Low temperature stress

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Chilling freezing stress

• Chilling
• Chilling injury occurs at temperatures lower than
  normal growth temperatures, but not freezing
  temperatures
• temps around 10C
• usually occurs in plants adapted to warm climates
  
• The main damage caused by chilling is to the
  membranes
• leakage of ions out of the cell
• inefficient functioning of photosynthetic or
  respiratory ATP synthesis
• leaf lesions, wilting

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Freezing

• Freezing damage occurs primarily due to the
  formation of ice crystals, which damage cell
  structure
• ice usually forms first in the cell walls and
  intercellular spaces
• damage occurs when ice crystals grow and
  puncture into the cytoplasm

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Physiology of low Temp.

• Chill-sensitive plants have a higher proportion
  of saturated fatty acids
• 1-saturated lipids solidify at higher
  temperatures
• 2-membranes change from fluid to gel as temps
  drop, lose function
• Plants can become acclimated by slow exposure to
  colder temps, so
• increase the proportion of unsaturated fats in
  the membranes

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Physics of ice formation

• In plants, the temperature of water will drop
  below its freezing temperature and will still
  remain liquid supercooling
• for the transition to solid phase to take place,
  need ice nucleation points
• when becoming solid, ice gives off heat, so the
  temperature rises
• when all of the water in the cell wall has
  frozen, then the temperature begins to drop again

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Avoidance through supercooling

• Many plants can avoid freezing injury, because
  they allow deep supercooling
• the liquid in the intercellular space never
  makes the transition to solid phase, so ice
  crystals don't form
• some can supercool down to -35C
• at -40C, ice crystal formation begins
  spontaneously
• occurs in hardwoods and some fruit trees

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Plants that live in environments colder than -40C
usually don't supercool

• ice crystal formation begins at -3 or -5C in the
  cell walls
• over time, water is pulled from the cytoplasm and
  accumulates on the growing ice crystals (if temps
  drop slowly)
• these plants must have sufficient intercellular
  space for ice crystals to grow
• this results in greater concentration of solutes
  in the cytoplasm (dehydration)
• these plants must also tolerate dehydration,
  similar effect during drought

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Bacteria and ice formation

• Some bacteria can promote ice crystal formation
  at warm temps (-3 or -5C)
• formation of ice crystals at warmer temps can be
  damaging to the plants
• can isolate mutant bacteria that do not promote
  ice formation, these can be sprayed on sensitive
  plants to protect them from frost damage
  (strawberry)
• Acclimation to cold involves the expression of
  some different genes
• isoforms of enzymes that are active at lower
  temps )antifreeze protein(

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According to lindow (1984) report, effective
bacteria for cold injury

• I ce nucleation active bacteria
• Pseudomonas syringae
• P. fluorescens
• Ervinia herbicola
• E. Stewartii
• Therefore, using bactericides inhibit freezing

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Cold Tolerance

• It is plain for all to see that some plants are
  able to survive low temperatures whereas others
  are not. While it is not know precisely what is
  required for cold tolerance, it is known that
  plants respond to low temperatures in a number of
  ways.
• These responses include synthesizing a large
  number of proteins that are not expressed under
  normal conditions. The functions of these
  cold-induced proteins are also not well understood

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Protein synthesize

• organisms typically do not waste their resources
  making proteins that do not serve an important
  and useful function. In this case, you might
  guess that the function of cold-induced proteins
  would be to help the plant survive low
  temperature conditions

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How can this set of proteins be activated in
transgenic plants?

• The approach to this question has been to
  identify what activates all the genes that are
  expressed when it gets cold. The activator is a
  protein that switches on transcription of
  cold-induced genes, referred to as a
  transcription activator. The diagram below
  illustrates how this activator functions.
• Low temperatures --gt Activator is induced --gt
  Cold-induced genes are expressed
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Calcium level and cold stress

• Some plants can adapt to chilling and
  dehydration,
• and some can actually develop a tolerance to
  freezing.
• Low temperature stress appears to be signaled by
  an increase in cellular levels of calcium.
• Sustained exposure to cold stress can cause
  chronic calcium elevation, resulting in
  organismal death.
• Thus, calcium levels must be carefully
  controlled.

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Calcium Cont.

• a connection between stress induced and
• increases in cellular calcium and the similar
  induction of calcium-binding proteins, suggesting
  a physiological role for these proteins in cold-
  and dehydration-stress management.

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calcium-binding proteins are requisite for plants
to respond to dehydration and cold-stress

• Testing whether these calcium-binding proteins
  are requisite for plants to respond to
  dehydration and cold-stress.
• The identification of the location of these
  proteins within the cell and the mechanisms by
  which their calcium-binding properties may be
  regulated will help us to understand the
  biochemical mechanisms that underlie plant
  responses to stress. Through this increased
  understanding we will be better able to manage
  plants in variable and adverse environments.

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• Whats relation between photoperiod acclimation
  and tolerance?
• How long will be hardiness?

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• freezing tolerance is not affected by the
  photoperiod but acclimation is mainly triggered
  by low nonfreezing temperatures.
• Cold acclimation is a rather rapid process in
  many herbaceous species. Upon return to normal
  growth temp. freezing tolerance is rapidly lost.

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• Except low temp. which kinds of other factors may
  be tolerated plant?
• Whats physiol. And Bioch. Changes after
  acclimation?

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• In addition to low non-freezing temperatures,
  cold acclimation can be
• triggered by exposing the plants to moderate
  water stress or exogenously applied abscisic acid
  (ABA).
• Cold acclimation is associated with several
• physiological and biochemical alterations in the
  plants.The best-characterized changes include
• alterations in gene expression, changes in
  hormone levels, increases in soluble sugars,
  amino acids and organic acids, accumulation of
  osmoprotectants and protective proteins as well
  as modification of membrane lipid
• composition

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• Which evidences show the role of ABA in
  acclimation?
• Whats primary sensor to cold stress?

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• Several lines of evidence indicate the role of
  ABA-controlled processes in cold acclimation
• (i) Exogenous application of ABA at normal
  growth temperature leads to increased freezing
  tolerance
• (ii) There is a transient increase in endogenous
  ABA at the onset of acclimation
• (iii) Several low temperature induced genes are
  also responsive to ABA

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Primary signal to low temperature

• primary signal might be the change in membrane
  fluidity, which is one of the most rapid effects
  of temperature on the plasma membrane.
• Reduction of membrane fluidity would lead to gene
  activation, are not understood.
• a clear correlation between temperature and
  calcium influx to the cells has been
  demonstrated.
• temperature modulated Ca channel could indeed be
  involved in the temperature sensing.

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• ABA
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• 3Co2 ? 6PGA PEP Co2 ? OAA (MA)
  
  
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