6.8
Antioxidant Enzymes
Formation of reactive oxygen species (ROS) has been associated with many types of
stress. It produces mainly superoxide radicals, generated from oxygen and electrons
that have leaked from the electron transport chain. ROS cause cellular damage, such
as protein inactivation, membrane damage due to lipid peroxidation, and damage to
DNA (Santoro and Thiele 1999). Increase in oxidative stress in cells grown at low
temperature was evidenced by increase in the activity of an enzyme and also in the
amount of free radicals generated, in the cold-grown cells. The association between
cold stress and oxidative stress demonstrated in this investigation bolsters the
concept of interlinked stress response in bacteria (Chattopadhyay et al. 2011). The
higher level of ROS in microbes nicely correlated as a result of freezing and thawing.
The activity of superoxide dismutase and other ROS detoxifying system has likewise
proven to be important for freeze tolerance in bacteria (Stead and Park 2000).
6.9
RNA Degradosomes
To protect from cold effect bacteria, synthesize RNA degradosome. This is a proteincomplex of several ribonucleases that serve as a major determinant factor for
stability of cellular RNA. The degradosome of an Antarctic bacterium Pseudomonas
syringae has been found to contain an endoribonuclease RNAse E and an RNA
helicase (Purusharth et al. 2005). But instead of polynucleotide phosphorylase,
which is the exoribonuclease found in mesophilic E. coli, the degradosome of the
Antarctic bacterium contains another exoribonuclease, called RNAse R. In E. coli
this enzyme is known to play an important role in ensuring the quality control of
rRNA. The significance of the association of this enzyme with RNAse E in the
Antarctic bacterium is not definitely known. But it is believed that RNAse R can
degrade RNA molecules with extensive secondary structures. This eliminates the
necessity of ATP, required by helicase, thereby helping the cell conserve energy at
low temperatures (Purusharth et al. 2005). This signifies that RNA metabolism is
highly influenced by the RNA secondary structures at low temperature.
6.10 Cold Shock Proteins (Csps)
A sudden drop of environmental temperature causes a number of physico-chemical
changes in bacteria that severely affect cellular function. The “cold shock” response
in microorganisms is a transient phenomenon that affects growth rate of cell,
membrane structure, and function and rates of DNA, RNA, and protein synthesis
(Herbraud and Potier 1999). Cold shock response is evidently not confined to
psychrophilic (cold loving) and psychrotrophic (cold tolerant) microorganisms but
constitutes the beginning of cold adaptation in all microbes. However, it has been
noticed that the responses are similar in both groups of organisms, except that the
actual temperature which induces them is much lower (0–4
C) in case of
6 Plant Growth Promoting Rhizobacteria: Mechanisms and Alleviation of Cold Stress. . . 197
Antioxidant Enzymes
Formation of reactive oxygen species (ROS) has been associated with many types of
stress. It produces mainly superoxide radicals, generated from oxygen and electrons
that have leaked from the electron transport chain. ROS cause cellular damage, such
as protein inactivation, membrane damage due to lipid peroxidation, and damage to
DNA (Santoro and Thiele 1999). Increase in oxidative stress in cells grown at low
temperature was evidenced by increase in the activity of an enzyme and also in the
amount of free radicals generated, in the cold-grown cells. The association between
cold stress and oxidative stress demonstrated in this investigation bolsters the
concept of interlinked stress response in bacteria (Chattopadhyay et al. 2011). The
higher level of ROS in microbes nicely correlated as a result of freezing and thawing.
The activity of superoxide dismutase and other ROS detoxifying system has likewise
proven to be important for freeze tolerance in bacteria (Stead and Park 2000).
6.9
RNA Degradosomes
To protect from cold effect bacteria, synthesize RNA degradosome. This is a proteincomplex of several ribonucleases that serve as a major determinant factor for
stability of cellular RNA. The degradosome of an Antarctic bacterium Pseudomonas
syringae has been found to contain an endoribonuclease RNAse E and an RNA
helicase (Purusharth et al. 2005). But instead of polynucleotide phosphorylase,
which is the exoribonuclease found in mesophilic E. coli, the degradosome of the
Antarctic bacterium contains another exoribonuclease, called RNAse R. In E. coli
this enzyme is known to play an important role in ensuring the quality control of
rRNA. The significance of the association of this enzyme with RNAse E in the
Antarctic bacterium is not definitely known. But it is believed that RNAse R can
degrade RNA molecules with extensive secondary structures. This eliminates the
necessity of ATP, required by helicase, thereby helping the cell conserve energy at
low temperatures (Purusharth et al. 2005). This signifies that RNA metabolism is
highly influenced by the RNA secondary structures at low temperature.
6.10 Cold Shock Proteins (Csps)
A sudden drop of environmental temperature causes a number of physico-chemical
changes in bacteria that severely affect cellular function. The “cold shock” response
in microorganisms is a transient phenomenon that affects growth rate of cell,
membrane structure, and function and rates of DNA, RNA, and protein synthesis
(Herbraud and Potier 1999). Cold shock response is evidently not confined to
psychrophilic (cold loving) and psychrotrophic (cold tolerant) microorganisms but
constitutes the beginning of cold adaptation in all microbes. However, it has been
noticed that the responses are similar in both groups of organisms, except that the
actual temperature which induces them is much lower (0–4
C) in case of
6 Plant Growth Promoting Rhizobacteria: Mechanisms and Alleviation of Cold Stress. . . 197