Chapter 38: Angiosperm Reproduction and Biotechnology Flashcards
Key Features of Angiosperms
Flowers, Double Fertilization, and Fruits, dominant sporophytes
Flowers of Angiosperms
The reproductive shoots of angiosperms that contain four types of floral organs: carpels (first/innermost whorl), stamens, petals, and sepals (last/outermost whorl)- all of which are attached to a part of the stem called the receptacles. Flowers are determinate shoots that cease growing after the flower and fruit is formed.
Carpel (reproductive organ of flowers)
Contains the ovary at its base, followed by a long, slender stalk called the style, at the top of which is a sticky structure that captures pollen known as the stigma. Within the ovary, ovule are found that become seeds when fertilized. A flower may have one or more carpels and the number of ovules depends on species . Carpels may fuse to produce a single structure containing one ovary with 2 or more chambers, each of which has one or more ovules.
Pistil
refers to a single carpel or two or more fused carpels
Stamen (reproductive structure of a flower)
Consists of a stalk called the filament and a terminal structure called the anther, within which are chambers called microsporangia which produce pollen.
Petals and Sepals
Petals advertise the flower to pollinators and are more brightly colored. Sepal protect the flower buds and resemble leaves.
Complete Flowers/ Incomplete flowers
Complete Flowers are those that have all four basic organs while incomplete flowers lack one or more of these 4 organs. Incomplete flowers may be sterile or unisexual as they lack either the stamen or the carpel.
Inflorescences
Showy clusters of flowers. Example: Sunflowers are inflorescences consisting of a central disk surrounded by sterile and incomplete flowers.
Gametophyte development in Angiosperms
They are microscopic and are the smallest gametophytes out of those of all plants. They are highly reduced and dependant on sporophyte for nutrients.
Development of a Female Gametophyte (Embryo sac)
embryo sac is a female gametophyte that develops within each ovule in a tissue called the megasporangium. Megasporocyte or the megaspore mother cell undergoes meiosis to produce 4 megaspores, only one of which survives. Then, the nucleus of the surviving megaspore undergoes mitosis 3 times without cytokinesis to produce a multinucleate structure, which is then divided by membranes to form the embryo sac. Cell fates of nuclei is determined by a gradient of the hormone Auxin that originates near the micropyle.
2 integuments
2 layers of integuments, tissues that will become the seed coat, surround the megasporangium within the ovule except at a gap called the micropyle.
Synergids (2)
2 cells that are at the micropyle edge of the embryo sac and that flank the egg and help attract and guide the pollen tube to the embryo sac
Antipodal cells (3)
3 antipodal cells are at the opposite end and their functions are unknown.
Polar nuclei (2)
2 nuclei that share the cytoplasm of the large central cell of the embryo sac
One egg cell
haploid and has one nuclei
Nuclei and Cells within the embryo sac
Embryo sac contains 8 nuclei within 7 cells. Eventually, the ovule, which will become the seed, consists of the embryo sac within the megasporangium (which will wither), all of which is surrounded by 2 integuments.
Development of male gametophyte in pollen grains
Each anther contains 4 microsporangia, each of which is called a pollen sac.
microsporocytes
Diploid cells within the microsporangia in the pollen grain that undergo meiosis to produce 4 microspores, each of which gives rise to a male gametophyte containing 2 cells through mitosis.
generative cell and tube cell and spore wall
Generative and tube cells are the 2 cells of a male gametophyte that, along with the spore wall, form a pollen grain. During maturation, generative cell passes into the tube cell. Spore wall has material produced from the anther and the microspore.
pollination
When a pollen grain is transferred to a receptive surface of a stigma.
pollen tube
a long cellular protuberance that delivers sperm to the female gametophyte. The generative cell´s nuclei divided by mitosis as it passes through the pollen tube to produce 2 sperm cells. Tube nucleus guides the sperm towards the micropyle as the pollen tube is attracted to the chemical signals released by the synergids. One of the synergids die when the pollen tube arrives at the micropyle, allowing the sperm to enter.
Fertilization
the fusion of gametes. One sperm forms a zygote and the other forms a triploid nucleus with the 2 polar nuclei of the large central cell, which will become the endosperm
endosperm
food storing tissue of the seed.
double fertilization
the union of 2 sperm cells with the 2 different nuclei of a female gametophyte
Ovule and ovary after fertilization
Ovule becomes seed and the ovary becomes the fruit, which aids in seed dispersal.
Sporophyte formation
Sporophyte embryo develops from zygote. Initially, nutrients are stored in the endosperm but later, the seed leaves called cotyledons may take over this function.
Methods of Pollination
Pollination can occur by wind, water or animal
Coevolution
the joint evolution of 2 interacting species, each in response to selection imposed by the other.
Endosperm development
Endosperm develops before embryo. The triploid nucleus divides to form a multinucleate supercell with a milky consistency. Individual cells form when membranes form between the nuclei. These cells secrete a cell wall and become solid.
Grains and other monocots have an endosperm that can store nutrients for the seedling even after germination . In others, all endosperm nutrient is transferred to the cotyledon and the mature seeds lack cotyledon.
Embryo Development
Mitosis of zygote produces 2 cells: basal and terminal cell. Terminal cell gives rise to most of embryo and the basal cell produces a thread of cells called the suspensor which anchors the embryo to the parent plant. The suspensor transfers nutrients from endosperm or parent plant to the embryo. Terminal cells gives rise to proembryo after mitosis. Cotyledons form as bumps on proembryo.
Shoot and Root apex
embryonic shoot apex is cradled between the two cotyledons and the embryonic root apex is located at the opposite end where the suspension attaches. Apices of roots and shoots sustain growth for the rest of the plant´s life.
Structure of Mature Seed
Seed dehydrates at the end of its maturation and the embryo enters dormancy. The seed is surrounded by a seed coat that may impose dormancy when intact.
dormancy
seed stops growing and its metabolism nearly ceases.
embryonic axis
an elongate structure that is attached to the cotyledons
hypocotyl
the embryo axis below where the 2 cotyledons are attached
Radicle or embryonic root
where the hypocotyl terminates. It is the first organ to emerge from a germinating seed
epicotyl
the embryonic axis above where the cotyledons are attached below the first pair of miniature leaves.
plumule
the epicotyl, young leaves, and shoot apical meristems are together called the plumule.
scutellum
a specialized cotyledon present in grasses like maize and wheat.
Coleoptile and coleorhiza
coleoptile is protective sheath that covers the young shoot of grasses while the coleorhiza is the protective sheath that covers the young root. Both aid in soil penetration.
Imbibition
the uptake of water due to low water potential of the dry seed. Causes the seed coat to rupture and the embryo to resume growth. In germination, nutrients from endosperm and cotyledons are used up in growing regions of the embryo
Germination of the seed
- Water is absorbed
- The radicle emerges
- the shoot tip breaks ground
- In garden beans and eudicots, the hypocotyl forms a hook which is pushed upwards
- epicotyl is exposed and it grows true leaves
- Cotyledons shrivel away
7 Vegetative growth, including primary and secondary growth, arises from activity of meristems. - Reproduction phases follows
flowering
Flowers occur simultaneously at specific times of the year, which promotes outbreeding.
Flower forms when the shoot apical meristem switches from a vegetative to a reproductive growth mode. This transition forms a floral meristem that is triggered by environmental cues and internal signals.
Fruit formation
As the ovules of a flower become a seed, the ovaries become the fruit, which enclose seeds and aid in their dispersal. This change is triggered by hormonal hormonal changes due to fertilization. As the fruit develops, the other parts of the flower wither and die
Pericarp
the thickened wall of the fruit that forms from the ovary wall. They ovary wall may dry out or remain fleshy. In peaches, the inner ovary becomes stony and the outside remains fleshy.
Simple Fruit
fruits derived from a single carpel or many fused carpel. Example: Pea Fruits
Aggregate Fruit
fruits that result from a single flower with more than one separate carpels, each of which form a small fruit.
Example: Raspberry fruit
Multiple Fruit
fruit that develops from an inflorescence, a group of flowers clustered together. As the wall of many ovaries thicken, they fuse and become a single fruit.
Example: Pineapple
Accessory Fruit
fruits that develop when other floral parts contribute to it.
Example: In apples, the ovary is embedded in the receptacle and the fleshy part of the apple is derived from an enlarged receptacle. Only the apple core is from the ovary.
Strawberry is an aggregate fruit consisting of an enlarged receptacle with many embedded fruits that each bear a seed.
Ripening of fruit
In dry fruit, ripening involves aging and drying of the fruit tissue. In fleshy fruit, hormonal interactions produce an edible fruit that entices animals. In these, the enzymes digest cell walls and the pulp becomes softer. There is also a color change from green to a more overt color. Organic acids and starch are converted to sugar, making the fruit sweeter
asexual reproduction
A type of reproduction where offspring are derived from a single parent without the fusion of an egg and a sperm.. This results in a clone.
Mechanisms of Asexual Reproduction
A. reproduction in plants is an extension of indeterminate growth which occurs in meristems, regions of undifferentiated and dividing cells.
Parenchyma cells throughout plant can develop into more specialized cell types, enabling regeneration of lost parts.
Fragmentation
a type of asexual reproduction in which the separation of parent plant into parts that can develop into whole plants. Detached vegetative fragments can develop into offspring. Seeds are not formed.
Apomixis
the asexual reproduction of seeds that occurs without pollination or fertilization where there is no production or joining of sperm and egg. This is found in dandelions. Here, a diploid cell in the ovule gives rise to an embryo , the ovule becomes a seed, which is distributed by windblown fruits.
Advantages and disadvantages of Sexual and Asexual Reproduction
Advantages of Asexual Reproduction:
There is no need for a pollinator. All the genetic info can be passed on to the progeny, which is advantageous when the plant is well suited to its environment. Progeny are stronger than seedlings produced by sexual reproduction since offspring arise from mature vegetative fragments of parent plant. This is why A. reproduction in plants is also known as vegetative reproduction. Seed germination produced fragile seedlings that are not likely to survive due to risk by the environment (which is why many seeds are produced)
Advantages of Sexual Reproduction:
In unstable environments, it is favorable because it produces genetic variation where evolving conditions affect survival rate. Seeds allow dispersal to more distant locations and allows for dormancy to be sustainable till the environment improves.
Some plants self fertilize because it ensures that every ovule develops into a seed.
Dioecious
Species whose plants cannot self fertilize because different individuals have either staminate flowers or carpellate flowers. Others may have carpels and stamens that mature at different times or are structurally arranged in a different way that ensures that pollinature will not transfer pollen to the carpel of the same plant.
Self-incompatibility
an anti selfing mechanism where where the plant has the ability to reject its own pollen and the pollen of closely related individuals. This is the most common. Biochemical block prevents pollen from completing development and fertilizing the egg. Self pollen is recognized based on S genes of which there are many alleles. If the alleles of the pollen grain match the allele of the stigma , the pollen tube fails to germinate or fails to grow through the style to the ovary.
Gametophytic Self-incompatibility
This governed by the S allele in the pollen grain. An S1 pollen grain from S1S2 sporophyte cannot fertilize an S1S2 flower but can fertilize an S2S3 flower. If the alleles are the same, then the style produces RNA hydrolyzing enzymes that enter the pollen tube and destroy its RMA.
Sporophytic Self-incompatibility
This is when fertilization is blocked by the S allele gene products in tissues of the parental sporophyte. Example, neither an S1 or an S2 pollen grain from an S1S2 flower can fertilize an S1S2 flower or an S2S3 flower because of of the S1S2 parental tissue attached to the pollen wall. This incompatibility involves a signal transduction pathway in epidermal cells of the stigma that prevent pollen germination.
Totipotent
a quality of a cell that can divide and asexually generate a clone of the original organism. This is usually associated with meristematic plants but other plant cells can de-differentiate and become totipotent.
Vegetative Propagation
The process where vegetative reproduction is facilitated or induced by humans.
In house plants, shoot cuttings are used. Callus develops at the cut end and gives rise to adventitious root. If shoot includes a node, roots form without a callus
Callus
a mass of dividing undifferentiated totipotent cells.
Modification of vegetative reproduction
A twig or bud from one plant can be grafted to another plant of related species or different variety of the same species. This combines best qualities of both plants. A callus first forms between the joining cut ends of the scion and stock, cell differentiation then unites the grafted individuals.
Stock
the plant that provides the roots
scion
the twig that is grafted onto the stock
Test-tube Cloning and Related Techniques
whole plants can be obtained by culturing small samples of tissues from a parent plant on an artificial medium that has nutrients and hormones.All tissues can be cloned, but the growth rate depends on the type of tissue. In media, a callus of undifferentiated totipotent cells may form and manipulation of the hormones and nutrients can allow the sprouting of roots and shoots with fully differentiated cells.
Test tube culture allows regeneration of genetically modified plants from a single plant cell into which the foreign DNA has been incorporated.
Maize
Maize cannot spread its seeds because maize kernels are permanently attached to the central axis and are protected by tough overlapping leaf sheaths. This arose due to artificial selection over time.
Wheat
The wheat species used by us today formed through the natural hybridization between different species of grass.
Plant Breeding
Beneficial traits that breeders look for usually arise through mutation but natural mutations rate are usually slow. Due to theis, breeders often treat large batches of seeds or seedlings with radiation or chemicals. If a wild plant with a beneficial trait is identifies, it is crossed with a domesticated variety. Offspring often inherit the beneficial trait as well as the undesirable traits of the wild parent. The offspring are continuously crossbred with a domesticated plant until an offspring arises which has both, the beneficial trait and well as the domesticated plants traits.
Plants from different species can also be crossbred- this usually results in abortion of the hybrid seed during development or the endosperm does not develop while the seed does. Hybrid embryos can be rescues by removing the embryo and culturing it in vitro.
Plant Biotechnology
Refers to the use of plants to make products that are useful to humans or more specifically, the use of GM organisms in agriculture and industry.
Plant biotechnologists can use techniques of genetic engineering to transfer genes between plant that are not limited to be closely related species or genera.
transgenic
organisms that have been engineered to express a gene from another species.
Reducing malnutrition and world hunger
transgenic crops can increase the yield of crops over a certain area of land. Examples include hybrids of cotton, maize, and potatoes that contain genes from the bacterium Bacillus thuringiensis. These transgenes encode a protein Bt toxin that is toxic to insect pests. This reduces the need for chemical pesticides and the Bt toxin is only activated by alkaline conditions like the guts of insects. It is harmless to humans because humans and vertebrates have highly acidic stomachs.
Transgenic plants can be made resistant to herbicides so that other plants can be weeded through herbicides without the transgenic crops being harmed. Weeding is a supplement toi soil tillage which causes soil erosion. Transgenic plants can also be engineered to resist certain diseases.
Golden rice
a transgenic variety supplemented with trangenes that enable it to produce grain with increased levels of beta Carotene, a precursor of Vitamin A, without which many children go blind.
Biofuels
fuels derived from living biomass. Biofuels from the plant biomass can be used as a replacement to fossil fuels because it would reduce the net amount of CO2 release. This is because biofuel crops resorb the CO2 released by the burning of fossil fuels through photosynthesis.
Biomass
the total mass of organic matter in a group of organisms in a particular habitat.
Biofuel crops
Biofuel crops can be made from wil precursors that are fast growing such as switchgrass and poplar, that grow on soil too poor for soil production. Plants would not be directly burned. Rather, cellulose and hemicellulose- the most abundant organic compounds on earth- will be broken down into sugars by enzymatic reactions. The sugars would be fermented to alcohol and distilled to yield biofuels. Reducing lignin content of the plant cell walls through genetic engineering will lower the cost of biofuel production
Issues of Human Health
genetic engineering may transfer allergens, a molecule to which some people are allergic, from one species that produces it to a plant used for food. Removal of genes that code for allergenic protein sis underway. Some transgenic plants may be healthier than the normal plant- Bt maize has 90% less fungal toxin called fumonisin, which is found in many processed maize products. Fumonisin is produced by a fungus that infects insect damaged c=maize and since Bt maize suffers less insect damage, it has less fumonisin.
Possible effects on non target organisms
Larvae of monarch butterflies responded adversely after eating milkweed leaves dusted with pollen from transgenic Bt maize. Research found that it was the floral parts and not the pollen that contained Bt toxin in high concentration which killed the larvae. Since only pollen is carried from plant to plant, this would not affect the butterflies naturally. Only one Bt maize line produces pollen with high concentrations of the Bt toxin. Spraying of non Bt maize with pesticides is even more harmful to the monarch populations.
Addressing the problem of transgene escape
introduced genes may escape from transgenic crops into their weed relative through crop to weed hybridizations.. This may cause weeds to gain selective advantage towards other wild weeds. Likelihood of transgene escape depend son the ability of crop and weed to hybridize and on how the transgene affects the fitness of the hybrids. A dwarf phenotype may be advantageous for for crops and disadvantageous for a weed in the wild. Wild relatives may not be available ( soybean has no wild relatives in the US)
Example: Transgenic variety of creeping bentgrass that was engineered to resist the herbicide glyphosate escaped during a windstorm. All the Argotis plants found nearby in the next three years were of the glyphosate variety
Strategies to prevent transgene escape
- Male sterility would allow transgenic plants to produce seed and fruit is pollinated by pollen from nearby plants but they themselves would not produce an viable pollen.
- Genetically engineering apomixis into transgenic plants would prevent transgene escape since in apomixis, the embryo and the endosperm develop without fertilization. this would reduce transgene escape via pollen since plants could be male sterile and still be able to produce seed and fruit.
- Transgene may be imbedded into the chloroplast DNA which is inherited strictly from the egg, which prevents transfer of transgenes via pollen.
- Flowers may be engineered so that they develop normally but fail to open, promoting self pollination and inhibiting the escape of pollen.
