Many organisms are highly adapted to environmental temperature

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Many organisms are highly adapted to
environmental temperature

• Psychrophiles (lt 20?C)
• Mesophiles ( 20-35?C)
• Thermophiles ( 35-70?)
• Hyperthermophiles ( gt 70-110?C)

There is growing interest in understanding how
organisms adapt to extreme cold.
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Adaptations to Cold

• Can produce high levels of osmotic molecules
• amino acids (e.g., proline)
• special amino acids (e.g., glycine betaine)
• sugars (trehalose (glu-glu disaccharide),
  mannitol
• glycerol
• Protein (enzymes) sequences
• Higher levels of certain key enzymes
• Many cryophilic plants also adapted to excess
  light stress (made worse by cold)

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Protein enzymes show evidence of adaptation to
low temperature

• Enzymes from psychrophilic organisms
• usually have a lower temperature optimum than
  their mesophilic counterpart
• are typically more thermolabile
• are usually more efficient catalysts
• are likely more flexible

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McMurdo Dry valley, Antartica
Glacier
Lake Bonney
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Red Snow
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Chlamydomonas nivalis

• Most common snow alga of Northern Hemisphere
  Red Snow is the aplanospore form
• May be more psychrotrophic than psychrophilic
  (some isolates can grow at gt 20?C)
• Red color from astaxanthin, a carotenoid,
  cytoplasmic, protects chloroplast from UV light

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Cold Tolerance

• Different cultivars of the same species vary in
  cold tolerance
• suggests a genetic basis
• Temperate perennial plants "overwinter" to
  survive - develop "cold hardiness
• a quantitative genetic trait (many loci that are
  additive)
• All temperate perennials and many annuals can
  "cold acclimate

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Cold acclimation (CA)

• CA is an inducible response, by cold (e.g., 4oC)
  and sometimes other environmental cues (e.g.,
  photoperiod)
• Cellular processes are adjusted to increase
  tolerance to cold
• e.g., exposure of Arabidopsis to 4oC for 1 day
  allows it to withstand -8 to -12oC
• Also affected by developmental stage
• CA has genetic basis - also a quantitative trait

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How does cold damage plants?

• Ice formation
• Intracellular cells die from the expansion
• Extracellular not as bad, but cells can
  collapse from dehydration
• Direct cold damage
• Membranes plasma membrane and chloroplast
  envelope probably main targets, cold and
  dehydration alters biophysical properties
• Enzymes - some are denatured by cold

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Membrane phospholipids change to gel-phase at low
temperature.
Inverted hexagonal phase can also form in the
cold due to dehydration.
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CA involves

• increased accumulation of certain small solutes
  (amino acids and sugars)
• help retain water
• help stabilize membranes and proteins
• Solutes usually neutral (zwitterions) or
  uncharged molecules derived from polymers (Fig.
  22.6)
• changes in lipid composition of membranes
• changes in gene expression

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Membrane lipid changes in response to cold.
Degree of unsaturation of the fatty acids (i.e,
how many with a CC double bond) increases at
lower temperature. Relative ratios of different
phospholipids also changes, with more PE at
lower temperature. These changes lower the temp.
of the liquid?gel-phase transition.
PE phosphatidylethanolamine PC
phosphatidylcholine
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Effects of cold on gene expression comparison
with heat stress

• Plants exposed to cold synthesize new proteins
• Sizes of strongly induced proteins not as well
  conserved as in heat shock, although some seen
  in many species.
• Housekeeping proteins continue to be
  synthesized.
• Some proteins synthesized transiently, whereas
  some synthesized for weeks.
• New proteins appear within a day after cold
  shift.

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Microarray analysis of cold regulated genes in
Arabidopsis. Seki et al. (2001), Plant Cell 13,
61-72 1300 genes (as cDNAs) immobilized on a
microarray, which is hybridized to mRNAs from
normal and cold-stressed plants (after reverse
transcription into cDNA with a fluorescently
labeled nucleotide). Cy5- red,
induced Cy3-green, depressed Yellow- unchanged
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Genes induced by cold

• Many of unknown function, but some interesting
  ones
• Lea(s) also expressed in seeds before
  dehydration (protective?)
• (Table 22.2 five groups of Leas)
• Antifreeze proteins - (e.g., kin1 - similar to a
  fish antifreeze protein), prevent or limit ice
  formation, some are extracellular
• other hydrophilic proteins (e.g., Cor proteins)
• Proteases
• Heat shock protein
• RNA-binding proteins
• Other regulatory proteins (transcription factors,
  Ca2-binding proteins, etc.)
• Appearance of many of these gene products
  correlate with CA!
• Over-expression of some have produced greater
  freezing tolerance.

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Regulation of Cold-induced genes

• Transcriptional and post-transcriptional
  regulation have been noted
• Many cold-regulated genes contain the
  DRE/C-repeat element in their promoters
• DRE elements bound by proteins, CBF/DREB1
• activate transcription from a DRE element in
  yeast
• DNA-binding domain homologous to apetala2

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Over-expression of CBF1 in Arabidopsis increased
expression of cold-related (COR) genes and
freezing tolerance.
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Role of ABA (stress hormone)

• ABA Abscisic acid, phytohormone induced by
  wilting, closes stomata by acting on guard cells
  
• Positive correlation between CA and ABA
• Treat plants with ABA, and they will be somewhat
  cold hardened
• possibly cold induces ABA, which induces genes?
• However, ABA does not induce all genes that cold
  will.
• Conclusion there are ABA-regulated and non-ABA
  regulated changes that are induced by cold.

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Possible pathway for gene regulation by cold
Cold ? Ca 2 uptake ? activates protein
kinase(s) ? phosphorylation of regulatory
proteins (e.g., transcription factors) ? changes
in gene expression
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Plants vary in ability to tolerate flooding

• Plants can be classified as
• Wetland plants (e.g., rice, mangroves, Hydrilla)
• Flood-tolerant (e.g., Arabidopsis, maize, wheat)
• Flood-sensitive (e.g., soybeans, peas, tomato)
• involves developmental/structural, cellular and
  molecular adaptations

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Pneumatophores weird roots In Mangroves For
gas exchange.
Fig. 22.20
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Flooding causes anoxia and an anaerobiotic
response in roots.
Maize Fig. 22.23
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ANAEROBIOSIS (low oxygen stress)

• mostly studied in maize and recently Arabidopsis
• roots can become anoxic when flooded
• Must shift carbohydrate metabolism from
  respiration to anaerobic glycolysis
• protein synthesis affected

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Aerobic Anoxic
Protein synthesis in aerobic versus anoxic maize
root tips. 5-hour labeling with 3H-leucine and
2-D gel electrophoresis.
Fig. 22.30
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Protein synthesis changes

• Dramatic
• slow response (hours)
• results in selective synthesis of 10-20
  proteins
• Response in two phases
• phase I (first few h) mainly a 33 kDa (ADH)
  protein synthesized
• phase II (12- 48h) 20 proteins dominate
  synthesis
• mRNAs for other proteins there but not translated
  well!

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Anaerobic proteins (ANPs)

• Many of the ANPs are enzymes in or associated
  with glycolysis and fermentation (examples)
• UDP-sucrose synthetase
• glucose-phosphate isomerase
• pyruvate decarboxylase
• alcohol dehydrogenase I and II
• Already being synthesized but rate increases
• Anaerobic response of roots similar in other
  species examined (cotton, soybeans, etc.)
• Some other tissues also show it, but not leaves

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Roots get sucrose from leaves during the day. At
night, they break down stored starch.
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ADH is one of the most prominent ANPs It is a
dimer with 2 main isozymic forms
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Regulation of ADH1 has been studied.
Increased transcription and efficient translation
Fig. 22.32
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• How is translational control achieved?
• Mechanism not yet clear, but phosphorylation of
  eIF4E (cap binding protein) and ribosomal
  protein S6 is induced by anoxia in maize roots.
• May explain repression of translation of the
  cellular mRNAs other than ANP mRNAs.
• Other questions
• What is the oxygen sensing mechanism?
• How does the low oxygen signal control the
  transcription factor(s) for the ANP genes?