fertilization and seed development depend on the controlled and directional growth of the pollen tube.
Strategies to avoid self-pollination
Plants have come up with various mechanisms to prevent self-pollination, thus ensuring genetic diversity in the following ways:
1. Self-Incompatibility: These are molecular recognition systems that are present in the stigma or style, and reject the pollen from the same plant.
2. Temporal Separation (Dichogamy): The male and female reproductive parts mature at different times.
3. Spatial Separation (Herkogamy): By creating a physical distance between anthers and stigma to reduce the chances of self-pollination.
4. Unisexual Flowers: Some plants produce separate male and female flowers so that they must cross-pollinate thus preventing self-pollination.
5. Genetic Sterility: Although self-pollens are compatible, some genetic mechanisms prevent fertilization of the ovule by its own pollen.
Recent findings
Some recent findings have emerged on the interaction between pollen and pistil:
1. LURE Peptides: Attractant molecules that are secreted by synergids and are species-specific, which are involved in the growth of pollen tubes, have been identified and are known as LURE peptides. They are cysteine-rich peptides.
2. S-Locus Genes: These genes are the best-understood regulators of self-incompatibility at the molecular level.
3. Exosome-like Vesicles: Nano-vesicles in the pistil facilitate the movement of signaling molecules to the pollen tube.
4. Calcium Signaling: The mechanisms of calcium ions regulating the growth and navigation of the pollen tube have been elucidated.
5. Role of Synergid Cells: Advances in imaging techniques have emphasized the role of synergid cells in guiding and facilitating pollen tube entry.
Pollen-pistil interaction affecting crop production
Efficient pollen-pistil interaction is essential for crop productivity because it ensures the realization of high fertilization rates and quality seed sets. These interactions improve compatibility by guiding the growth of pollen tubes and preventing selfing, which directly influences crop yields in cross-pollinated crops. Gaining insights into the molecular machinery underlying these interactions has promoted innovations in hybrid seed production and improved artificial pollination techniques. In its broad essence, knowledge supports breeding programs by eliminating incompatibility barriers and enhancing crop robustness with adverse stress factors for environmental stresses-precisely addressing food-security challenges within a changing climate.
Conclusion
Interaction between pollen and pistil is a basic factor of plant reproduction that provides the success of fertilization and seed development. The complexity of this process includes molecular signaling, compatibility recognition, and guided growth of pollen tubes. Advances in this knowledge not only extend our knowledge of plant biology but also provide practical uses in agriculture, improving crop yields, ensuring food security, and producing high-yielding hybrid varieties. Further studies of pollen-pistil dynamics will help solve world challenges on biodiversity and agriculture.
References
- Dresselhaus, T., & Franklin-Tong, V. (2013). Receptor-like kinases in pollen-pistil interactions. Plant Journal.
- Higashiyama, T., & Yang, W. (2017). Chemical signaling in pollen tube guidance. Current Opinion in Plant Biology.
- Chapman, L. A., & Goring, D. R. (2010). Pollen-pistil interactions regulating successful fertilization in the Brassicaceae. Journal of Experimental Botany.
- Johnson, M. A., & Preuss, D. (2002). Mechanisms of pollen tube guidance in plants. Annual Review of Plant Biology.
- POLLEN GERMINATION. (2022, March 31). Unacademy. https://unacademy.com/content/neet-ug/study-material/biology/pollen-germination/
- Bharamappanavara, M. (2021). Pollen pistil interactions. ResearchGate. https://www.researchgate.net/publication/355058408_Pollen_Pistil_Interactions
- Vedantu. (n.d.). Pollen grains. VEDANTU. https://www.vedantu.com/biology/pollen-grains
- Wheeler, M. J., Franklin‐Tong, V. E., & Franklin, F. C. H. (2001). The molecular and genetic basis of pollen–pistil interactions. New Phytologist, 151(3), 565–584. https://doi.org/10.1046/j.0028-646x.2001.00229.x
- G, S. (2016, November 28). Pollen-Pistil Interaction and its Significance. Biology Discussion. https://www.biologydiscussion.com/palynology/pollen-pistil-interaction-and-its-significance/64518
- Pollen pistil interaction.pptx. (2023, March 1). [Slide show]. SlideShare. https://www.slideshare.net/slideshow/pollen-pistill-interactionpptx/256181742
