Plants to feed the world

1
Plants to feed the world

• (Chapter 11)

2
Plants to feed the world

• Hunger, starvation, and malnutrition are endemic
  in many parts of the world today.
• Rapid increases in the world population have
  intensified these problems!
• ALL of the food we eat comes either directly or
  indirectly from plants.
• Cant just grow more plants, land for cultivation
  has geographic limits
• Also, can destroy ecosystems!

3
Plants to feed the world

• At the latest count there are between 250,000 and
  400,000 plant species on the earth.
• But three - maize, wheat and rice - and a few
  close runners-up, have become the crops that feed
  the world. All produce starch, helping to provide
  energy and nutrition, and all can be stored.
• Maize converts the suns energy into sugar
  faster, and potentially produces more grains,
  than any of the other major staples.

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Plants to feed the world

• The term Green Revolution is used to describe the
  transformation of agriculture in many developing
  nations that led to significant increases in
  agricultural production between the 1940s and
  1960s
• Scientists bred short plants that converted the
  suns energy into grain rather than stem, so
  preventing the mass starvation in the developing
  world predicted before the 1960s, at a cost of
  higher inputs from chemical fertilizers and
  irrigation.

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Plants to feed the world

• Disease-resistant wheat varieties with high yield
  potentials are now being produced for a wide
  range of global, environmental and cultural
  conditions.
• The Green Revolution has had major social and
  ecological impacts, which have drawn intense
  praise and equally intense criticism.

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Plants to feed the world

• The Green Revolution is sometimes misinterpreted
  to apply to present times.
• In fact, many regions of the world peaked in food
  production in the period 1980 to 1995, and are
  presently in decline, since desertification and
  critical water supplies have become limiting
  factors in a number of world regions.

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A few of the many medicinal plants
8
Energy flow through an ecosystem

• Energy enters as sunlight
• Producers convert sunlight to chemical energy.
• Consumers eat the plants (and each other).
• Decomposer organisms breakdown the organic
  molecules of producers and consumers to be used
  by other living things
• Heat is lost at every step So Sun must provide
  constant energy input for the process to continue!

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Photosynthesis

• Very little of the Suns energy gets to the
  ground
• gets absorbed by water vapor in the atmosphere
• The absorbance spectra of chlorophyll.
• Absorbs strongly in the blue and red portion of
  the spectrum
• Green light is reflected and gives plants their
  color.
• There are two pigments
• Chlorophyll A and B

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Photosynthetic pigments

• Two types in plants
• Chlorophyll- a
• Chlorophyll b
• Structure almost identical,
• Differ in the composition of a sidechain
• In a it is -CH3, in b it is CHO
• The different sidegroups 'tune' the absorption
  spectrum to slightly different wavelengths
• light that is not significantly absorbed by
  chlorophyll a, will instead be captured by
  chlorophyll b

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Photosynthetic pigments

• Chlorophyll has a complex ring structure
• The basic structure is a porphyrin ring,
  co-coordinated to a central atom.
• This is very similar to the heme group of
  hemoglobin
• Ring contains loosely bound electrons
• It is the part of the molecule involved in
  electron transitions and redox reactions of
  photosynthesis

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The Chloroplast

• Membranes contain chlophyll and its associated
  proteins
• Site of photosynthesis
• Have inner outer membranes
• 3rd membrane system
• Thylakoids
• Stack of Thylakoids Granum
• Surrounded by Stroma
• Works like mitochondria
• During photosynthesis, ATP from stroma provide
  the energy for the production of sugar molecules

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General overall reaction

• 6 CO2 6 H2O C6H12O6
  6 O2
• Carbon dioxide Water
  Carbohydrate Oxygen

Photosynthetic organisms use solar energy to
synthesize carbon compounds that cannot be formed
without the input of energy. More specifically,
light energy drives the synthesis of
carbohydrates from carbon dioxide and water with
the generation of oxygen.
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The chemical reaction of photosynthesis is driven
by light

• The initial reaction of photosynthesis is
• CO2 H2O (CH2O) O2
• Under optimal conditions (red light at 680 nm),
  the photochemical yield is almost 100
• However, the efficiency of converting light
  energy to chemical energy is about 27
• Very high for an energy conversion system

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The chemical reaction of photosynthesis is driven
by light

• Quantum efficiency Measure of the fraction of
  absorbed photons that take part in
  photosynthesis.
• Energy efficiency Measure of how much energy in
  the absorbed photons is stored as chemical
  products
• ¼ energy from photons stored the rest is
  converted to heat

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The light reactions

• Step 1 chlorophyll in vesicle membrane capture
  light energy
• Step 2 this energy is used to split water into
  2H and O.
• Step3 O released to atmosphere. Each H is
  further split into H ion and an electron (e-).
• Step 4 H ion build up in the stacked vesicle
  membranes.

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The light reactions

• Step 5 The e- move down a chain of electron
  transport proteins that are part of the vesicle
  membrane.
• Step 6 e- ultimately delivered to the molecule
  NADP - forming NADPH
• Step 7 - Some membrane proteins pump H into the
  interior space of the vesicle
• Stored energy
• Step 8 These make ATP!

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Summary of light reactions

• Plants have two reaction centers
• PS-II
• Absorbs Red light 680mn
• makes strong reductant ( weak oxidant)
• oxidizes 2 H2O molecules to 4 electrons, 4
  protons 1 O2 molecule
• Mostly found in Granum
• PS-I
• Absorbs Far-Red light 700nm
• strong oxidant ( weak reductant)
• PS-I reduces NADP to NADPH
• Mostly found in Stroma

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The Carbon reactions

• The NADPH and ATP move into the liquid
  environment of the Stroma.
• The NADPH provides H and the ATP provides energy
  to make glucose from CO2.
• The Calvin cycle thus fixes atmospheric CO2 into
  plant organic material.

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Overview of the carbon reactions

• The Calvin cycle
• The cycle runs six times
• Each time incorporating a new carbon . Those six
  carbon dioxides are reduced to glucose
• Glucose can now serve as a building block to
  make
• polysaccharides
• other monosaccharides
• fats
• amino acids
• nucleotides

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Photorespiration

• Occurs when the CO2 levels inside a leaf become
  low
• This happens on hot dry days when a plant is
  forced to close its stomata to prevent excess
  water loss
• If the plant continues to attempt to fix CO2 when
  its stomata are closed
• CO2 will get used up and the O2 ratio in the leaf
  will increase relative to CO2 concentrations
• When the CO2 levels inside the leaf drop to
  around 50 ppm,
• Rubisco starts to combine O2 with
  Ribulose-1,5-bisphosphate instead of CO2

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The C4 carbon Cycle

• The C4 carbon Cycle occurs in 16 families of both
  monocots and dicots.
• Corn
• Millet
• Sugarcane
• Maize
• There are three variations of the basic C4 carbon
  Cycle
• Due to the different four carbon molecule used

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The C4 carbon Cycle

• This is a biochemical pathway that prevents
  photorespiration
• C4 leaves have TWO chloroplast containing cells
• Mesophyll cells
• Bundle sheath (deep in the leaf so atmospheric
  oxygen cannot diffuse easily to them)
• C3 plants only have Mesophyll cells
• Operation of the C4 cycle requires the
  coordinated effort of both cell types
• No mesophyll cells is more than three cells away
  from a bundle sheath cells
• Many plasmodesmata for communication

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How the rest of plant works

• Roots absorb water from the soil as well as
  many mineral nutrients
• Xylem transports water from the roots to the
  rest of the plant
• Phloem transports sugars made in the leaves via
  photosynthesis to the pest of the plant
• Leaves Site of gas exchange CO2 brought in and
  O2 out. Have structures called Stomata which
  also control water loss.

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Water across plant membranes

• There is some diffusion of water directly across
  the bi-lipid membrane.
• Auqaporins Integral membrane proteins that form
  water selective channels allows water to
  diffuse faster
• Facilitates water movement in plants
• Alters the rate of water flow across the plant
  cell membrane NOT direction

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Water transport in Plants

• Xylem
• Main water-conducting tissue of vascular plants.
• arise from individual cylindrical cells oriented
  end to end.
• At maturity the end walls of these cells dissolve
  away and the cytoplasmic contents die.
• The result is the xylem vessel, a continuous
  nonliving duct.
• carry water and some dissolved solutes, such as
  inorganic ions, up the plant

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Water transport in Plants

• Phloem
• The main components of phloem are
• sieve elements
• companion cells.
• Sieve elements have no nucleus and only a sparse
  collection of other organelles . Companion cell
  provides energy
• so-named because end walls are perforated -
  allows cytoplasmic connections between
  vertically-stacked cells .
• conducts sugars and amino acids - from the
  leaves, to the rest of the plant

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Osmosis and Tonicity

• Osmosis is the diffusion of water across a plasma
  membrane.
• Osmosis occurs when there is an unequal
  concentration of water on either side of the
  selectively permeable plasma membrane.
• Remember, H2O
• CAN cross the plasma membrane.

• Tonicity is the osmolarity of a solution--the
  amount of solute in a solution.
• Solute--dissolved substances like sugars and
  salts.
• Tonicity is always in comparison to a cell.
• The cell has a specific amount of sugar and salt.

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Tonic Solutions

• A Hypertonic solution has more solute than the
  cell. A cell placed in this solution will give
  up water (osmosis) and shrink.
• A Hypotonic solution has less solute than the
  cell. A cell placed in this solution will take
  up water (osmosis) and blow up.
• An Isotonic solution has just the right amount of
  solute for the cell. A cell placed in this
  solution will stay the same.

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Plant cell in hypotonic solution

• Flaccid cell in 0.1M sucrose solution.
• Water moves from sucrose solution to cell
  swells up becomes turgid
• This is a Hypotonic solution - has less solute
  than the cell. So higher water conc.
• Pressure increases on the cell wall as cell
  expands to equilibrium

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Plant cell in hypertonic solution

• Turgid cell in 0.3M sucrose solution
• Water movers from cell to sucrose solution
• A Hypertonic solution has more solute than the
  cell. So lower water conc
• Turgor pressure reduced and protoplast pulls away
  from the cell wall

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Plant cell in Isotonic solution

• Water is the same inside the cell and outside
• An Isotonic solution has the same solute than the
  cell. So no osmotic flow
• Turgor pressure and osmotic pressure are the same

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Water transport

• Transpiration
• Evaporation of water into the atmosphere from the
  leaves and stems of plants.
• It occurs chiefly at the leaves while their
  stomata are open for the passage of CO2 and O2
  during photosynthesis.
• Transpiration is not simply a hazard of plant
  life. It is the "engine" that pulls water up from
  the roots to
• supply photosynthesis (1-2 of the total)
• bring minerals from the roots for biosynthesis
  within leaf
• cool the leaf.

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Stomatal control

• Almost all leaf transpiration results from
  diffusion of water vapor through the stomatal
  pore
• waxy cuticle
• Provide a low resistance pathway for diffusion of
  gasses across the epidermis and cuticle
• Regulates water loss in plants and the rate of
  CO2 uptake
• Needed for sustained CO2 fixation during
  photosynthesis

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Stomatal control

• When water is abundant
• Temporal regulation of stomata is used
• OPEN during the day
• CLOSED at night
• At night there is no photosynthesis, so no demand
  for CO2 inside the leaf
• Stomata closed to prevent water loss
• Sunny day - demand for CO2 in leaf is high
  stomata wide open
• As there is plenty of water, plant trades water
  loss for photosynthesis products

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Stomatal control

• When water is limited
• Stomata will open less or even remain closed even
  on a sunny morning
• Plant can avoid dehydration
• Stomatal resistance can be controlled by opening
  and closing the stomatal pores.
• Specialized cells The Guard cells

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Stomatal guard cells

• Guard cells act as hydraulic valves
• Environmental factors are sensed by guard cells
• Light intensity, temperature, relative humidity,
  intercellular CO2 concentration
• Integrated into well defined responses
• Ion uptake in guard cell
• Biosynthesis of organic molecules in guard cells
• This alters the water potential in the guard
  cells
• Water enders them
• Swell up 40-100

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Relationship between water loss and CO2 gain

• Effectiveness of controlling water loss and
  allowing CO2 uptake for photosynthesis is called
  the transpiration ratio.
• There is a large ratio of water efflux and CO2
  influx
• Concentration ratio driving water loss is 50
  larger than that driving CO2 influx
• CO2 diffuses 1.6 times slower than water
• Due to CO2 being a larger molecule than water
• CO2 uptake must cross the plasma membrane,
  cytoplasm, and chloroplast membrane. All add
  resistance

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water status of plants

• Cell division slows down
• Reduction of synthesis of
• Cell wall
• Proteins
• Closure of stomata
• Due to accumulation of the plant hormone Abscisic
  acid
• This hormone induces closure of stomata during
  water stress
• Naturally more of this hormone in desert plants

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Plants and water

• Water is the essential medium of life.
• Land plants faced with dehydration by water loss
  to the atmosphere
• There is a conflict between the need for water
  conservation and the need for CO2 assimilation
• This determines much of the structure of land
  plants
• 1 extensive root system to get water from soil
• 2 low resistance path way to get water to leaves
  xylem
• 3 leaf cuticle reduces evaporation
• 4 stomata controls water loss and CO2 uptake
• 5 guard cells control stomata.

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Nitrogen in the environment

• Many biochemical compounds present in plant cells
  contain nitrogen
• Nucleoside phosphates
• Amino acids
• These form the building blocks of nucleic acids
  and protein respectively
• Only carbon, hydrogen, and oxygen are nor
  abundant in plants than nitrogen

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Nitrogen in the environment

• Present in many forms
• 78 of atmosphere is N2
• Most of this is NOT available to living organisms
• Getting N2 for the atmosphere requires breaking
  the triple bond between N2 gas to produce
• Ammonia (NH3)
• Nitrate (NO3-)
• So, N2 has to be fixed from the atmosphere so
  plants can use it

43
Nitrogen in the environment

• This occurs naturally by-Lightning
• 8 splits H2O the free O and H attack N2
  forms HNO3 (nitric acid) which fall to ground
  with rain
• Photochemical reactions
• 2 photochemical reactions between NO gas and O3
  to give HNO3
• Nitrogen fixation
• 90 biological bacteria fix N2 to ammonium
  (NH4)

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Nitrogen in the environment

• Once fixed in ammonium or nitrate -
• N2 enters biochemical cycle
• Passes through several organic or inorganic forms
  before it returns to molecular nitrogen
• The ammonium (NH4) and nitrate (NO3-) ions
  generated via fixation are the object of fierce
  competition between plants and microorganisms
• Plants have developed ways to get these from the
  soil as fast as possible

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How do plants get their nitrogen?

• Some plant species are Legumes.
• Legumes seedlings germinate without any
  association to rhizobia
• Under nitrogen limiting conditions, the plant and
  the bacteria seek each other out by an elaborate
  exchange of signals
• The first stage of the association is the
  migration of the bacteria through the soil
  towards the host plant

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How do plants get their nitrogen?

• Nodule formation results a finely tuned
  interaction between the bacteria and the host
  plant
• Involves the recognition of specific signals
  between the symbiotic bacteria and the host plant
• The bacteria forms NH3 which can be used directly
  by the plant
• The plant gives the bacteria organic nutrients.

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Figure 11.8 (1)
How do plants get their nitrogen?

• Some plants obtain nitrogen from digesting
  animals (mostly insects).
• The Pitcher plant has digestive enzymes at the
  bottom of the trap
• This is a passive trap Insects fall in and can
  not get out
• Pitcher plants have specialized vascular network
  to tame the amino acids from the digested insects
  to the rest of the plant

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Figure 11.12 (2)
How do plants get their nitrogen?

• The Venus fly trap has an active trap
• Good control over turgor pressure in each plant
  cell.
• When the trap is sprung, ion channels open and
  water moves rapidly out of the cells.
• Turgor drops and the leaves slam shut
• Digestive enzymes take over

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Figure 11.13
Increasing crop yields

• To feed the increasing population we have to
  increase crop yields.
• Fertilizers - are compounds to promote growth
  usually applied either via the soil, for uptake
  by plant roots, or by uptake through leaves. Can
  be organic or inorganic
• Have caused many problems!!
• Algal blooms pollute lakes near areas of
  agriculture

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Figure 11.13
Increasing crop yields

• Algal blooms - a relatively rapid increase in the
  population of (usually) phytoplankton algae in an
  aquatic system.
• Causes the death of fish and disruption to the
  whole ecosystem of the lake.
• International regulations has led to a reduction
  in the occurrences of these blooms.

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Figure 11.17
Chemical pest control

• Each year, 30 of crops are lost to insects and
  other crop pests.
• The insects leave larva, which damage the plants
  further.
• Fungi damage or kill a further 25 of crop plants
  each year.
• Any substance that kills organisms that we
  consider undesirable are known as a pesticide.
• An ideal pesticide would-
• Kill only the target species
• Have no effect on the non-target species
• Avoid the development of resistance
• Breakdown to harmless compounds after a short time

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Figure 11.17
Chemical pest control

• DTT was first developed in the 1930s
• Very expensive, toxic to both harmful and
  beneficial species alike.
• Over 400 insect species are now DTT resistant.
• As with fertilizers, there are run-off problems.
• Affects the food pyramid.
• Persist in the environment

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Figure 11.18
Chemical pest control

• DTT persists in the food chain.
• It concentrates in fish and fish-eating birds.
• Interfere with calcium metabolism, causing a
  thinning in the eggs laid by the birds break
  before incubation is finished decrease in
  population.
• Although DTT is now banned, it is still used in
  some parts of the world.

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Genetically modified crops

• All plant characteristics, such as size, texture,
  and sweetness, are determined on the genetic
  level.
• Also
• The hardiness of crop plants.
• Their drought resistance.
• Rate of growth under different soil conditions.
• Dependence on fertilizers.
• Resistance to various pests and diseases.
• Used to do this by selective breeding

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Figure 11.20
Genetically modified crops

• Corn plants have been selective breed to increase
  oil yields or protein
• content for over 70 years.
• Attempts to change one trait at a time can lead
  to the production of an
• inferior strain.
• Breeding plants with high oil content changes
  inherited characteristics
• of a given strain

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Genetically modified crops

• 1992- The first commercially grown genetically
  modified food crop was a tomato - was made more
  resistant to rotting, by adding an anti- sense
  gene which interfered with the production of the
  enzyme polygalacturonase.
• The enzyme polygalacturonase breaks down part of
  the plant cell wall, which is what happens when
  fruit begins to rot.

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Figure 11.21
Genetically modified crops

• So to modify a plant
• Need to know the DNA sequence of the gene of
  interest
• Need to put an easily identifiable maker gene
  near or next to the gene of interest
• Have to insert both of these into the plant
  nuclear genome
• Good screen process to find successful insertion
• Clone the genetically altered plant

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Figure 11.22 (1)
Genetically modified crops
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Genetically modified crops

• Particle-Gun Bombardment
• Selected DNA sticks to surface of metal pellets
  in a salt solution (CaCl2).
• Loaded up into a shot gun cartridge
• Fired into plant material
• The DNA sometimes was incorporated into the
  nuclear genome of the plant
• Gene has to be incorporated into cells DNA where
  it will be transcribed
• Also inserted gene must not break up some other
  necessary gene sequence

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Genetically modified crops

• Agrobacterium method
• Uses the natural infection mechanism of a plant
  pathogen
• Agrobacterium tumefaciens naturally infects the
  wound sites in dicotyledonous plant causing the
  formation of the crown gall tumors.
• Capable to transfer a particular DNA segment
  (T-DNA) of the tumor-inducing (Ti) plasmid into
  the nucleus of infected cells where it is
  integrated fully into the host genome and
  transcribed, causing the crown gall disease.
• So the pathogen inserts the new DNA with great
  success!!!

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Genetically modified crops

• The vir region on the plasmid inserts DNA between
  the T-region into plant nuclear genome
• Insert gene of interest and marker in the
  T-region by restriction enzymes the pathogen
  will then infect the plant material
• Works fantastically well with all dicot plant
  species
• tomatoes, potatoes, cucumbers, etc
• Does not work as well with monocot plant species
  - corn
• As Agrobacterium tumefaciens do not naturally
  infect monocots

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Figure 11.21
Genetically modified crops

• So to modify a plant
• Need to know the DNA sequence of the gene of
  interest
• Need to put an easily identifiable maker gene
  near or next to the gene of interest
• Have to insert both of these into the plant
  nuclear genome
• Good screen process to find successful insertion
• Clone the genetically altered plant

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Figure 11.22 (2)
Genetically modified crops
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Genetically modified crops

• Can alter nutritional content
• Potatoes with 21-22 more starch
• Resistance to pathogens
• Less damage to crops better total yield lower
  retail cost
• Herbicide-resistant plants
• Spraying the fields only kills weeds
• Longer shelf-lives
• More attractive to buy in bulk

65
Genetically modified crops

• Issues
• Destroying ecosystems tomatoes are now growing
  in the artic tundra with fish antifreeze in them!
• Destroying ecosystems will the toxin now being
  produced by pest-resistance stains kill
  friendly insects such as butterflies.
• Altering nature should we be swapping genes
  between species?

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Genetically modified crops

• Issues
• Vegetarians what about those tomatoes?
• Religious dietary laws anything from a pig?
• Cross-pollination producing a super-weed
• Human health what of the antibiotic marker gene?

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The End.

• Any Questions?