paramount role of cbrA/B in central metabolism it is possible that these genes do
have a role to play in regulation of genes responsible for cold tolerance. Cold shock
proteins (Csp) and Cold acclimation proteins (Cap) include many diverse kind of
proteins, which suggest that the genes are involved in all major aspects of cellular
metabolism that are affected by low temperatures.
6.13 Freeze Tolerance in Bacteria
In bacteria, several cellular constituents synthesize to survive in freezing temperature. Most of these freeze protection mechanisms include synthesis of
cryoprotectants, ice nucleators, and antifreeze proteins.
6.13.1 Cryoprotectants
Cryoprotectants are chemical substances, which are known to accumulate in the
cellular fluid of bacterial cell during cold stress. These substances include both sugar
and amino acids. Such cryoprotectants are thought to act as chemical chaperones at
cold temperatures (Margesin and Schinner 1999a, b; Russell 1998). These
cryoprotectants have been repeatedly shown to protect proteins and other
macromolecules against different types of stresses (Chattopadhyay 2006).
6.13.1.1 Sugar Cryoprotectants
Cold tolerant bacteria are endowed with the ability to synthesize several sugars
(glucose, fructose, sucrose, and trehalose) and sugar alcohols (glycerol, mannitol,
sorbitol, erythritol, threitol) cryoprotectants (Chattopadhyay 2006). The cryoprotective role of glucose was reported by Koda et al. (2002) in Pantoea ananas KUIN-3.
They found higher glucose in cells at 10
C when cells were subjected to cold stress
from 30
C (optimum temperature) to 10
C. The cryotolerance of this bacterium
reached 80% after cold acclimation at 10
C. This high level of freezing tolerance in
bacteria was correlated with the cryoprotective role of glucose and high activity of
glucose-6-phosphatase during cold acclimation.
Trehalose is non-reducing disaccharide (α-O-glucopyranosyl-1, 1-2 D
glucopyranoside) found in many prokaryotic and eukaryotic organisms, known to
be important protectants against stress condition (Kandror et al. 2002). Trehalose
plays a vital role in protecting cell against adverse environmental condition. The
function of trehalose is stabilizing the membrane and proteins by replacing water and
preservation of intracellular water structure (Sano et al. 1999). Exogenous trehalose
helps to protect a variety of organisms against freezing and the maximum protection
is found when trehalose is present on both sides of the cell membrane, while the
exogenous trehalose enhances viability of bacteria during freezing temperature
(Herbraud and Potier 1999). Trehalose synthesis is regulated by the genes otsA
and otsB that encode the enzymes, trehalose-6-phosphate synthases and trehalose-6phosphatase, respectively (Kaasen et al. 1992). A mutant of E. coli strain unable to
6 Plant Growth Promoting Rhizobacteria: Mechanisms and Alleviation of Cold Stress. . . 203
have a role to play in regulation of genes responsible for cold tolerance. Cold shock
proteins (Csp) and Cold acclimation proteins (Cap) include many diverse kind of
proteins, which suggest that the genes are involved in all major aspects of cellular
metabolism that are affected by low temperatures.
6.13 Freeze Tolerance in Bacteria
In bacteria, several cellular constituents synthesize to survive in freezing temperature. Most of these freeze protection mechanisms include synthesis of
cryoprotectants, ice nucleators, and antifreeze proteins.
6.13.1 Cryoprotectants
Cryoprotectants are chemical substances, which are known to accumulate in the
cellular fluid of bacterial cell during cold stress. These substances include both sugar
and amino acids. Such cryoprotectants are thought to act as chemical chaperones at
cold temperatures (Margesin and Schinner 1999a, b; Russell 1998). These
cryoprotectants have been repeatedly shown to protect proteins and other
macromolecules against different types of stresses (Chattopadhyay 2006).
6.13.1.1 Sugar Cryoprotectants
Cold tolerant bacteria are endowed with the ability to synthesize several sugars
(glucose, fructose, sucrose, and trehalose) and sugar alcohols (glycerol, mannitol,
sorbitol, erythritol, threitol) cryoprotectants (Chattopadhyay 2006). The cryoprotective role of glucose was reported by Koda et al. (2002) in Pantoea ananas KUIN-3.
They found higher glucose in cells at 10
C when cells were subjected to cold stress
from 30
C (optimum temperature) to 10
C. The cryotolerance of this bacterium
reached 80% after cold acclimation at 10
C. This high level of freezing tolerance in
bacteria was correlated with the cryoprotective role of glucose and high activity of
glucose-6-phosphatase during cold acclimation.
Trehalose is non-reducing disaccharide (α-O-glucopyranosyl-1, 1-2 D
glucopyranoside) found in many prokaryotic and eukaryotic organisms, known to
be important protectants against stress condition (Kandror et al. 2002). Trehalose
plays a vital role in protecting cell against adverse environmental condition. The
function of trehalose is stabilizing the membrane and proteins by replacing water and
preservation of intracellular water structure (Sano et al. 1999). Exogenous trehalose
helps to protect a variety of organisms against freezing and the maximum protection
is found when trehalose is present on both sides of the cell membrane, while the
exogenous trehalose enhances viability of bacteria during freezing temperature
(Herbraud and Potier 1999). Trehalose synthesis is regulated by the genes otsA
and otsB that encode the enzymes, trehalose-6-phosphate synthases and trehalose-6phosphatase, respectively (Kaasen et al. 1992). A mutant of E. coli strain unable to
6 Plant Growth Promoting Rhizobacteria: Mechanisms and Alleviation of Cold Stress. . . 203