7.1 Pollination and fertilization

Pollination and fertilization are crucial processes in plant reproduction. These mechanisms allow plants to create genetic diversity and produce offspring. From the intricate structures of flowers to the complex interactions between plants and pollinators, this topic explores the fascinating world of plant sexual reproduction.

The journey from pollen grain to seed involves multiple steps, including pollination, pollen tube growth, and double fertilization. This process highlights the remarkable adaptations plants have developed to ensure successful reproduction in diverse environments. Understanding these concepts is key to grasping plant biology and ecology.

Flower structure and function

• Flowers are the reproductive structures of angiosperms (flowering plants) that facilitate sexual reproduction
• The main function of flowers is to produce male and female gametes and provide a platform for pollination and fertilization
• Flower structure varies widely among species but typically consists of sepals, petals, stamens (male organs), and carpels (female organs)

Male reproductive organs

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• The stamen is the male reproductive organ of a flower which produces pollen grains
• Each stamen consists of a filament (stalk) and an anther, which contains microsporangia (pollen sacs)
• The number and arrangement of stamens vary among species (e.g., monadelphous stamens in Malvaceae, didynamous stamens in Lamiaceae)
• Pollen grains contain the male genetic material (sperm cells) necessary for fertilization

Female reproductive organs

• The carpel is the female reproductive organ of a flower which produces ovules and provides a site for pollen reception and germination
• A carpel consists of a stigma (pollen receptive surface), style (elongated neck), and ovary (enlarged base containing ovules)
• Carpels can be either free (apocarpous) or fused (syncarpous) to form a single pistil
• The ovary contains one or more ovules, each enclosing an embryo sac with the female gamete (egg cell)

Nectaries and other attractants

• Many flowers possess specialized structures that attract pollinators, such as nectaries, scent glands, and visual cues
• Nectaries are glands that secrete sugary nectar, which serves as a food reward for pollinators (e.g., floral nectaries in Brassicaceae, extrafloral nectaries in Fabaceae)
• Scent glands produce volatile organic compounds that attract pollinators by olfactory cues (e.g., osmophores in Orchidaceae)
• Visual cues such as petal color, shape, and size can also attract pollinators (e.g., UV patterns in Asteraceae, mimicry in Ophrys orchids)

Pollination mechanisms

• Pollination is the transfer of pollen grains from the anther to the stigma, which is a prerequisite for fertilization
• Pollination mechanisms can be broadly classified into self-pollination and cross-pollination, depending on the source of pollen
• Flowers can be pollinated by abiotic agents (wind, water) or biotic agents (animals), each with specific adaptations

Self-pollination vs cross-pollination

• Self-pollination occurs when pollen is transferred from the anther to the stigma of the same flower or between flowers on the same plant
• Self-pollination can be advantageous in stable environments or when pollinators are scarce but may lead to inbreeding depression
• Cross-pollination involves the transfer of pollen between flowers of different plants, which promotes genetic diversity and hybrid vigor
• Many flowers have evolved mechanisms to promote cross-pollination, such as self-incompatibility, dichogamy (temporal separation of male and female functions), and herkogamy (spatial separation of anthers and stigmas)

Wind pollination

• Wind pollination (anemophily) is a form of pollination where pollen is carried by wind currents from one flower to another
• Wind-pollinated flowers are typically small, inconspicuous, and lack showy petals or nectaries (e.g., grasses, sedges, and many trees)
• Wind-pollinated plants produce large quantities of lightweight, dry pollen grains that are easily dispersed by wind
• The stigmas of wind-pollinated flowers are often large, feathery, or sticky to efficiently capture airborne pollen

Animal pollination

• Animal pollination (zoophily) involves the transfer of pollen by animals, such as insects, birds, bats, and other vertebrates
• Animal-pollinated flowers have evolved various adaptations to attract specific pollinators, such as showy petals, scent, nectar rewards, and specialized shapes
• Insects (particularly bees, butterflies, and moths) are the most common animal pollinators, accounting for the pollination of nearly 75% of all flowering plants
• Birds (e.g., hummingbirds, sunbirds), bats, and other mammals (e.g., rodents, primates) are also important pollinators in some ecosystems

Pollinator adaptations and syndromes

• Pollinators have evolved various adaptations to efficiently locate and exploit floral resources, such as specialized mouthparts, sensory abilities, and foraging behaviors
• Bees have branched body hairs and pollen baskets (corbiculae) on their hind legs to collect and transport pollen, as well as long tongues (proboscises) to access nectar
• Butterflies and moths have long, coiled proboscises to reach nectar in tubular flowers, and some (e.g., Sphingidae) can hover while feeding
• Hummingbirds have long, slender bills and tongues to probe flowers for nectar, as well as the ability to hover and fly backwards
• Pollination syndromes are suites of floral traits that have co-evolved with specific groups of pollinators (e.g., bee-pollinated flowers are typically blue or yellow, with a landing platform and nectar guides)

Pollen structure and development

• Pollen grains are the male gametophytes of seed plants, which develop within the anthers and give rise to sperm cells
• Pollen structure and development are crucial for successful pollination and fertilization, as well as for plant reproduction and evolution
• The process of pollen formation involves two main stages: microsporogenesis (formation of microspores) and microgametogenesis (development of male gametophytes)

Microsporogenesis and microgametogenesis

• Microsporogenesis occurs within the microsporangia of anthers and involves the formation of microspores through meiosis
• Each diploid microsporocyte (pollen mother cell) undergoes meiosis to produce four haploid microspores, which are arranged in a tetrahedral tetrad
• The microspores then separate and develop into pollen grains through a process called microgametogenesis
• During microgametogenesis, each microspore undergoes mitosis to produce a generative cell and a vegetative cell, which constitute the mature pollen grain
• The generative cell further divides to form two sperm cells, either within the pollen grain or after pollination during pollen tube growth

Pollen wall composition

• The pollen wall is a complex structure that protects the male gametophyte and plays a crucial role in pollen dispersal, recognition, and germination
• The pollen wall consists of two main layers: an outer exine and an inner intine
• The exine is composed of sporopollenin, a highly resistant biopolymer that can withstand harsh environmental conditions and enzymatic degradation
• The exine often has a sculptured surface with various patterns (e.g., reticulate, echinate) that are species-specific and can be used for taxonomic identification
• The intine is a thin, cellulosic layer that lies beneath the exine and is involved in pollen tube germination and growth

Pollen viability and longevity

• Pollen viability refers to the ability of pollen grains to germinate and produce functional male gametes
• Pollen viability can be affected by various factors, such as environmental conditions (temperature, humidity), nutrient availability, and genetic factors
• The longevity of pollen grains varies among species, ranging from a few hours to several months or even years in some cases
• Pollen longevity is influenced by storage conditions (temperature, humidity) and the presence of protective compounds (e.g., polyamines, antioxidants)
• Assessing pollen viability and longevity is important for plant breeding, conservation, and studies of plant reproductive biology

Pollination process

• The pollination process involves a series of steps that lead to the transfer of pollen grains from the anther to the stigma, setting the stage for fertilization
• Successful pollination requires coordination between pollen dispersal, pollen-pistil interactions, and pollen tube growth
• The pollination process is influenced by various factors, such as the timing of pollen release, the receptivity of the stigma, and the compatibility between pollen and pistil

Pollen dispersal and transfer

• Pollen dispersal is the first step in the pollination process, which involves the release of pollen grains from the anther and their transport to the stigma
• Pollen dispersal can be mediated by abiotic factors (wind, water) or biotic factors (animals), depending on the pollination syndrome of the plant species
• In wind-pollinated plants, pollen grains are typically small, light, and produced in large quantities to increase the chances of reaching a receptive stigma
• In animal-pollinated plants, pollen grains are often larger, sticky, and produced in smaller quantities, as they are directly transferred by pollinators
• Pollen transfer occurs when pollen grains are deposited on the stigma, either by direct contact (e.g., insect pollination) or by airborne deposition (e.g., wind pollination)

Pollen-pistil interactions

• Once pollen grains land on the stigma, they interact with the pistil tissues in a series of recognition and signaling events that determine the compatibility between pollen and pistil
• The stigma surface can be dry or wet, with specific proteins and lipids that facilitate pollen adhesion and hydration
• Pollen recognition involves interactions between pollen coat proteins and stigmatic receptors, which trigger a cascade of signaling events that lead to pollen acceptance or rejection
• In compatible pollinations, the pollen grain germinates and produces a pollen tube that grows through the stigma and style towards the ovary
• In incompatible pollinations (e.g., self-incompatibility, interspecific incompatibility), pollen germination and/or pollen tube growth are inhibited, preventing fertilization

Pollen tube growth and guidance

• Pollen tube growth is a highly regulated process that involves the polarized extension of the pollen tube through the pistil tissues towards the ovule
• The pollen tube is a specialized structure that emerges from the germinating pollen grain and contains the male gametes (sperm cells)
• Pollen tube growth is driven by the deposition of new cell wall materials at the tip and the directional transport of secretory vesicles containing growth-promoting factors
• Pollen tube guidance involves a complex interplay of chemical, mechanical, and electrical cues that direct the pollen tube towards the ovule
• Ovular guidance cues, such as chemotropic signals (e.g., LURE peptides) and calcium gradients, attract the pollen tube towards the micropyle of the ovule
• Successful pollen tube growth and guidance are essential for delivering the sperm cells to the embryo sac, where double fertilization takes place

Double fertilization in angiosperms

• Double fertilization is a unique feature of angiosperms (flowering plants) that involves the fusion of two sperm cells with two female gametes, resulting in the formation of the zygote and the endosperm
• The process of double fertilization is a key innovation in angiosperm evolution, as it provides nutrients for the developing embryo and influences seed development
• Double fertilization occurs within the embryo sac of the ovule, following the entry of the pollen tube and the release of sperm cells

Pollen tube entry into ovule

• Once the pollen tube reaches the ovule, it enters through the micropyle (a small opening in the integuments) and penetrates the embryo sac
• The pollen tube grows towards the synergid cells, which are specialized cells located at the micropylar end of the embryo sac
• The synergid cells secrete attractants and guidance cues that direct the pollen tube towards the egg apparatus (egg cell and synergids)
• Upon contact with the synergids, the pollen tube ruptures and releases the two sperm cells into the embryo sac

Syngamy and triple fusion

• Syngamy is the fusion of one sperm cell with the egg cell, which gives rise to the diploid zygote that develops into the embryo
• The second sperm cell fuses with the two polar nuclei of the central cell, a process called triple fusion, which gives rise to the triploid endosperm
• The endosperm is a nutritive tissue that supports the growth and development of the embryo, similar to the placenta in mammals
• The formation of the endosperm through triple fusion is a unique characteristic of angiosperms and plays a crucial role in seed development

Development of zygote and endosperm

• Following fertilization, the zygote undergoes a series of mitotic divisions and differentiation events to form the embryo
• The first division of the zygote is typically asymmetric, giving rise to a small apical cell (which forms the embryo proper) and a large basal cell (which forms the suspensor)
• The embryo develops through several stages, such as globular, heart, and torpedo stages, which are characterized by specific patterns of cell division and differentiation
• The endosperm also undergoes rapid cell division and differentiation, forming a nutrient-rich tissue that supports embryo growth
• In most angiosperms, the endosperm is consumed by the developing embryo, leaving only a thin layer of endosperm in the mature seed (e.g., cereals)
• In some species, the endosperm persists in the mature seed as a storage tissue (e.g., coconut) or as a source of secondary metabolites (e.g., coffee, cocoa)

Post-pollination events

• Post-pollination events refer to the changes that occur in the flower and ovary following successful pollination and fertilization
• These events involve the development of the ovary into a fruit, the maturation of ovules into seeds, and the preparation for seed dispersal
• Post-pollination events are regulated by a complex network of hormonal and genetic factors that coordinate the growth and differentiation of reproductive tissues

Ovary and ovule changes

• After fertilization, the ovary undergoes significant changes in size, shape, and texture as it develops into a fruit
• The ovary wall (pericarp) differentiates into distinct layers, such as the exocarp (outer layer), mesocarp (middle layer), and endocarp (inner layer), which contribute to fruit texture and protection
• The ovules within the ovary develop into seeds, which contain the embryo, endosperm (in most cases), and protective seed coats
• The integuments of the ovule give rise to the seed coat (testa), which can be hard, smooth, or ornamented, depending on the species
• The growth and development of the ovary and ovules are regulated by hormones, such as auxins, gibberellins, and cytokinins, which are produced by the developing seeds and surrounding tissues

Fruit and seed development

• Fruit development involves the coordinated growth and differentiation of the ovary tissues, as well as the accumulation of storage compounds (e.g., sugars, starch, oils) in the pericarp or seeds
• Fruits can be classified as dry or fleshy, depending on the water content and texture of the pericarp at maturity
• Dry fruits, such as capsules, legumes, and nuts, have a dry pericarp that may split open at maturity to release the seeds
• Fleshy fruits, such as berries, drupes, and pomes, have a succulent pericarp that attracts animals for seed dispersal
• Seed development involves the accumulation of storage reserves (e.g., proteins, lipids, carbohydrates) in the endosperm or cotyledons, which support embryo growth and germination
• The seed coat also undergoes differentiation and may develop specific features, such as a hard testa, a waxy cuticle, or a mucilaginous layer, which aid in seed protection and dispersal

Seed dispersal mechanisms

• Seed dispersal is the process by which seeds are transported away from the parent plant to colonize new habitats and reduce competition
• Plants have evolved a wide range of seed dispersal mechanisms, which can be classified based on the dispersal agent (e.g., wind, water, animals) or the morphological adaptations of the seeds or fruits
• Wind dispersal (anemochory) is common in plants with small, lightweight seeds or fruits that have wings, plumes, or parachute-like structures (e.g., dandelion, maple, ash)
• Water dispersal (hydrochory) occurs in plants with buoyant seeds or fruits that can float on water currents (e.g., coconut, water lily, mangrove)
• Animal dispersal (zoochory) involves the transport of seeds or fruits by animals, either externally (epizoochory) or internally (endozoochory)
• Epizoochory occurs when seeds or fruits attach to the fur, feathers, or skin of animals using hooks, barbs, or sticky substances (e.g., burdock, cocklebur)
• Endozoochory involves the ingestion of seeds or fruits by animals, which are later dispersed in the feces (e.g., berries, figs, nuts)

Pollination ecology

• Pollination ecology is the study of the interactions between plants and their pollinators, as well as the ecological and evolutionary consequences of these interactions
• Pollination ecology encompasses a wide range of topics, such as plant-pollinator coevolution, pollinator diversity and conservation, and the effects of environmental changes on pollination systems
• Understanding pollination