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Angiosperm Life Cycle Overview

• Angiosperms undergo a life cycle, involving both sporophyte and gametophyte generations.
• Flowers of mature sporophytes contain the stamen, which is the male part, and the carpel, the female part.

Stamen and Anther Development

• The stamen consists of an anther and a filament.
• A typical stamen is made of a filament attached to an anther composed of four microsporangia.
• Each microsporangium pair is separated by sterile tissue containing a vascular bundle.
• Microsporangium development sequence varies among species.
• The archesporial cells in Arabidopsis undergo meiosis and are surrounded by the epidermis, endothecium, middle layer, and tapetum.
• The floral meristem's three layers:
  • L1 becomes the epidermis.
  • L2 gives rise to archesporial and inner surrounding layers.
• The locule is the central region containing the archesporial cells.

Microsporogenesis

• Microsporogenesis and microgametogenesis are two phases in the development of the male gametophyte or pollen grain.
• During microsporogenesis, archesporial cells differentiate into microsporocytes or pollen mother cells (2n).
• Microsporocytes undergo meiosis, producing a tetrad of haploid microspores (n) joined by cell walls, which are largely composed of callose.
• Tapetum surrounds the locule and secretes:
  • Callase
  • Cell-wall-degrading enzymes into the locule
• The tapetum digests cell walls and separates the tetrad into individual microspores.
• Some insect-pollinated species shed pollen as tetrads, as in the common heather (Calluna vulgaris), or as polyads, like in Acacia.
• Tetrad dissolution is blocked in quartet mutants of Arabidopsis, yet their pollen grains develop normally and are fertile, despite the blocked dissolution.
• Microsporogenesis ends once microspores form within the anther locules and then they start microgametophyte development.

Microgametogenesis

• The haploid microspore develops mitotically into the mature male gametophyte.
• The mature male gametophyte contains a vegetative (or tube) cell and two sperm cells.
• Microspores expand and undergo cell wall biosynthesis, leading to a large vacuole.
• The microspore nucleus migrates to the cell wall, producing a polarized microspore.
• The microspore then undergoes asymmetric cell division resulting in:
  • Vegetative cell
  • Small generative cell (or male germ cell)
• Callose separates the generative cell from the vegetative cell while the generative cell remains attached to the microspore cell wall.
• The callose layer breaks down, and the generative cell is engulfed by the vegetative cell, forming a "cell within a cell" structure or bicellular stage.
• The generative cell takes on an elongated shape, aiding its passage through the dynamic protoplasm of the growing pollen tube.
• Pollen grains accumulate carbohydrate or lipid reserves for metabolism during germination and pollen tube growth.
• Pollen is then released by dehiscence or the opening of the anther wall, and only after landing on a stigma do the generative cells divide to produce two sperm cells.
• Some plants undergo pollen mitosis II while the generative cell is inside the anther (tricellular stage).
• Microgametogenesis ends with two sperm cells' production.
• Many flowering plants produce bicellular pollen and many food crops like rice, wheat and maize produce short-lived, tricellular pollen grains.

Tapetal Cells

• Tapetal cells perform a secretory function and release their contents into the locule undergoing a programmed cell death.
• Depending on the species, tapetal cells either remain alongside the locule periphery or migrate into it, intermingling with developing microspores.
• Tapetal cells that supply essential materials to developing pollen grains defects to the tapetum result.
• Defects can cause abnormal pollen development and decreased fertility.

Microspore Structure

• Microspores or pollen grains are non-motile, unicellular, and haploid.
• Exine:
  • Thick
  • Made of sporopollenin
  • Offers resistance to chemicals.
  • Has smooth or sculptured pattern.
• Sporopollenin has fatty acids and phenylpropanoids and is one of the most resistant biopolymers known.
• The exine is not evenly distributed over the pollen grain’s surface regions with reduced or missing sporopollenin form apertures.
• Germ pores are used as sites for growing pollen tubes.
• Intine is within the exine
  • Made of cellulose and pectin
• The pollen wall and its coatings isolate, protect, and mediate complex communication.
• Sporoderm:
  • Outer layer of exine
  • Inner layer of intine

Angiosperm Female Reproduction

• The carpel is the unit or building block of the gynoecium.
• The carpel is a modified leaf that contains:
  • Ovary (swollen base containing ovules)
  • Style (a stalk connects stigma to ovary)
  • Stigma (sticky part which receives pollen)
• The pistil contains one carpel or multiple fused carpels.

Ovary, Septum, and Placenta

• The gynoecium of Arabidopsis consists of two carpels, or valves, separated by a medial partition called the septum.
• The valves and septum connect to the replum controlling dry fruit dehiscence.
• Two strips of placental tissue associate with the septum on either side of the gynoecium.
• Ovule primordia arise in ovary tissues called the placenta.
• Marginal placental tissue types consist of:
  • Parietal
  • Axile
  • Basal
  • Free-central

Ovule Structure

• Integuments encompass the Nucellus
• Nucellus is a mass of diploid cells that is:
  • Located in the center of the ovule of parenchyma cells.
  • Nutritive and surrounds embryo sac.
• The Central core of the ovule is also known as the embryo sac in angiosperms
• The Embryo sac has seven cells, containing egg cells
• Micropyle:
  • Small opening at the top of the ovule
  • A pore is created when the integuments meet that allows the pollen tube to enter.
• Chalaza is the basal part of the ovule, uniting the integuments and nucellus
• A Small stalk connected to the placenta is a funicle
• The point of attachment where the funicle connects to the body of the ovule is known as the hilum.

Female Gametophyte Development

• Development of the female gametophyte or embryo sac, is more complex compared to the mal e gametophyte.
• There are over 15 variations of embryo sac development in angiosperms.
• Polygonum-type is a highly common embryo sac development.
• The archesporial cell differentiates into the megaspore mother cell inside the nucellus
• Visible because of cytoplasm and large cells.

Polygonum-type Embryo Sac Development

• Meiosis of the diploid megaspore mother cell produces 4 haploid megaspores.
• Three megaspores undergo programmed cell death at the micropelar end leaving one functional megaspore.
• These functional megaspores undergo three rounds of free nuclear mitotic divisions (mitoses without cytokinesis) forming a syncytium cell.
• The syncytium multinucleate matures into an eight-nucleate, immature embryo sac.
• Four nuclei migrate towards the chalazal and four migrate to the micropylar pole.
• Three nuclei at the chalazal and micropylar poles, undergo cellularization.
• The remaining two nuclei, polar nuclei, migrate to the central region of the embryo sac containing a large vacuole.
• Plasma membrane and cell wall develop in this part of the cytoplasm and the polar nuclei giving rise to a binucleate.
• Embryo sac matures the female gametophyte or embryo sac.
• Polygonum-type embryo sac consists of seven cells and eight nuclei at maturity.
• Three cells that the chalazal cells become antipodal cells
• Many species the antipodal degrade prior to fertilization having no real part, but in other ultrastructural studies prove it provides in signaling.
• At the micropylar cells are:
  • Egg cells of the female gamete
  • Two synergid cells which make the egg apparatus that contributes to the cells of a middle part called the central cell
  • Has a binucleate

Hormonal Signaling in Embryo Sac Development

• Roles of auxin, cytokinin, and brassinosteroids in female gametophyte development steps have been implicated in plant growth factors.
• Having a role in auxin is having cell fate in female gametophytes
• Synthesized cytokinin in the chalazal and if the plant did not have a cytokinin response because of mutation of the cytokinin production or cytokinin receptor fails to develop some megaspores.
• Brassinosteroids shows that cells of megaspores have initiation of mitosis

Pollination in Angiosperms

• Pollination relies on delivering pollen grains from the anther to the stigma,
• Pollination can be self pollination
  • occurs between the pollen and the stigma are from the same plant material.
• Cross pollination is between a male and female that are separate.
• Factors that influence this:
  • Ambient temperature
  • Timing
  • Receptivity of a compatible flower

Pollen Grains

• Many pollen grains resists heat or dryness. But if such as a tomato, they are in the correct environments when they are exposed and transferred.
• Understanding its tolerance can ensure food supply is safe at tolerating climate changes.

Pollen Tube Delivery

• Six Phases:

1. The pollen grain has to adhere
2. The pollen grain has to invade which it grows in the stigma, style, and ovary.
3. Grown in the nutrients needed.
4. Exits near an ovule.
5. Grows on the septum surface and enters the micropyle.
6. Grows through the micropyle and discharges sperm cells.

Pollen Adhesion

• Adhesion relies on recognition of pollens and stigmas.
  • Proteins and surface make a flower adhere to it.
  • Others are rejected if not compatible.
• Flowers have to have a:
  • Wet stigmas that have proteins, carbohydrates, etc.
  • Dry stigma have a protein in which hydration depends on.
• After landing the stigma and proteins come together form the "foot" structure:
• Has lipids that reorganize and bring water leading to their germination.
• Lipids are crucial to pollen hydration. Mutants in Arabidopsis, without a pollen coat, could be rescued either by high applications of certain lipids
• Influx of calcium kicks off pollen tube formation
• Calcium kicks starts reorganized leading to polarization
• To the germination pore occurs Cytosolic and elevated concentrations through actin filaments
• Grow in tip growth where the regulating tips grow and helps the signals

Fertilization

• Pollen tube releases chemicals that is senses to trigger
• Micropyle
• Embryo
• Synergid cells
• Live imaging of florescent, in a sperm cell
• Breaks in synergids, pollen stops, and breaks synergid
• Sperm releases for 7 seconds in boundary region.
• Has a protein on both sides for cell, facilitating union

Embryogenesis

• Single is cell transferred into multi cell
• Happens only in ovule, the flower pistil
• During embryogenesis, cells make vascular systems
• Forms apical meristem to enable root and shoot developments
• Changes make it more resistant and goes through embryogenesis between eudicots and single cell.
• Stages of division by what they undergo
• 5 cells with 5 main stages.

Arabidopsis Embryogenesis Stages

1. Zygotic: Egg and sperm fuse to single-celled zygote.
2. Globular: The eight-celled, creates division to create the protoderm for what the epidermis becomes.
3. Heart: Division in what shoots apical mesmisters become
4. Torpedo: Distinctions through what they are made up of
5. Divisions of cells give embryo for more functions.

• Long vacuoles and helps suspension.
• A smaller cell is produced where all parts such from.
• Tier gives to the SAM what cotleydons make
• Low tier which:
  • Embryonic
  • Root
  • Some stem
• The blue is when they create
• Apical is known what the embryo has
• Helps form to the cap.