PLANT GROWTH AND DEVELOPMENT

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PLANT GROWTH AND DEVELOPMENT

• General Features
• Developmental Responses to Plant Hormones
• Auxins
• Cytokinins
• Gibberellins
• Abscisic Acid
• Ethylene
• Other Plant Hormones
• Cellular Basis of Plant Growth
• Developmental (Growth) Responses to Environment
• Photoreceptors
• Signal Transduction Mechanisms

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Assuming you already know

• Genes and gene organization in eukaryotes
• Introns and exons
• Promoters
• Genetic code (codons)
• Transcription
• mRNA
• Translation
• Ribosomes
• tRNA
• ER

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Terminology
Growth irreversible increase in mass involves
cell division cell enlargement/expansion Differ
entiation change from generalized cell type to
specialized one with specific functions - 50
different types of plant cells Development
Growth Differentiation Morphogenesis the
development of form/shape cell, organ, entire
plant depends on what were talking
about
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Plant Growth
1. Localized, Arithmetic Growth (copy error rate
high?) 2. Open Growth continue to initiate new
organs (roots, stems, leaves) throughout
lifespan 3. Indeterminate Growth many continue
to grow throughout lifespan 4. Cell
differentiation is readily reversible
Totipotency single differentiated cell can
behave like zygote and give rise to whole new
plant
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Scientific Cloning of Plants
F.C. Steward (1950s) carrot root phloem cells
can behave like zygotes! regenerate whole fertile
carrot plants genetically identical to original
one! tissue culture callus mass of
undifferentiated cells - can propagate it
forever! (like HeLa cells)
differentiated cells ? callus ? form roots/shoots
? entire plantlet (presence of plant growth
hormones)
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Practical Value of Tissue Culture
1. propagate individual plants with desirable
traits/genomes (orchids, etc.) 2. develop new
genotypes with desired traits genetic
engineering or cloning 3. study genetics of
somatic cells (instead of germ cells) IMMORTALITY
?
some plants become very old bristlecone pine in
Southern California 5,000 years old! oldest
living organism
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Immortality when is a plant dead?
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Cellular Basis of Plant Growth
cell division, enlargement differentiation done
in different parts of plant
axillary bud pocket of meristematic cells left
behind (also vascular cambium) remain
undifferentiated
Zone of cell division Meristem
Zone of cell elongation ( some
differentiation!)
Cell differentiation mostly further away from
apical meristem lot of overlap not sharply
demarcated
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Cell Division the cell cycle
S
G1
G0
G2
interphase
M
some plant growth hormones stimulate cell
division (e.g., cytokinins) causing them to
break dormancy and re-enter cell cycle
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Cell expansion
Turgor Pressure (osmotic properties are driving
force for plant cell growth)
central vacuole
H pumped into cell wall (?pH) 1. weakens
interactions between wall components (pectins,
cellulose, etc.) 2. stimulates expansins
(proteins involved in wall loosening)
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Cell Differentiation
occurs after cell division and enlargement/elongat
ion so it keeps progenitor-type cell
(meristem) intact
meristematic cell maintained
meristematic cell
(arithmetic growth)
protoderm cell
epidermal cell
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Cell Differentiation
cell division itself CAN be involved in
differentiation
trichome initial
asymmetric cell division
elongate epidermal cell
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Genetic Basis of Differentiation
study gene expression during differentiation ce
rtain genes up-regulated, others
down-regulated some genes are specific for
development also! involves temporal (time)
spatial (space) regulation of gene
expression changes in gene expression must by
highly coordinated among genes 1. maintain
proper function in cells/tissues 2. form
complex cellular structures (pts. apparatus,
spindle, etc.)
Patterns of Gene Expression SPATIAL 1.
every cell type (housekeeping genes) 2.
organ-specific 3. tissue-specific 4.
cell-specific TEMPORAL 1. constituitive
on all the time 2. regulated/modulated
a. inducible b. repressible c. rhythmic
(e.g., circadian day/night, on 24-hour clock)
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DIFFERENTIATION GENE EXPRESSION
molecular cloning of genes
permitted study of individual gene expression
provided sequences and probes/tools for such
studies
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Differentiation Gene Expression
Know that developmental regulation of genes
occurs at 1. transcription almost always
2. mRNA stability/degradation 3. translation
4. protein stability/degradation 5.
posttranslational modification (phosphorylation,
etc.) How study gene expression?
Northern Blots Nuclease Protection Assay
Microarrays, etc.
gel
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Gene Expression Microarrays
permit genome-wide gene expression studies
EVERY GENE! but need to know all/most of
possible genes that exist for a plant - get
this from whole-genome sequencing or from
expressed sequence tags (ESTs) mRNA? cDNA ?
clone partially sequence ? identify by
many of them sequence homology
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Microarray
separately spot single-stranded DNA for each gene
on microscope slides (1,000s) or computer
chips ideally want to know which gene each spot
encodes
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mRNA
make cDNA and incorporate fluorescently-labeled
nucleotides
digest RNA to generate single-stranded cDNAs
hybridize to microarray
wash away unbound DNAs
Gene A Gene B Gene C Gene D
MICROARRAY
therefore, Genes A B were being expressed when
isolated mRNA Genes C D were not being
expressed when isolated mRNA
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root parenchyma mRNA
mesophyll mRNA
most expressed genes are not involved in
development
Gene A Gene B Gene C Gene D
only Gene B expressed in both cell types
candidate for developmental gene!
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Microarray
Slide or Chip
intensity of fluorescence proportional to amount
of mRNA!
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Microarray Data
What kinds of data do you get from these
studies? e.g., tobacco (Nicotiana tabacum)
- has 106 kb of DNA/nucleus naïve estimate
average gene/mRNA 1 kb, so 106 genes in
total leaf mRNA - 30,000 genes expressed -
NOT all developmental ones if compared another
organ, might find that 10,000 genes in common
many of these may represent genes involved in
development
maybe 8,000-10,000 genes that are organ-specific
(e.g., leaf) but organs contain MANY kinds of
cells technology improving - can look at mRNA
from few cells- all one type amplify mRNA and
proceed w/probing microarray
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FLOWER DEVELOPMENT
floral parts modified leaves shoot apical
meristem produces primordia giving rise to 4
whorls in order sepals, petals, stamens,
carpels plant expresses 3 classes of homeotic
genes (A,B,C) differentially
class A
class B
class C
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FLOWER DEVELOPMENT
expression of gene classes in primordia of four
different whorls of floral parts
1 2 3 4
A
B
C
sepal petal stamen carpel
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FLOWER DEVELOPMENT
Class B mutant (B not expressed)
1 2 3 4
A
B
C
sepal petal stamen carpel
1 2 3 4
A
B
C
sepal sepal carpel carpel
wild-type
B mutant
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Gene Class B mutant (B is not expressed)
2 outer whorls sepals (A only) 2 inner whorls
carpels (C only)
what would mutant in C produce?
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Gene Class C mutant (C is not expressed)
Note A C are mutually antagonistic so if one
not expressed, the other is expressed
1 2 3 4
A
B
C
sepal petal stamen carpel
1 2 3 4
A
B
C
sepal petal petal sepal
wild-type
C mutant
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Gene Class C mutant (C is not expressed)
all sepals and petals!
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Triple mutant (A, B C)
all 3 activities missing produces only leaves!
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MAJOR PHOTORECEPTORS (PIGMENTS)
photosynthetic accessory pigments
chlorophylls, carotenoids, xanthophylls anthocyani
ns color/attractiveness to pollinators
photoprotection?
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MAJOR PHOTORECEPTORS

• PHYTOCHROME (PHY) - absorbs red/far-red light
• essentially involved in determining daylength
  shading (quality)
• such phenomena as
• a. stem elongation in young seedlings
  emergence from soil
• b. seed germination (on surface of soil)
  seasonal
• c. promoting stomatal opening
• d. promoting leaf development - seasonal
• proplastid ? etioplast ? chloroplast (if light)
• etioplasts have no PS activity lack major
  thylakoid proteins
• Two Forms
• red-absorbing (660 nm) Pr usu.
  inactive form
• far-red absorbing (730 nm) Pfr usu. active
  form
• are interconvertible
• Pr Pfr
• is slow conversion Pfr form to Pr form
  in dark
• red light (660-700 nm) is useable for
  photosynthesis, longer ?s not
• pass through leaves so they represent SHADE!

red
far-red
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MAJOR PHOTORECEPTORS
Pfr is more hydrophobic binds membranes better
is why is active? Phytochrome molecule 120,000
Daltons, 1,000 amino acids
phosphorylation activity
chromophore linear tetrapyrrole (A-D)
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MAJOR PHOTORECEPTORS
Phytochrome experimental test for its
involvement red light ? is response (e.g. leaf
production) far-red light ? no response red
light ? far-red light ? ?? (no
response) whatever plant sees last!
shaded plant on right receives higher ratio of
far-red to red ? increased stem elongation (
delay in leaf production) should eventually
reach white light (which includes red
light) Arabidopsis has 5 PHY genes with
overlapping functions different sensitivities
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Control of flowering

• Long and short-day plants
• Studies of flowering induction
• some plants are sensitive to the period of
  darkness that they are exposed to
• Clearly a mechanism to time flowering.
• Why bother?
• Consider tradeoff between vegetative and
  reproductive growth in a plant.
• Critical night length (varies by species)
• Short night flower when night is less than the
  critical length
• Long night flower when night is longer than
  critical length
• Day neutral night length is not responsible for
  flowering

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Control of Flowering
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Control of flowering

• Reception is in young leaves
• Shine light on leaves will induce flowering
• shine light and immediately remove flower not
  induce flowering
• shine light on apical meristem no induction of
  flowering
• Not simply the reversion of Pfr to Pr
• Reversion in 3 to 4 hours
• Other mechanisms must be part of the clock
  mechanism too

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Control of flowering the signal?

• Grafting experiments
• N. silvestris is a long day plant
• N. tabacum is either day neutral (Trabezond) or
  short day (Maryland Mammoth)
• Graft either long or short to day neutral and
  give correct night length to long and short and
  will induce day neutral to flower
• Grafting of day neutral plants does not increase
  flowering of either plant
• Graft short day and day neutral and give long
  days
• Day neutral flower normally, but not short day
• Graft long day and day neutral and give short
  days
• Nobody flowered

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Diagram of grafting experiment
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Control of flowering

• Interpretation long-day plant must produce
  something that inhibits flowering
• Apparently need to produce an inhibitor of the
  inhibitor to get flowering.
• Not known what substances are involved. Possibly
  gibberellins

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MAJOR PHOTORECEPTORS
2. CRYPTOCHROME (CRY) - absorbs blue light
(350-450 nm) protein 650 amino
acids chromophore a flavin (FAD)
1st discovered in mutant Arabidopsis deficient
in blue light-stimulated leaf development
plays a role in circadian rhythms (part of
biological clock?)
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MAJOR PHOTORECEPTORS
3. PHOTOTROPIN absorbs blue light also
phototropism growth toward light chloroplast
migration in bright light move to side
walls protein - 1,000 amino acids similar
to cryptochrome flavin for photoreceptor
(FMN) but has kinase activity also
LOV domains in genes responding to light,
oxygen or voltage in phytochrome and
cryptochrome too!
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SIGNAL TRANSDUCTION MECHANISMS
a. cGMP - formed GTP---gt cGMP 2Pi by
guanylate cyclase destroyed by cGMP
phosphodiesterase discovered this by studying
mutant tomato plants that did not have good
leaf or plastid development, even in light -
injected cells with various potential 2o
messengers
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SIGNAL TRANSDUCTION MECHANISMS
b. calmodulin - common secondary messenger in
plants binds 4 Ca2 ions and becomes
activated Ca2 - very low levels in cytoplasm
(lt10-7M), tightly regulated increase Ca2 in
influx or release from internal stores
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SIGNAL TRANSDUCTION MECHANISMS
c. trimeric G-proteins one subunit (alpha) is a
GTPase (GTP-binding/hyrdolyzing) becomes
activated when binds GTP beta/gamma subunits
dissociate, and alpha moves to target GTP
hydrolysis releases it from binding to
target beta/gamma reassociated target goes back
to uninduced state
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SIGNAL TRANSDUCTION MECHANISMS
anthocyanins
cGMP
PS I Cyt Bf/B6
PHY
PS II ATP Synthetase LHC II
Ca2
Calmodulin
can be fairly complex with multiple
players! Response system differential gene
expression