Flowering Plants: Reproduction

Chapter 27: Flowering Plants: Reproduction Lecture Outline

• Flowering plants are explored, covering from sexual reproductive strategies to asexual methods like tissue culture.

27.1 Sexual Reproductive Strategies

• Plants have a two-stage alternating life cycle
• The diploid sporophyte is dominant, producing haploid spores through meiosis
• Flowers produce microspores and megaspores
• Spores divide mitotically into haploid gametophytes
• Gametophytes produce gametes
• Gametes fuse to produce a diploid zygote
• Zygote divides mitotically into a diploid sporophyte

Flower Structure and Function

• Flowers are unique to angiosperms
• Flowers attract pollinators and produce fruits that enclose seeds
• Flowering responds to environmental cues like day length
• In some plants, leaf-forming meristems stop leaf production to create flowers within a bud
• Monocots have flower parts in threes or multiples of three
• Eudicots have flower parts in fours/fives or multiples of four/five
• A typical eudicot flower contains four whorls of modified leaves that are attached to a receptacle at the end of a flower stalk called a peduncle
• Sepals protect the bud
• Petals attract pollinators
• Stamens are the male part of the flower, with an anther (sac-like container) and filament (slender stalk)
• Carpel is the female portion of the flower, which consists of a stigma (enlarged sticky knob), style (slender stalk), ovary (enlarged base that encloses ovules) , and ovules
• Ovules play a vital role in megaspore production (female gametophyte)
• Complete flowers contain sepals, petals, stamens, and a carpel
• Incomplete flowers miss one or more of those parts
• Perfect (bisexual) flowers have both stamens and carpels
• Imperfect (unisexual) flowers have one, but not the other
• Monoecious plants contain staminate and carpellate flowers on the same plant
• Dioecious plants contain staminate and carpellate flowers on separate plants
• Holly trees are dioecious

Male Gametophytes

• Microspores are produced in anthers
• Each anther has 4 pollen sacs with microspore mother cells
• Microspore mother cells undergo meiosis into to four haploid microspores
• Microspores undergo mitosis to produce pollen grains
• An immature gametophyte initially consists of a tube cell and a generative cell
• The tube cell divides and will produce a pollen tube while the generative cell produces 2 sperm by mitosis.
• The pollen grain becomes a mature make gametophyte

Ovary and Pollination

• The ovary contains one or more ovules
• An ovule has a central mass of parenchyma cells covered by integuments
• One parenchyma cell enlarges to become a megaspore mother cell
• The megaspore mother cell undergoes meiosis to produce four haploid megaspores, of which three are nonfunctional
• The functional megaspore divides mitotically until eight nuclei are present in the female gametophyte
• The female gametophyte, also known as the embryo sac, contains one egg cell associated with 2 synergid cells, one central cell containing two polar nuclei, and 3 antipodal cells
• Pollination is pollen transfer from an anther to a carpel's stigma
• Self-pollination occurs when pollen comes from the same plant
• Cross-pollination occurs when pollen comes from a different plant of the same species

Fertilization and Pollinators

• Pollen grains landing on a stigma germinate, forming a pollen tube
• The pollen tube passes between the stigma and style to reach the ovule's micropyle
• Double fertilization takes place
• One sperm nucleus unites with the egg nucleus to produce a 2n zygote
• The other sperm nucleus unites with the polar nuclei, creating a 3n endosperm nucleus that will develop into the endosperm
• A mature seed contains an embryo, stored food, and seed coat
• Insect-pollinated flowers are often a color other than red because bees can't detect this color
• Butterfly pollinated flowers are often composites with room for the pollinator to land

27.2 Seed Development

• Development is a series of programmed stages from a simple to a more complex form
• Cellular differentiation occurs as development proceeds
• After double fertilization, the zygote divides repeatedly to form a proembryo and a suspensor in development of a eudicot embryo.
• The root-shoot axis has been established
• During the globular stage, the proembryo is a ball of cells
• The outermost cells will become dermal tissue that protects the plant from desiccation and include stomata
• The embryo is heart-shaped when cotyledons appear
• The embryo enlarges, elongates, and becomes what is known as torpedo shaped
• The shoot apical meristem facilitates growth aboveground.
• The root apical meristem facilitates growth underground
• The epicotyl is the portion between cotyledons responsible for shoot development
• The hypocotyl is the section below that facilitates stem development
• The radicle is the embryonic root
• After fertilization, the eudicot embryo undergoes multiple stages of differentiation beginning with the zygote stage and proembryo stage
• The axis for shoot and root development is established, and the embryo takes on a globe-like appearance at the globular stage of development
• The heart stage indicates the formation of cotyledons.
• In torpedo stage , the shoot, root, and ground meristems are present and actively dividing
• In the mature embryo, a rudimentary shoot and root develop with continued division from the apical meristems
• The seed coat comes from the integuments of the original ovule hardening
• Monocots only possess one cotyledon, while eudicots posses 2
• The monocot cotyledon contains certain nutrients, and it absorbs nutrients from the endosperm and transfers them to the embryo.
• The eudicot cotyledons store all the nutrients the embryo needs.

Monocot vs Eudicot Seeds

• The endosperm disappears in a bean seed (eudicot)
• The bean embryo’s cotyledons take over food storage
• The hypocotyl becomes the shoot system, to include the plumule (first leaves)
• The radicle becomes the root system
• The corn kernel (monocot) has an endosperm at maturity
• The coleoptile protects the shoot system and the coleorhiza protects the future root system
• The pericarp develops from the ovary wall

27.3 Fruit Types and Seed Dispersal

• A fruit is known as a mature ovary
• Simple fruits come from a single ovary
• Compound fruits develop from several groups of ovaries
• Aggregate fruits describe multiple ovaries from a single flower, originating on one receptacle
• Multiple fruits come from multiple separate flowers, clustered together
• The ovary thickens and becomes the pericarp with 3 layers
• The outer skin also known as the exocarp
• The mesocarp is the fleshy tissue between the exocarp and the endocarp
• The endocarp is the boundary around the seeds
• Dry fruits contain a pericarp that is papery, leathery, or woody at maturity
• Dehiscent fruits split when ripe
• Indehiscent fruits do not split open when ripe
• Dry fruits are dispersed by wind
• Fruits attract animals and provide them with food
• Dry fruits attach to animal fur and human clothing with hooks and spines
• Plants can disperse seeds by a forced ejection
• When seed germination takes place, the embryo begins growth/metabolic activity.
• Length of time seeds retain their viability can vary.
• Some seeds will only germinate once they’ve been through a dormant period
• No growth happens
• The temperate areas require cold weather to break dormancy
• Fleshy fruits contain inhibitors to prevent germination when the fruit is on the plant
• Oxygen must be available for metabolic needs
• Warmth and water are required

Germination

• Bean (eudicot) seeds germinate, revealing 2 cotyledons
• Corn (monocot) seeds germination showcase an emerging coleoptile.

27.4 Asexual Reproduction in Plants

• Plants reproduce asexually using:
• Stolons which are horizontal stems running aboveground
• Rhizomes are underground stems to facilitate new, asexual reproduction
• Nodes of stems are regions where new growth will take place
• New plans arise at the nodes of stolons & rhizomes
• White potatoes eyes, are buds which can produce a new plant, as long as they are planted with a portion of the tuber (rhizome).
• Meristem tissues at nodes creates new plants, for example when stolons of strawberry plants.

Tissue culture

• Tissue culture means growing a tissue in an artificial liquid or solid culture
• Many plant cells are totipotent
• Each one is capable of becoming a plant
• Meristem tissue is put on a sterile media , and an undifferentiated mass or callus, forms
• organogenesis can also take place from the callus, then shoots or roots will form
• Then the callus becomes a plantlet
• Plantlets can be stores, then shipped to soil for development inside of sterile containers
• Meristem results in clonal plans that have nearly identical traits to that of the parent
• The tissue is cultivated in a medium which has auxin, cytokinis and other vital nutrients
• Rapidly growing calluses can be cut in pieces and then placed into liquid
• Cells or smaller clumps get separated and will form a suspension
• Chemcials can be withdrawn from genetically altered plant cells
• Digitals can make pharmaceuticals in the management of cardiovascular diseases, and tissue culture will reduce overselection of them in their native natural habitats
• Tissue culture is a viable option for re establishing rare plants into their native environment