All living organisms perceive and process information from the environment at the cellular level through different kinds of receptors, which transfer the signal inside the cell and activate downstream signalling cascades. For example, in humans, ‘thermoTRP’ proteins are involved in thermosensing and are activated by the cool temperatures (10-23°C) and by compounds that cause a sensation of coolness, such as menthol or eucalyptol. However, to date no cold sensor has been found in any higher plant. Furthermore, it has been shown that cold-responsive gene expression in plants increases in response to gradual decreases in temperature, as well as to sudden shifts to cold temperatures, indicating that plants can sense and respond both to the magnitude and the rate of the temperature change, making it probable that there are multiple mechanisms present to detect and mediate plant thermal responses.  

Fig14.22_0.png

Figure 14.22. Simplified model illustrating cold sensing and the induction of stress gene expression during the early alarm stage of cold hardening in Arabidopsis. ICE1 is a constitutive, nuclear localized, transcription factor that is activated by phosphorylation and controls the expression of a range of secondary transcription factors, such as the CBF family of cold-induced transcription factors, that regulate the expression of the functional, delayed response genes. These delayed response genes, in turn, play important roles in cold acclimation and the acquisition of freezing tolerance in both herbaceous and woody perennial species.

One of the more favoured candidates for sensing low temperature is a change in membrane fluidity (i.e. viscosity of the lipid bilayer of cell membranes). This reflects the fact that the structure and fluidity of lipid membranes is dependent on their composition and temperature. Low temperatures decrease the fluidity of lipid membranes as the hydrogen bonding between adjacent fatty acids is increased. The temperature at which membranes undergo a conversion from a fluid state (that exists at warm-moderate temperatures) to a gel-like state (that exists in membranes at low temperatures) is defined as the ‘transition temperature’ (T_m). T_m values as high as 15–20^oC have been reported for some species, with increases in the unsaturated fatty acid content of membranes decreasing the T_m. Below the T_m, the function of membrane-bound enzymes and transport of substrates is often reduced owing to the gel-like state of the membrane.  

The mechanism via which the decreases in membrane fluidity are linked to subsequent downstream responses is still unclear, but it is likely that fluidity-mediated increases in cytosolic concentrations of calcium ([Ca²⁺]cyt) play a role (Figure 14.22). This is because decreases in fluidity are often associated with a fast, cold-induced increase in [Ca²⁺]cyt, mainly as a result of Ca²⁺ transfer across the plasma membrane from the extracellular spaces. Ca²⁺ is integral to normal cell function and its signalling network is extensive.

Increases in [Ca²⁺]cyt are thought play a role in the cold acclimation process, with [Ca²⁺]cyt acting as a secondary messenger. The increase of [Ca²⁺]cyt results from the activation of calcium channels primarily located in plasma membranes, and it has been suggested that the Ca²⁺ channels may either directly respond to cold or in some way sense changes in the plasma membrane or the cytoskeleton, triggering the molecular response to the change in temperature. The Ca²⁺ signal is decoded in plants by calmodulins (Ca²⁺-binding messenger proteins) and Ca²⁺-dependent protein kinases (Figure 14.22). Mitogen-associated protein kinase (MAPK) cascades participate in the signalling pathways of an array of abiotic stress responses and act as downstream transducers of the cold stress-induced Ca²⁺ signal.  Cold Ca²⁺ signalling is affected by the circadian rhythm and depends partly on cell type and sub-cellular location, potentially allowing specific responses from this generic signal.

While Ca²⁺ is known to regulate the activity of many signalling components, including phospholipases and protein kinases, and lead to the induction of cold-induced gene expression or repression, many questions remain. For example, the identity of the Ca²⁺-dependent protein kinases and the genes whose expression they control are generally unknown. Finally, while this represents one of the more intensively investigated options to explain thermal sensing and signal transduction, it is important to note that Ca²⁺-signalling is not the only sensing mechanism, with Ca²⁺-signalling often exhibiting extensive cross-talk with other signalling systems, including that of reactive oxygen species (ROS). For example, an increase in [Ca²⁺]cyt activates mitochondrial external NAD(P)H dehydrogenases, thus lowering the reductive power of the inter-membrane space and potentially reducing ROS formation. Such a combination of signalling networks most likely allows generic stress responses to occur in response to a variety of stimuli, leading to cross tolerance. Finally, Ca²⁺-signalling events may take place at different locations within the cell (e.g. within the different organelles), will have different physiological causes (e.g. production of ROS in the chloroplast or mitochondria) and, as a result, will occur at different specific times following exposure to a chilling stress event.

Once the change to cold temperatures is perceived and the signal transduced to the nucleus, there follows a substantial reprogramming of the transcriptome, proteome and metabolome of the plant cell (i.e. cold acclimation). The cold acclimation response is best understood in herbaceous annuals such as Arabidopsis thaliana where members of the C-repeat binding factor (CBF or DREB1) family of transcriptional activators, which bind the cis-element known as the C-repeat (CRT)/dehydration-responsive element (DRE), have been shown to control the transcription of a suite of genes that play important roles in the development of freezing tolerance. Subsequent to their discovery in Arabidopsis, many CBF homologues have been found in both monocots and dicots, including perennial species such as aspen, birch and Eucalyptus. The CBF/DREB transcription factors are inducible by stress events but are not normally expressed under non-stress conditions. The plants response to stress therefore requires that there are components within the signal transduction pathway that are constitutively present but only active following the perception of the stress signal.  For plant cold responses, a MYC-type bHLH (basic Helix-Loop-Helix) transcription factor, ICE1 (inducer of CBF expression 1), serves as an upstream activator of CBF expression. Transcriptional profiling of the ice1 mutant showed impaired expression of 40% of cold-regulated genes, suggesting ICE1 is one of the main regulators in the cold stress response, but also that it is not the only regulator. ICE1 is constitutively localized to the nucleus and induces CBF expression in a cold-dependent fashion (Figure 14.22). The ability of ICE1 to activate gene transcription in response to cold may be dependent on protein phosphorylation, making ICE1 a likely target of the MAPK cascades activated by the transient increase in [Ca²⁺]cyt, although the signalling components responsible for this activation are yet to be discovered.

The accumulation of CBF transcripts and the activity of the CRT regulatory motif in Arabidopsis is also modulated by the presence and quality of light during cold stress, and it also appears to be mediated by the circadian gate; with extent to which CBF transcripts accumulate in response to low temperatures being dependent on the time of day plants are exposed to low temperatures. Temperatures are typically at their lowest point in the diurnal cycle at night, and the ability to anticipate this day-night rhythm may give plants the ability to anticipate night frosts and thus confer an adaptive advantage. Entrainment of the circadian clock has been shown to be affected by temperature as well as by light and it appeared that this entrainment might be linked to the regulation of the cold stress response. Plants in temperate regions are also able to anticipate the onset of winter by detecting the shortening photoperiod. This light signalling, or measurement of the critical day-length, is mediated by the phytochromes and it is not surprising that sensing and signalling mechanisms controlling developmental processes such as bud set, seasonal senescence, growth cessation and dormancy overlap with cold acclimation. This potential for overlap is consistent with the observation that cold acclimation can be modified by the red and far-red light (R/FR) ratio.  A decrease in R/FR ratio at the end of the day promotes cold acclimation, including increasing the expression of CBF genes; conversely, exposure to red pulses of light can undo this effect and a high R/FR reduces the accumulation of CBF-regulated gene expression.  Finally, it is clear that there are ICE/CBF-independent pathways that participate in cold acclimation (e.g. expressions of ZAT, HOS and ESK); however, evidence pointing to the identity of the components in these pathways is scarce. Furthermore, in field or natural conditions it is likely that the transcriptomic, proteomic and metabolomic changes in response to alterations in the plants thermal regime will be more complex than those revealed by controlled laboratory experiments. Responses to additional stressors such as light and drought will overlap and will introduce different signalling pathways, particularly those involving ABA, but also brassinosteriods and jasmonic acid. As result, constructing a spatiotemporal network linking these different components in order to understand how these factors come together to limiting plant growth, productivity and distribution, is going to be an enormous challenge.

One of the most characteristic changes associated with cold acclimation are increases in the degree of saturation of membrane fatty acids, leading to greater membrane fluidity and an increase the functionality of membrane-bound substrate transporters and membrane-bound proteins at low temperatures. Moreover, cold-induced modifications in gene expression, subsequent changes in the abundance and activity of proteins, and alterations in other processes such as source-sink relationships and accumulation of metabolic intermediates, collectively lead to increases in the concentration of many metabolites. Thus, sustained exposure to cold leads to major modifications to the metabolome of plants.  Enhanced concentrations of soluble sugars are foremost amongst these, the most studied being the monosaccharides glucose and fructose, the disaccharide sucrose, and the trisaccharide raffinose; levels of each rise within a few hours of the onset of a cold treatment. Starch also accumulates in cold-exposed leaves.  The increase in sucrose is particularly rapid, despite the fact that cold often has strong inhibitory effect on one of the enzymes (sucrose phosphate synthase, SPS) responsible for its synthesis. Levels of sucrose continue to rise during the first few days of cold acclimation, aided by a long-term increase in the abundance and activity of SPS. Moreover, overall sugar levels generally remain elevated in plants exposed to sustained cold; this accumulation reflects shifts in source:sink relationships, underpinned by changes in the balance between carbon uptake by photosynthesis, carbon use by catabolic processes (e.g. respiration) and carbon export to other parts of the plant.  In some plants, increased concentrations of sugars may convey cryoprotective properties, reducing the incidence of membrane lesions and increasing survival during freezing (see Section 14.6). 

Cold treatment also leads to the accumulation of a range of other metabolites, including compatible solutes (i.e. small, highly soluble molecules that are non-toxic at high concentrations). The most studied of these is proline, which accumulates dramatically following cold exposure. Proline appears to act as a cryoprotectant, as evidenced by the fact that freezing tolerant mutants of Arabidopsis accumulate proline to very high levels, even in the absence of a cold stimulus.

Cold is known to markedly inhibit rates of photosynthesis, via both its kinetic effects on protein activity and membrane fluidity. Rapid increases in soluble sugars also contribute to an inhibition of photosynthesis through feedback inhibition and down-regulation of nuclear-encoded photosynthetic gene expression. Extended cold-treatment of pre-existing leaves can also result in photodamage; low temperature slows the consumption of ATP and NADPH by the Calvin cycle and the resulting over-reduction of the photosynthetic electron transport chain may cause oxidative damage to the light-harvesting machinery. A further (and important) limitation is the exhaustion of chloroplastic orthophosphate (Pi) required for ATP regeneration and maintenance of the thylakoid membrane proton gradient, which in turn drives the chloroplastic electron transport chain. Calvin cycle turnover also decreases (explaining the observed decline in carbon assimilation) as this relies on ATP to drive the regeneration of ribulose-1,5-bisphosphate (RuBP) reduction.

Recovery of photosynthesis occurs during cold acclimation via an increase in the export of chloroplastic triose phosphates across an antiporter that exchanges these Calvin cycle intermediates for cytosolic Pi. As Pi is produced in the cytosol during sucrose synthesis, the recovery of sucrose metabolism via increases in the abundance and activity of SPS and cytosolic fructose 1,6 bisphosphatase (cFBPase) is an important step in the restoration of photosynthetic function during cold acclimation, and has also been suggested to be partly responsible for the decrease in sensitivity to photoinhibition that occurs following cold acclimation.  Also contributing to the recovery of photosynthesis is the increase in the abundance and activity of several proteins directly involved in photosynthesis.

The photosynthetic phenotype of cold-developed leaves has been suggested to be distinct from that of pre-existing leaves. Cold-developed leaves exhibit marked physical changes compared to warm-grown leaves (Figure 14.23). Leaves developed in the cold tend to be thicker, with a higher leaf mass per area and higher nitrogen and protein concentrations than leaves developed at warmer temperatures.

Fig14.23.jpg

Figure 14.23. (A) Warm-grown (left), 10-day cold-treated (centre) and cold-developed (right) shoot phenotypes of Arabidopsis thaliana.  Warm-grown plants experienced 25/20°C day/night temperatures, whereas cold-treated and cold-developed plants were exposed to constant 5°C. (B) and (C) show transverse sections of representative warm-grown and cold-developed leaves, respectively (Source: Atkin et al. 2006). 

Cold developed leaves can accumulate soluble sugars without the suppression of carbon assimilation typically associated with an abundance of photosynthates, and the transcript and protein abundance of most of the Calvin cycle enzymes have been found to be greatly increased in these leaves. The abundance of ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) increases, whereas the other Calvin cycle enzymes tend to decline with cold acclimation relative to Rubisco. Further recovery of sucrose synthesis in cold developed leaves assists in the recovery from cold stress and photoinhibition; SPS transcript abundance, protein and the activity of the enzyme increases relative to the Calvin cycle and starch synthesis during acclimation. Associated with the increase in photosynthetic capacity in cold developed leaves is a change in electron transport capacity relative to carboxylation capacity, and an alleviation of triose phosphate utilization (TPU) limitation. Alleviation of TPU limitation during acclimation to low temperatures has been ascribed to increasing activity and transcription of SPS and cFBPase, and the redistribution of inorganic phosphate between cellular compartments. The net result of these changes is an increase in photosynthetic capacity in newly developed, relative to pre-existing, leaves.  This phenomenon may therefore optimise the function of these new leaves to their environment.

For over a century, it has been known that short-term exposure to cold in measurements lasting minutes to a few hours results in reduced rates of respiratory CO₂ release and O₂ uptake. When measured over a range of moderate temperatures (e.g. 15-25^oC), the relationship between respiration and temperature is near exponential, with respiration exhibiting short-term Q₁₀ values near 2.0 (i.e. respiration increases two-fold for every 10^oC increase in temperature). However, when viewed over a wider range of temperatures, the shape of the temperature response tends to be more dynamic, reflecting the fact that the functional form of the short-term temperature response curve of respiration departs significantly from a simple exponential. As a result, short-term Q₁₀ values typically increase with short-term decreases in measuring, commonly reaching values >3.0 at temperatures in the 0-10^oC range. Theory and empirical evidence suggests that the increasing temperature-sensitivity of respiration as measurement temperatures decrease is linked to shifts in the control exerted by substrate limitations at moderate-high temperature to maximum enzyme activity at low temperature. This is either because of the inhibitor effect of cold on potential enzyme activity per se (both in soluble and membrane-bound compartments) and/or limitations on the function of enzymes embedded in membranes at temperatures below the T_m. At moderately high temperatures (e.g. 25^oC), respiratory flux is less limited by enzymatic capacity because of increases in the V_max [i.e. maximal flux through the respiratory system as a whole, or parts thereof, in the absence of other limiting factors (e.g. substrate supply and adenylates)] of enzymes in soluble and membrane-bound compartments; here, respiration is likely to be limited by substrate availability and/or adenylates (in particular the ratio of ATP to ADP and the concentration of ADP per se, which are in turn influenced by the demand for respiratory energy by growth and cellular maintenance processes). Increased leakiness of membranes at temperatures above the T_m (particularly at high temperatures) could further contribute to substrate limitations. Changes in growth temperature that last several days can also alter the short-term Q₁₀, with Q₁₀ values also varying seasonally in some ecosystems. 

A further factor that could contribute to variability in the sensitivity of respiration to low temperatures is the extent to which cold differentially suppresses activity by the two terminal pathways via which electrons are transferred to oxygen in the inner mitochondrial membrane [i.e. the phosphorylating cytochrome oxidase pathway (COP) versus the non-phosphorylating alternative oxidase pathway (AOP)]. To date, results from studies with intact tissues and isolated mitochondria from a range of plant species and organs have yielded conflicting conclusions, with some studies suggesting that the AOP might be less sensitive to cold that the COP, while others have reported the opposite or little difference between the temperature sensitivity of the two pathways. Thus, at this stage it is unclear to what extent differential temperature sensitivities of the AOP and COP play a role in influencing variations in the short-term Q₁₀ of plant respiration.

Fig14.24.png

Figure 14.24. Theoretical examples of two types of respiratory thermal acclimation: (a) Type I and (b) Type II. (a) in Type I acclimation, changes in growth temperature result in changes in the Q₁₀ of respiration with no change in the value of respiration at low temperatures (‘B’) (i.e. the intercept remains unchanged). Rather, changes in respiration are only observed at moderate to high temperatures (‘A_c’>’A_w’>’A_h’). Shifting to low growth temperatures for an extended period typically results in an increase in the Q₁₀, whereas the Q₁₀ decreases following shift to high growth temperatures. (b) Type II acclimation results in changes in respiration at both low and high temperatures (i.e. the overall elevation of the temperature response curve is affected). No changes in the Q₁₀ of respiration are necessary for Type II acclimation. Type II acclimation will result in a greater degree of homeostasis of respiration than Type I acclimation. Source: Atkin and Tjoelker (2003)

With sustained exposure to cold, respiratory rates (when measured at a low temperature) start to recover quickly (within hours), and continue to increase as the plant acclimates. Often, the capacity for respiratory enhancement is relatively low in pre-existing tissues; here, acclimation is associated with a change in the rate of respiration primarily at moderate to high measuring temperatures, with little or no change in respiration at low measuring temperatures (i.e. Type I acclimation; Figure 14.24, reflecting a change in the availability of respiratory substrate and/or degree of adenylate restriction of respiration.  Changes in gene expression may also occur, but are not essential for the overall change in respiratory flux. In other cases, acclimation is associated with an increase in the rate of respiration over a wide range of measurement temperatures (‘Type II acclimation’; Figure 14.24). Type II acclimation is likely associated with temperature-mediated changes in respiratory capacity that can be maximally realized through growth of new tissues with altered morphology and biochemistry. Increases in respiratory capacity in the cold appears to be altered as a result of increases in the density of mitochondria mitochondrial number per unit volume of tissue and/or amount of total protein invested in the respiratory chain. The observation that mitochondrial density is higher in in situ alpine than lowland plants further supports the notion that an increase in mitochondrial number is a driving force behind increased respiratory rate in cold-acclimated leaves.  Intermediate cases of acclimation (i.e. between Types I and II) are likely, particularly in individual plants that experience long-term changes in temperature depending on the extent to which respiratory capacity is altered in pre-existing and newly formed leaves and roots.  Another characteristic of acclimation (particularly Type II acclimation) is that it can result in respiratory homeostasis [i.e. identical rates of R in plants grown and measured in contrasting temperatures (Figure 14.24).

Previous work examining the impact of sustained exposure to cold on respiration has suggested that the AOP might play a critical role in the cold acclimation response. In some studies, alternative oxidase transcript abundance, protein abundance, and capacity have all been shown to increase following growth in the cold. In addition, there are examples where in vivo partitioning of electrons to the AOP has been shown to increase following sustained exposure to cold. Such studies have led to the proposal that cold induced increases in AOP activity function to prevent the over-reduction of the mitochondrial electron transport chain, and thus the accumulation of ROS, at low temperatures. However, in reality the response of the AOP to cold may be more complex. For example, in some cases, the recovery of AOP activity in cold acclimated plants is transient (e.g. reaching a maximum after few days of sustained cold treatment), while others have reported that re-establishment of respiratory flux in the cold is associated not with an increase in AOP capacity, but rather with an increase in energy-conserving COP capacity. Such variability in the AOP response suggests that different plant species employ different strategies for coping with cold.  

Finally, consideration should be given to the fact that plant mitochondria possess two additional non-phosphorylating bypasses of the mitochondrial electron transport chain: the alternative NAD(P)H dehydrogenases (NDHs) and the uncoupling proteins (UCPs). Both of these proteins function to reduce the extent to which mitochondrial electron transport is coupled to the production of ATP. The NDHs oxidize matrix and cytosolic NAD(P)H, but do not contribute to proton pumping, while the UCPs facilitate proton flux back through the inner mitochondrial membrane, thereby partially dissipating the proton gradient across this membrane. Both the NDHs and the UCPs are thought to play a role in preventing oxidative stress, with sustained cold treatment increasing transcripts for both types of proteins.  Moreover, there is growing evidence that transcript levels of these alternative respiratory bypasses exhibit coordinated increases in abundance following sustained cold treatments. Collectively, the above suite of modifications in metabolic processes contribute to the ability of many plants to cope with sustained exposure to low temperatures.

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