9.3 Plant Growth & 9.4 Plant Reproduction

Notes taken from Bioninja

What are meristems?

tissues in a plant consisting of undifferentiated cells capable of indeterminate growth

How do meristems benefit plants?

allow plants to regrow structures or even form new plants (vegetative propagation)

Distinguish between the two types of meristematic tissue

apical meristems: occur at shoot and root tips, responsible for primary growth (plant lengthening), gives rise to new leaves/flowers

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lateral meristems: occur at cambium, responsible for secondary growth (plant widening/thickening), responsible for the production of bark

What cellular processes allows for apical growth?

a combination of cell enlargement and repeated cell division aka mitosis and cytokinesis

Describe how apical growth occurs

growth occurs in sections called nodes, the remaining meristem tissue forms an inactive axillary bud which can form new branching shoots with leaves and flowers

What is the main group of plant hormones involved in shoot and root growth? Where is it released?

auxins; released from the shoot apex

Describe how auxins promote apical growth

* production of auxins promotes growth in the shoot apex via elongation and division
* causes apical dominance meaning that it prevents growth in lateral buds, ensuring that a plant will use its energy to grow up towards the light
* as the distance between the terminal bud and axillary bud increases, the inhibition of the axillary bud diminishes
* different plant species have different levels of apical dominance

Describe how plant pruning occurs

removing the terminal bud by cutting off the top of the plant allows for dormant lateral buds to develop, allowing for the production of wider plants

Describe how auxin efflux pumps can control the direction of plant growth

they can determine which regions of plant tissue have high auxin levels and hence set up a concentration gradient

Distinguish between the role of auxin in the roots vs the shoots of plants

* shoots: auxin stimulates cell elongation so high concentrations of auxin promote growth
* roots: auxin inhibits cell elongation so high concentrations of auxin limit growth

Describe how auxin increases the flexibility of the cell wall to promote growth

* auxin activates a proton pump in the plasma membrane
* H+ ions are secreted into the cell wall
* pH decreases which causes cellulose fibers in the cell wall to loosen as bonds are broken
* auxin also upregulates the expression of expansins which increases the cell wall elasticity
* the influx of water causes the cell to increase in size as the cell wall is now more flexible

What is phototropism

a growth movement in response to a unidirectional light source

What is geotropism

growth movement in response to gravitational forces

aka gravitropism

List 2 other tropisms besides phototropism and geotropism and describe them

hydrotropism: responding to a water gradient

thigmotropism: responding to a tactile stimulus

Describe the change in auxin distribution to enable geotropism

* auxin accumulates on the lower side of the plant in response to gravity
* high auxin concentrations promote cell elongation in the shoot so the dark side elongates and the shoot grows upwards
* high auxin concentrations inhibit cell elongation in the roots so the lower side of the root becomes shorter causing the roots to turn downwards

Describe the change in auxin distribution to enable phototropism

* light receptors (phototropins) trigger the redistribution of auxin to the dark side of the plant
* the dark side of the shoot elongates and grows towards the light as high auxin concentrations in the shoot promotes growth
* the dark side of the root becomes shorter and the roots grow away from the light as high auxin concentrations in the root decreases growth

What is micropropagation

a technique used to produce large numbers of identical plants (clones) from a selected stock plant; plant tissues are cultured in the lab to reproduce asexually

What is vegetative propagation

when a plant cutting is used to reproduce asexually in the native environment

Describe the process of micropropagation

* specific plant tissue (usually from the undifferentiated shoot apex) is selected from a stock plant and sterilized
* the tissue sample is grown on a sterile nutrient agar gel
* the explant is treated with growth hormones like auxin to stimulate shoot and root development
* growing shoots can be continuously divided and separated to form new samples
* after development, the cloned plant can be moved to soil

Describe how micropropagation can be used for rapid bulking

* desirable stock plants are cloned via micropropagation to conserve the fidelity of the selected characteristic
* more reliable than selective breeding because new plants are genetically identical to the stock plant
* also used to produce large quantities of plants created via genetic modification

Describe how micropropagation is used to produce virus-free strains

* plant viruses have the potential to decimate crops
* can cripple economies and lead to famine
* viruses typically spread through infected plants via vascular tissue
* meristems don’t have vascular tissue
* propagating plants from non-infected meristems allows for the reproduction of virus-free plant strains

Describe how micropropagation can be used for the propagation of rare species

* used to increase rare or endangered plant species
* increases the numbers of species that are difficult to breed sexually
* can be used to increase numbers of plant species that are in commercial demand

List the methods of plant reproduction

* vegetative propagation (asexual reproduction from a plant cutting)
* spore formations
* pollen transfer (occurs in flowering plants)

List the three phases in plant sexual reproduction

pollination, fertilization, and seed dispersal

Describe how pollination occurs

* pollen grains from an anther (male plant structure) to a stigma (female plant structure)
* many plants possess both male and female structures (monoecious); thus can potentially self-pollinate
* cross-pollination is preferable as it improves genetic diversity

Describe how fertilization occurs

* fusion of a male gamete nuclei with a female gamete nuclei to form a zygote
* male gamete is stored in the pollen grain and the female gamete is found in the ovule

Describe how seed dispersal aids in plant reproduction

* fertilization of gametes results in the formation of a seed which moves away from the parental plant
* seed dispersal reduces competition for resources between germinating plant and parental plant
* seed dispersal mechanisms: wind, water, fruits, animals

Describe the relationship between the pollinator and the flowering plant

* mutualistic relationship
* flowering plant gains a means of reproduction via the transference of pollen between plants
* animal gains a source of nutrition through nectar

List examples of pollinators

birds, bats, insects (bees, butterflies)

* some flowers are structured to optimize access for certain pollinators like tube shaped flowers for birds with long beaks

Describe the production of flowers

* develop from the shoot apex
* changes in gene expression trigger the enlargement of the shoot apical meristem
* tissue differentiates to form the different flower structures (sepals, petals, stamen, pistil)

What causes the activation of the gene responsible for flowering

* typically linked to the seasons
* typically come into bloom when a suitable pollinator is most abundant
* the most common trigger is day/night length (photoperiodism)

Distinguish between the terms monoecious and dioecious

monoecious: possesses both male and female structures

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dioecious: may only possess one structure

Label the parts of a flower

List the parts of a stamen

anther and filament

Describe the function of the anther

pollen producing organ of the flower, pollen is the male gamete of a flowering plant

Describe the function of the filament

slender stalk supporting the anther, makes the anther accessible to pollinators

List the parts of the pistil

stigma, style, ovule

Describe the structure & function of the stigma

sticky, receptive tip of the pistil that is responsible for catching the pollen

Describe the structure and function of the style

tube-shaped connection between the stigma and ovule (elevates the stigma to catch pollen)

Describe the function of the ovule

contains the female reproductive cells (after fertilization it develops into a seed)

List other flower reproductive structures excluded from the pistil and stamen and describe their function

* petals: brightly colored modified leaves that attract pollinators
* sepal: outer covering which protects the flower when in bud
* peduncle: stalk of the flower

Describe the function of phytocromes

leaf pigments that are used by the plant to detect periods of light and darkness

Describe what occurs when phytocromes detect light

* inactive form of phytochrome is converted into the active form when it absorbs red light
* active form of phytochrome is broken down into the inactive form when it absorbs far red light
* active form gradually reverts to the inactive form in the absence of light

Distinguish between the flowering of short and long day plants

Short-day plants

* flower when the days are short meaning that they require the night period to exceed a critical length
* flowering occurs at at low levels of Pfr (the active form of phytochrome)

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Long-day plants

* flower when the days are long meaning that the night period must be less than a critical length
* flowering occurs at high levels of Pfr

Describe how horticulturalists can trigger long-day plants and provide an example

* typically don’t flower during winter and autumn months
* can trigger flowering by exposing the plant to a light source during the night
* ex) carnations

Describe how horticulturalists can trigger long-night plants and provide an example

* typically don’t flower during the summer months
* can trigger by covering the plant with an opaque black cloth for around 12 hours each day
* ex) crysanthemums

List the features of a seed

testa, micropyle, cotyledon, plumule, radicle

State the function of the testa of a seed

an outer seed coat that protects the embryonic plant

State the function of the micropyle of a seed

a small pore in the outer covering of the seed that allows for the passage of water

State the function of the cotyledon of a seed

contains the food stores for the seed and forms the embryonic leaves

State the function of the plumule of a seed

the embryonic shoot aka the epicotyl

State the function of the radicle of a seed

embryonic root

List the factors needed for germination of all seeds and the reasons for requirement

* oxygen: for aerobic respiration as the seed needs lots of ATP
* water: for metabolic activation as it triggers the synthesis of gibberellin
* temperature: for optimal function of enzymes
* pH: for optimal function of enzymes

List the specialized conditions for germination and the reasons for them

fire, freezing, digestion to erode the seed coat, washing to remove inhibitors, scarification to weaken the seed coat