Download File - Ms. Richards IB Biology HL

Topic 9 Plant Biology
Part II
Why? We love plants
9.3 Growth in plants
Nature of science: Developments in scientific research follow improvements in analysis and
deduction—improvements in analytical techniques allowing the detection of trace amounts
of substances has led to advances in the understanding of plant hormones and their effect
on gene expression. (1.8)
Understandings
• Undifferentiated cells in the meristems of plants allow indeterminate growth
• Mitosis and cell division in the shoot apex provide cells needed for extension of the stem
and development of leaves
• Plant hormones control growth in the shoot apex
• Plant shoots respond to the environment by tropisms
• Auxin efflux pumps can set up concentration gradients of auxin in plant tissue
• Auxin influences cell growth rates by changing the pattern of genes
9.3 Growth in plants
Applications and Skills
• Application: Micropropagation of plants using tissue from the shoot
apex, nutrient agar gels, and growth hormones
• Application: Use of micropropagation for rapid bulking up of new
varieties, production of virus-free strains of existing varieties and
propagation of orchids and other rare species
9.4 Reproduction in plants
Nature of science: Paradigm shift—more than 85% of the world’s 250,000 species
of flowering plant depend on pollinators for reproduction. This knowledge has led
to protecting entire ecosystems rather than individual species. (2.3)
Understandings
• Flowering involves a change in gene expression in the shoot apex
• The switch to flowering is a response to the length of light and dark periods in
many plants
• Success in plant reproduction depends on pollination, fertilization, and seed
dispersal
• Most flowering plants use mutualistic relationships with pollinators in sexual
reproduction
9.4 Reproduction in plants
Applications and Skills
• Application: Methods used to induce short-day plants to flower out of
season
• Skill: Drawing internal structure of seeds
• Skill: Drawing of half-views of animal-pollinated flowers
• Skill: Design of experiments to test hypotheses about factors affecting
germination
Meristems
• There are two types of meristems
1. Apical meristems
2. Lateral meristems
• Apical meristems: primary meristems
• Lateral meristems: cambium
• Meristems generate new cells for growth of the
plant throughout the plant’s life
• This property is termed indeterminate growth
• Two main types of indeterminate growth:
1. Primary growth
2. Secondary growth
Primary Growth
• Primary growth allows roots to extend throughout the soil and
shoots to increase exposure to light and carbon dioxide
• This is performed by the apical meristems located at the tips of roots
and in the buds of shoots
• Apical meristems are found in all flowering plants and enable the
plant to grow in length
Secondary Growth
• Performed by lateral meristems and is only present in dicots
• Growth in thickness = secondary growth
• Caused by activity of lateral meristems called the vascular cambium
and cork cambium
• Extend along the length of roots and stems
• Vascular cambium adds layers of vascular tissue called secondary
xylem (wood) and secondary phloem
• Cork cambium replaces the epidermis with periderm, which is thicker
and tougher
• Mitosis and cell division in the shoot apex (apical meristem) provide
cells needed for extension of the stem and development of leaves
• Some cells always remain in the meristem and continue through the
cell cycle
• With each division, one cell remains in the meristem while the other
increases in size and differentiates as it is pushed away from the
meristem region
• These cells are displaced to the edge of meristem
Things apical meristem can give rise to:
• Protoderm (epidermis);
Procambium (vascular tissue);
ground meristem (pith)
• Cells at edge stop dividing and
undergo growth and
differentiation to become
either stem or leaf tissue
• Leaves are initiated as small
bumps at the side of the
apical dome and are called
leaf primordia
Growth and Plant Hormones
• Plant development involves extensive coordination among individual
cells within a plant
• This requires communication within the plant
• Many factors affect plant development and growth
Environmental factors such as day length and water availability
Receptors which allow the plant to detect certain environmental
factors
Genetic makeup of plant
Hormones which are chemical messengers
Plant Hormones
• Plant hormones have varying effects depending on the receptor’s
location in the plant
• There is often a great deal of interaction between the different
hormones to bring about the most appropriate response
• In plants, a hormone may be produced throughout the plant, not just
in an individual organ
Auxin and Phototropism
• Tropism is the growth response that results in curvatures of whole plant
organs toward or away from stimuli
• Growth of a shoot toward light is called positive phototropism and
gravitropism is growth in response to gravitational forces
• Phototropism was discovered from studies of grass seedlings (oats)
• Think photo= light, tropism= growth thus growth toward light
• Shoot of a sprouting grass seedling is enclosed in a sheath called the
coleoptile
• Coleoptile grows straight up if kept in dark or lit uniformly from all sides
• If illuminated from one side, it grows toward light as a result of a
differential growth of cells on opposite sides of the coleoptile
• Cells on darker side elongate faster than cells on the brighter side
Phototropism
Auxin and Phototropism
• Both phototropism and gravitropism are auxin dependent
• First, light is absorbed by photoreceptors
• Proteins within these called phototrophins absorb the light
• When phototrophins absorb a light of a certain wavelength they will
change conformation and will bind to receptors and induce
transcription of certain genes
• These genes most likely code for glycoproteins in the plasma
membrane of the cells in the stem that transport auxin
• These glycoproteins are called PIN3 proteins
• The type and position of PIN3 proteins is variable to the where auxin
is needed
• Example: light is shining on the tip of a plant stem more intensely on
one side vs the other
• Phototrophins will detect the light on one side of the stem and auxin
will be transported laterally to the side receiving LESS light
• Thus there is a higher concentration of auxin on the shaded side
• This will cause greater growth of the cells on the shadier side which
will cause the stem to curve toward the light allowing the leaves to
receive more light
Auxin and Phototrophism
Auxin
• Auxin – any chemical substance that promotes elongation
• It was purified by Kenneth Thimann at Cal Tech and determined to be
indoleacetic acid (IAA)
• Auxins are found in the embryo of seeds, the meristems of apical buds,
and young leaves
• IAA is produced by all cells in the stem on the side towards the light
• It is transported into the nuclei of cells on the side of the stem opposite
the light
• Caused by distribution of auxin transport proteins (auxin efflux carriers)
in the cell. Synthesized by apical meristem of a shoot
• The auxin and a receptor in the nuclei form a complex that activates a
proton (hydrogen ion) pump
• Proton pumps play a major role in growth response of cells to auxin. Auxin
stimulates plasma membrane’s proton pumps
• Proton pump moves hydrogen ions into the spaces of the cell wall
• Pumping of H+ increases voltage across membrane and lowers the pH in cell
wall which activates enzymes called expansins that break the cross-links
(hydrogen bonds) between cellulose microfibril. Increased ion uptake, uptake of
water, increased turgor, increased cell wall plasticity
• This results in the elongation of the cells on the side away from the light
• Auxin also alters gene expression and causes new protein production
Auxins have many functions within a plant
• Stimulation of cell division in most meristematic tissues
• Differentiation of xylem and phloem
• Development of lateral roots in tissue culture
• Suppression of lateral bud growth when present in the apical bud
• Stimulation of growth of flower parts
• Induction of fruit production without pollination
Auxin and Gravitropism
• The upward and downward growth of a plant depends on gravity
• If a root is placed on its side, gravity will cause organelles called
statoliths to accumulate on the lower side of the cells
• PIN3 proteins will direct auxin transport to the bottom cells
• High amounts of auxin in the roots will inhibit cell elongation
• The top cells elongate faster than the bottom cells causing the roots
the bend down
• So… this is opposite of how auxin works in the stem
Application: Micropropagation of plants
• Micropropagation is an in vivo procedure that produces large numbers of
identical plants. Much faster and takes less space than traditional methods
of propagation
1. Cells from the shoot apex are cultured on nutrient agar. (derived from
plant with some desirable feature). Depends on totipotency (ability of a
cell to divide and produce all cells in an organism) of plant tissues
2. Growth hormones are used to achieve maximum growth and quantity of
particular types of plants (auxin and cytokinin- plant growth hormones,
phytohormones, that promote cell division in plant roots and shoots)
3. Rare and endangered species can be maintained
Micropropagation
4. Used in the case of orchids, which have very small seeds, to grow
plants more reliably in sterile cultures
5. Must maintain pathogen-free environment when culturing
meristematic tissue
6. Has been used to develop virus-free strains of existing plants. This
allows international exchange of plant materials
7. Can be used in conjunction with gene transfer creating GMO plants
8. Micropropagation is very expensive
Micropropagation
9.4 Reproduction in plants
9.4 Reproduction in plants
Nature of science: Paradigm shift—more than 85% of the world’s 250,000 species
of flowering plant depend on pollinators for reproduction. This knowledge has led
to protecting entire ecosystems rather than individual species. (2.3)
Understandings
• Flowering involves a change in gene expression in the shoot apex
• The switch to flowering is a response to the length of light and dark periods in
many plants
• Success in plant reproduction depends on pollination, fertilization, and seed
dispersal
• Most flowering plants use mutualistic relationships with pollinators in sexual
reproduction
9.4 Reproduction in plants
Applications and Skills
• Application: Methods used to induce short-day plants to flower out of
season
• Skill: Drawing internal structure of seeds
• Skill: Drawing of half-views of animal-pollinated flowers
• Skill: Design of experiments to test hypotheses about factors affecting
germination
Flower Structure
Skill: Drawing of half-views of animal-pollinated flowers
• Review guide: Lamium album, Plum flower, Azalea
Reproductive parts of flowering plants
• Flowers are specialized shoots bearing the
reproductive organs of the angiosperm
sporophyte (2N stage… do you remember what
2N is?)
• Female gametes are contained in ovules in the
ovaries of the flower
• Pollen grains are produced by stamens contain
the male gametes
• The 4 kinds of flower organs, in sequence from
the outside to the inside of the flower are the
sepals, petals, stamens, and carpels
Stamen
• Stamen are the male reproductive organ
consisting of a stalk called a filament and
a terminal structure called the anther
• Think staMEN  MEN  MALE
• The male gametophytes (1N stage) are
sperm-producing structures called pollen
grains which form within the pollen sacs
of anthers
Carpel
• Carpel is the female reproductive organ
and has ovaries at its base and a slender
neck called the style
• At the top of the style is a sticky
structure called the stigma that serves as
a landing platform for pollen
• Within the ovary are one or more ovules
• The female gametophytes are eggproducing structures called embryo sacs
which form within the ovules in ovaries
Sepals and Petals
• Non-reproductive structures
• Sepals enclose and protect the floral bud
before it opens and are usually green and
more leaf-like in appearance than the
other floral organs
• Petals are more brightly colored than
sepals and advertise the flower to insects
and other animal pollinators
Fertilization
• Fertilization definition- the formation of a zygote by the union of a
male gamete with a female gamete inside the ovule
• For the egg within the embryo sac to be fertilized the male and
female gametophytes must meet and their gametes must fuse
Pollination
• Pollination definition- the transfer of pollen from an anther to a stigma
• Pollination occurs when pollen, released from anthers and carried by wind
or animals, lands on a stigma (not necessarily on the same flower or plant)
• Each pollen grain produces a structure called a pollen tube, which grows
down into the ovary via the carpel and discharges sperm nucleus into the
embryo sac resulting in fertilization of the egg
• The pollen tube completes its growth by entering an opening (micropyle)
at the bottom of the ovary
• The sperm moves from the tube to combine with the egg of the ovule to
form a zygote
Pollen
Mutualism regarding pollination
• 85% of the world’s 250,000 species of flowering plants depend on
insects or other animal pollinators for reproduction
• Appears that the first angiosperms were pollinated by insects.
Convincing fossil evidence that angiosperms and insects coevolved
• Most flowering plants use mutualistic relationships with pollinators
in sexual reproduction
Mutualism
• Plant benefits- flowers are pollinated so that they
form seeds and pass on their genes
• Pollinators benefit- obtains nectar which is a source
of energy and pollen, which is a source of protein
• Co-evolution often entails developing a mutualistic
relationship for pollination with one specific species
of insect
• Example: Vanilla orchid pollinated by a species of
Melipona bee. Advantage: insect will transfer pollen
from flower to flower of the species and not to other
species
• Orchid mimics bee and has scent to attract males
How does pollinator find flower?
• Flowers can employ various means to attract their vector
• Red flowers are conspicuous for birds
• Yellow and orange flowers are noticed by bees
• Heavily scented flowers can be located at night
• If wind pollinated, often have inconspicuous, odorless flower parts
Fertilization
• Actually a double fertilization
• One of the two sperms produced by the pollen grain combines with
the egg
• The other combines with two polar nuclei within the ovary to
produce a triploid (3N) endosperm
• The endosperm has the function of storing nutrients for the young
plant
Reproductive Adaptations: Fertilized ovules
develop into seeds
• Include reduced gametophyte, advent of the seed, and the evolution
of pollen
• A seed consists of a sporophyte embryo packaged along with a food
supply within a protective tough coat
• Ovaries containing fertilized ovules develop into fruits
Skill: Drawing internal structure of seeds
Bean Seed
• Parts of the seed include:
1. Seed coat called the testa
2. Embryo root or radicle
3. Embryo shoot or epicotyl which has at its tip
4. Plumule which consists of the shoot tip with a pair of miniature leaves
5. Cotyledons
• The cotyledons of the bean plant are fleshy before the seed germinates
because they absorbed food from the endosperm when the seed developed
• The endosperm is a food-storing tissue of the seed
• Hypocotyl is part of the developing stem
External Structure
For this class you
will be expected to
DRAW internal AND
external structures
of a seed!!
Hilum- scar on seed
where it was
attached to seed
vessel
Internal Structure
Internal Structure
Embryotic Development
• The zygote gives rise to an embryo, and as the embryo grows, the
ovule that contains it develops into a seed
• The entire ovary, meanwhile, develops into a fruit containing one or
more seeds
• Fruits, carried by wind or by animals, help disperse seeds some
distance from their source plants
• So short and sweet… the function of fruit is to disperse seeds
Germination
• As a seed matures, it dehydrates and enters a phase referred to as
dormancy, a condition of extremely low metabolic rate and a
suspension of growth and development
• Conditions required to break dormancy vary between plant species
• Some seeds germinate as soon as they are in a suitable environment
• Others remain dormant and will not germinate, even if sown in a
favorable place, until favorable environmental cue causes them to
break dormancy
Breaking dormancy
• Desert plants germinate only after a substantial rain
• Where natural fires are common, many seeds require intense heat to
break dormancy
• Where winters are harsh, seeds may require extended exposure to
cold
• Some require light for germination
• Some seeds have coats that must be weakened by chemical attack as
they pass through an animal’s digestive tract
General Factors Needed for Seed
Germination
1. Water to rehydrate the dry tissues of the seed
2. Oxygen for aerobic cell respiration
• Some seeds respire anaerobically if oxygen is not available but ethanol
produced in anaerobic respiration usually reaches toxic levels
3. Temperature suitable for germination
• Since germination involves enzymes, temperature cannot be too low (low
collision of enzyme’s active site with substrate) or too high (denaturation
of enzyme). Seeds stay dormant when temps are too high or too low
Metabolic events in germination
1.
2.
3.
4.
5.
6.
Seed absorbs water
Gibberellic acid is produced by the embryo
Gibberellic acid stimulates production of amylase
Amylase catalyzes the breakdown of starch to maltose
Maltose diffuses to embryo
Maltose is used for energy production and growth
Skill: Design of experiments to test hypotheses
about factors affecting germination
• Be aware of safety
• Some of the factors you should consider- temperature, plant
hormones, salt concentrations, light, wavelengths of light, water
levels, possible toxins (heavy metals, pesticides)
• Independent, dependent (length of shoot or root at X days),
controlled variables (temp., oxygen, water, # seeds/dish/container,
amount of time, method of measurement)
• Hypothesis
• Materials and procedures
• Data collected and what method of analysis (t-test, regression, etc)
Metabolic events during germination
• Imbibition- the uptake of water causes the seed to expand and
rupture its coat and also triggers metabolic changes in the embryo
that enable it to resume growth
• Gibberellin production- the plant hormone, gibberellin, is
synthesized in the cotyledons of the seed.
• Gibberellin stimulates the production of the digestive enzyme,
amylase, which catalyzes the digestion of starch into maltose in the
food stores of the seed
More metabolic events
• Maltose (like any good disaccharide used for transport) is transported
from the food stores to the growth regions of the seedling, including
the embryo root and the embryo shoot
• Maltose conversion- maltose is converted to glucose, which is either
used in aerobic cell respiration as a source of energy, or is used to
synthesize cellulose or other substances needed for growth
• Photosynthesis kicks in as soon as the leaves of the seedling have
reached light and have opened. Photosynthesis supplies the seedling
with food and the food stores of the seed are no longer needed
9.4.2 Control of flowering
• Plants can detect not only the presence of light but also its direction,
intensity, and wavelength (color)
• Two major classes of light receptors
1. Blue-light photoreceptors
2. Phytochromes- photoreceptors that absorb mostly red light
Phytochromes
• Consists of two identical proteins joined to form
one functional molecule
• Each of these proteins has two domains
1. Photoreceptor- covalently bonded to a nonprotein pigment or chromophore
2. Kinase- photoreceptor domains interact with
kinase domains to link light reception to
cellular responses triggered by the kinase
• Chromophore of a phytochrome is photoreversible
meaning it can be converted back and forth between 2
isomers
• Pr is inactive absorbs red light of wavelength 660 nm or
white light (400-700 nm)
• Pfr absorbs far-red light 730 nm
• When Pr absorbs red it is converted to Pfr
• When Pfr absorbs far-red light, it is converted back to Pr
• In normal sunlight: Pr rapidly converted to Pfr
• Pr is more stable than Pfr, so in darkness Pfr very gradually
changes into Pr
Phytochrome Swing
9.4.1 Control of Flowering
• When a plant has roots, stems, and leaves only  vegetative phase
• When a plant produces flowers  reproductive phase
• This change occurs when meristems in the shoot start to produce
parts of flowers instead of leaves
• This involves a change to the pattern of gene expression in cells
produced by the meristem
• In some plants, the trigger is a particular day length
• Experiments done in growth chambers where the length of light and
dark periods are controlled showed that it is the length of darkness
that matters, not the length of daylight
Long-day (short night) vs. short-day (long night) plants
• Long-day plants: flower in summer when nights have become short
enough (asters, lettuce, spinach, and potatoes)
• Short-day plants: flower in autumn, when nights have become long
enough (chrysanthemums, poinsettias, and Christmas cactus)
• Pfr is the active form and binds to receptor proteins in cytoplasm. Pr
does not bind.
• In long-day plants, large enough amounts of Pfr remain at end of short
nights to bind to the receptor, which then promotes transcription of
genes needed for flowering. Pfr is a promoter in these plants
• In short-day plants, the receptor inhibits the transcription of genes
needed for flowering when Pfr binds to it. Pfr is an inhibitor in these
plants
• However, at the end of long nights, very little Pfr remains, so the
inhibition fails and the plant flowers
Application: Methods used to induce shortday plants to flower out of season
• Example: Poinsettias, Chrysanthemum
• Can maintain plants in pots in greenhouses with blinds
• When the nights are not long enough to induce flowering, the blinds
are closed to extend the nights artificially
• Some growers will cover individual plants with black cloth for 12-15
hours a day until the flower buds begin to show color
Detecting Plant Hormones
• Hormones in plants, like hormones in animals, are regulators of gene
transcription (transcription factors)
• Concentrations at which plant hormones are active can be as low as
picograms/gram of plant tissue (1 million millionth of a gram)
• Five groups of hormones which are very diverse and require different
extraction methods
Methods- ELISA
• ELISA (Enzyme linked immunosorbent assays)- an antibody is synthesized
to the hormone (which acts as the antigen)
• A primary antibody will bind to a sample of phloem or tissue that has been
absorbed onto a plastic well
• A secondary antibody that has an enzyme linked to it will bind to the
primary antibody/hormone complex
• The substrate of the enzyme is added so that there is a color change
• The color is read and interpreted
• If you inject an antigen (hormone) into an animal (goat, rabbit, mouse) the
animal will make antibodies from B lymphocytes which can be cultured and
antibodies extracted. These antibodies are used to detect the “antigen”
Methods- Gas/Liquid Chromatography
• Gas chromatography (GC) is a common type of chromatography used
in analytical chemistry for separating and analyzing compounds that
can be vaporized without decomposition
Methods- Microarray
• Microarray analysis detects changes in the pattern of gene
expression
• Begins by isolating RNA samples from cells and synthesizing DNA
copies
• Hybridizing cDNAs linked to fluorescent dyes to small surface with
millions of DNA probes
• Laser light makes the fluorescent dyes give off light