Reproduction in Flowering Plants
REPRODUCTION IN FLOWERING PLANTS
Introduction
Reproduction is the ability of an organism to produce a new generation of itself.
Importance of reproduction:
Passing of genes to the next generation ensures survival and evolution.
Two types of reproduction:
Asexual (vegetative): Production of a new generation by one parent.
Sexual: Production of a new generation by joining genetic material from two parents.
Similarities Between Asexual and Sexual Reproduction
Both result in the production of the same kind of organism, preventing species extinction.
Both contribute to food production, helping to feed the world population.
Differences Between Asexual and Sexual Reproduction
Asexual Reproduction
Number of parents: One (all individuals produce offspring).
Processes: One stage involving mitosis only and no fusion of gametes.
Gamete formation: No gametes are formed.
Rate: Quicker than sexual reproduction.
End-result: Genetically identical offspring, identical to the parent with no variation between alleles (no shuffling occurred).
Environment: In stable environments, well-adapted individuals thrive; reproduction is possible even with few or no mates.
Energy input: Efficient as no energy is required.
Outside agents: None required.
Ability to adapt: No.
Possibility of evolution: Low due to no genotype variation.
Sexual Reproduction
Number of parents: Two.
Processes: Two stages involving meiosis and fusion of gametes.
Gamete formation: Gametes are formed.
Rate: Slower than asexual reproduction.
End-result: Genetically different offspring, not identical to parents, due to allele shuffling during meiosis and fertilization.
Environment: In unstable environments, individuals with variations adapt to new conditions.
Energy input: Less efficient due to gamete production and mate seeking.
Outside agents: Pollinators are needed for pollination.
Ability to adapt: Yes.
Possibility of evolution: Good genotype variation present.
Advantages of Asexual Reproduction
All individuals can reproduce, saving energy by not needing to find mates or produce offspring.
Fast and simple process involving only mitosis.
Useful in stable conditions where a well-adapted parent will produce well-adapted offspring.
Rapid spread of a favorable mutation, leading to rapid adaption to new conditions, such as resistance to antibiotics or pesticides.
Disadvantages of Asexual Reproduction
No variation in offspring; without mutation, changing environments can lead to population inability to adapt.
Possibility of overcrowding, leading to limited resources such as food and water.
Advantages of Sexual Reproduction
Genetic variations result in:
Basis for evolution.
Increased chance of survival as offspring can adapt to an unstable environment.
Prevention of disease spread as offspring may be genetically resistant.
Reduced chance of inheriting a disease from a parent.
Disadvantages of Sexual Reproduction
High energy expenditure due to the need for specialized plant organs like the flower.
Slower reproduction process as gamete production and meeting require time.
Unfavorable mutations and recessive genes may be expressed.
Outside agents are required, such as pollen or seeds.
Process of Sexual Reproduction
Haploid (n): Contains one set of chromosomes.
Diploid (2n): Contains two sets of chromosomes.
Step 1: Diploid parent produces gametes (sex cells) in the gonads using meiosis. Gametes are haploid, containing one set (n) of chromosomes.
Step 2: Male and female gametes are brought together via pollination in plants and mating in animals.
Step 3: Nuclei of the gametes fuse in a process known as fertilization.
Step 4: A new cell is formed, called a zygote. The zygote is diploid (2n) because it has two sets of chromosomes.
Step 5: Zygote grows into a new individual through mitotic cell division.
Female gamete = egg cell (ovum) in an embryo sac.
Male gamete = nucleus in a pollen grain.
Angiosperm Reproduction
Flowering plants produce ovules enclosed in an ovary.
The flower is the organ of sexual reproduction, containing reproductive organs and attracting pollinators.
Most flowers are hermaphrodite or bisexual, containing both male and female gametes.
Male gametes are inside pollen grains, produced by anthers.
Female gametes are inside ovules enclosed by the ovary.
Structure of a Flower
Series of modified leaves arranged in four whorls or circles:
Calyx (green) protects unopened buds.
Corolla (colored) attracts the pollinators.
Stamen (male whorl) is the male part.
Carpels (female whorl) is the female part.
The corolla and calyx combined are known as the perianth.
Female Parts of a Flower
Ovary:
Contains ovules.
Each mature ovule contains a female gamete.
After fertilization, the ovule becomes the seed and the ovary becomes the fruit.
Style:
Slender section connecting stigma to ovary.
Holds stigma in a favorable position for receiving pollen.
The pollen tube (carries male gamete) grows along the style to the ovary.
Stigma:
Sticky.
Receives pollen.
Male Parts of a Flower
Anther:
Forms the pollen grains in which the male gametes are found.
Filament:
Holds the anther in the best position for pollen release.
Difference Between Pollination and Fertilization
Pollination:
Transfer of pollen from anther to stigma.
Enables fertilization and reproduction.
Occurs when the mature anther splits open, pollen is discharged, and reaches the stigma via wind and insects.
Fertilization: joining of two haploid gametes (male and female) to form a diploid zygote
Types of Pollen Transfer
Cross-pollination:
Pollen transferred from anther of a flower to stigma of another flower of the same species.
Results in genetic variation in offspring.
Self-pollination:
Pollen is transferred from the anther to the stigma of the same flower or another flower of the same plant.
No genetic variation in offspring.
Prevention of Self-Pollination
In Nature:
Bisexual flowers: Anthers and stigmas ripen at different times (anthers ripen first).
Unisexual flowers: Unable to pollinate themselves; stigma is positioned above the anthers.
In Plant breeders:
Remove anthers
Fertilization
Fertilization = joining of two haploid gametes (male and female) to form a diploid zygote.
Occurs after pollination:
Pollen grain lands on stigma of flower.
Pollen tube develops from pollen grain.
Pollen tube grows along the style and into the ovary to the ovule.
Nuclei of both gametes fuse to complete fertilization.
Formation of Seeds and Fruit
Seed
Zygote develops into embryo that consists of:
Seed leaves (cotyledons) – take food from parent plant for themselves.
Radicle – embryonic root
Plumule – embryonic shoot.
Remaining ovule develops into endosperm tissue (stored food such as starch, oils or proteins).
Outer ovule covering thickens and hardens, forming the seed coat (testa).
Fruit
Grows as the seed is being formed.
Function:
Contain and protect seeds.
Seed dispersal from parent plant.
Sexual/Asexual Reproduction and Improved Food Crop
Wild plants = grow naturally (in nature) without assistance from humans
Domestication = skillful breeding leading to:
Increased phenotypic changes and altered genotypes
Development of new varieties
Examples: wheat, maize and rice
Asexual Reproduction and Crop Improvement
Normal gamete formation and fertilization does not occur
Techniques include: division, grafting, using storage organs (bulbs, corms, tubers and rhizomes) and cuttings.
Asexual reproduction: genetic stability and no variation in the offspring
Mutation may produce a plant with a superior trait (bigger seeds, fruit or tubers)
More plants produced with this superior trait; eventually crop will consist of entirely of plants with this trait.
Crop has been improved through asexual reproduction
Grafting
Tissue from one plant is inserted into those of another → tissues will then join
Rootstock = plant selected for its roots due to good traits (resistance to pests, hardy in difficulty soil conditions)
Scion = other fruit chose for superior fruit traits (larger, increased yield) → mature plant is more productive
Used in commercially grown crops of avocados, grapes, and pears
Sexual Reproduction and Crop Improvement
Normal gamete formation and fertilization does occur
Offspring is very different from parent and one another
Beneficial traits include larger yield, bigger seeds, tubers and fruit, resistance to pests and poor soil
Breeders selected and planted seeds from plants with beneficial traits – eliminating plants with poor traits
Repeated selection improved crops
Cross-Pollination
Using cross pollination:
New species with improved traits
Cultivar = plant or grouping of plants chosen for their traits that can be maintained by propagation
Example: mildew-resistant pea crossed with a high-yielding mildew susceptible pea → pea resistance to mildew and has larger yield. Plants were known as cultivars
Self-Pollination
Self-pollination:
Corn with more pips
Seeds planted from this plant will be self-pollinated
Next generation has this new trait
Repeated breeding eventually leads to all maize having more pips.
Plant Breeders Using Asexual Reproduction and Engineering to Improve Crops
Why?
Climate change will cut crops
Limited land available for crops
Best soil is protected (environmental concerns)
Need for stress-tolerant plants.
Breeders and Challenges of Future Food Security
Ensuring food security
Higher yield than present
Resistance to pests and diseases
Drought-resistant or regionally adapted to different environments
Asexual Reproduction and Benefit of Future Crop Production
Propagation is faster than seeds – no gamete production/pollination needed
Larger, quicker, cheaper, easier from cuttings, bulbs, or tubules
Consistent superior trait
Grafting – fruit trees reach maturity quicker
Micro-propagation (tissue culture) – new identical plants (clones) are produced
Micro-propagation
Why micro-propagation?
Mass propagation of commercially important plants in a short time
Selecting disease-free cells produces disease-free plants – cultured in sterile conditions
Occurs all year around – not limited to seasons
Used together with genetic engineering to propagate transgenic plants from genetically modified cells
Micro-propagation = process where small amounts of plant tissue is cultured in a growing medium to produce a callus and then plantlets
Tissue culture (TC) is the cultivation (growing) of plant cells, tissues, or organs on specific nutrient-rich media. This allows for an entire plant to be regenerated from a single cell.
Genetic Engineering Benefits Crop Production
Genetic engineering = process where gene from one organism is placed into DNA of another organism
Results in a transgene or genetically modified organism (GMO)
New variety with desired trait is produced quicker
No interspecies barriers – as all have same genetic code (example: genes from bacteria will produce correct protein in a maize plant)
NOTE: Proteins produced by transgenes are same as those in original species – as genetic code is universal
Signals for gene expression are different – as they are plant-specific, not universal
Examples of GMOs
Bt maize – Bacillus thuringiensis – soil bacterium
The Bt maize contains a gene from an insect pathogen, Bacillus thuringiensis (Bt)
The gene encodes a protein that is toxic to the European corn borer (ECB)
The corn borer will then die when kit eats this maize.
Most of USA’s maize crops contain this protein
Roundup Ready soybeans
Roundup = brand name of glyphosate (herbicide)
Bacterial gene with resistance to this herbicide is transplanted into crops.
Herbicide kills the pests, but not the crops
Traits Incorporated Using Biotechnology
Resistance against certain diseases and pesticides – Example: Maize and rice - China; potatoes and maize - Kenya
Increased tolerance to pests – Example: Maize (against stork- borer) – SA
Enrichment of nutrient content – Example: Iron, proteins, vitamins, zinc, carotenoids. Golden Rice (Vitamin A)
Increased tolerance for environmental pressure – Example: salinity, drought (DroughtGard), extreme temperatures
Flood tolerance – Example: Rice variant (Swarna-Sub1) in Asia.
Longer storage life of harvested crops – Example: Strawberries
Improved flavour
Using Sexual Reproduction for Improved Food Crop Varieties
Using hybridized plants
Hybridization = When two organisms of different species mix or breed.
Offspring is known as a hybrid
NOTE: Hybrids are NOT GMOs.
Hybrid crops are formed by cross-pollinating two inbred plants with different genotypes
Seeds produced are hybrids → hybrid crops
Improvements Due to Hybridization
Plants are more vigorous (strong, robust and grows well) – less agriculture land required to grow them
Improved disease resistance – commonly sought after trait. Diseases affect productivity. Example: hybrid tomatoes are resistant to Fusarium (fungus)
Increased yield – Example: rice
Mature earlier and extended growing season – Example: strawberries and tomatoes
Quality improvement – Example: hybrid watermelons have a crispier taste
What is a Polyploidy Plant?
Plants that have more than two homologous sets of chromosomes
Agriculturally important plants such as broccoli, cabbage, cauliflower
Causes of diploid to polyploidy: – Disturbance in mitosis or meiosis (during crossing of two hybrids) – Seeds treated with colchicine (chemical)
Wheat = many different strains due to years of hybridization and modification:
Diploid strains
Tetraploid strains (four sets of homologous chromosomes). Example: Durum wheat
Hexaploid strains (six sets of homologous chromosomes). Example: bread wheat
Advantages of Polyploidy in Agriculture
Forms seedless varieties of fruit
Plants are bigger, robust, larger fruit and larger flowers
Mutagenesis
Free of regulatory restrictions placed on genetically modified organisms
Also called variation breeding
Mutation breeding = process where seeds are exposed to mutagens (chemicals that cause mutations) so that mutants with desired trait is generated.
Example: larger seeds or sweeter fruit – not found in nature
Also known as mutagenic plants
Examples of crops worldwide where this is used: maize, wheat, barley, pears, cotton, peppermint, sunflowers, peanuts, grapefruit, cassava, and sorghum
Seed Banks
All life on earth depends on plants
60 000- 100 000 plant species under threat due to:
Human population growth
Socio-economic factors
Seed banks = maintain biodiversity
Seed bank = facility used to store seeds of various wild plants and crops in an effort to maintain biodiversity
Examples of Seed Banks
Kew’s Millennium Seed Bank Project (MSBP) – in the UK
Conserved up to 10% of world’s seed plants
Has seeds from extinct plants in the wild
Working with South African National Biodiversity Institute (contributed 2500 indigenous species)
Focuses on: Endangered and endemic species; Species that may become endangered due to over-exploitation
International Seed Vault – Sweden
Located on Svalbard Islands, close to North Pole
Re-enforced tunnel in a mountain
Stores seeds from every country
Stored at -180°C (remain viable for thousands of years)
How Seed Banks Maintain Biodiversity
Offers protection against loss of a species in the wild – Due to habitat loss, climate change, over-exploitation of species
Re-establish damaged or destroyed habitats and ecosystems – Dry-land species are maintained. Important for food, medicine, fuel, wood and forage for life stock
Re-introduction of newly extinct, endangered or threatened species
Production of plant material as a source of research for over exploited plant species. Prevents extinction in the wild
Importance of Seeds as a Food Source
Have a good food value
Practical source of food (easy transport and storage)
Form staple diet (grains and pulses)
High nutritional value
Cheap
Value of Grains
Rich in starch (stored in endosperm)– very good source of energy
Source of fibre – seed coat of whole grains. Prevents colon cancer, maintains healthy bowel (prevents constipation and diverticulitis)
Source of vitamin B and other minerals – seed coat (bran)
Contain small amounts of proteins and fats
Value of Pulses
Rich in protein
Source of vitamin B and other minerals – seed coat (bran)
Regulate blood sugar levels – low glycaemic index (lowers the rate at which sugar is released from starches during digestion)
Examples of pulses: lentils, legumes, peanuts, soyabeans
Value of Nuts
Good source of energy – most calorie-rich food (apart from animal fats)
Rich in monounsaturated or polyunsaturated fatty acids - lower cholesterol
Best natural source of vitamin E – antioxidant properties
Examples of nuts: almonds, pecans and cashews
Value of Oil Seeds
Rich in monounsaturated or polyunsaturated fatty acids - lower cholesterol
Contain omega-3 fatty acids - overall health (brain and heart)
Examples of oil seeds: flaxseeds, soyabeans, peanuts, sunflower seeds, canola seeds
Use of Growth Factors in Agriculture
Natural (plant hormones) or synthetic
Types of plant hormones es:
Auxins = cell growth and plant elongation
Gibberellins = germination, elongation growth, flower development, flowering time
Cytokinins = cell division, leaf cell development
Ethylene = regulates growth and senescence (deterioration with age – cell loses ability to divide and grow)
Abscisic acid = regulate plant growth
Flowering hormones – florigen – boosts flowering
Use of Growth Factors in Agriculture
Increased productivity due to:
Successful propagation – auxins and cytokinins – control growth and development of shoots and roots
Increased fruit size – gibberellins
Induce early flowering and number of flowers – auxins. More flowers = more fruit
Break dormancy of some seeds
Increased yield of oil content in seeds and nuts
Control ripening of some fruits - ethylene