Vaughan Hurry

Photosynthesis, photoinhibition and low temperature acclimation in cold tolerant plants.

Cold acclimation requires adjustment to a combination of light and low temperature, conditions which are potentially photoinhibitory. The photosynthetic response of plants to low temperature is dependent upon time of exposure and the developmental history of the leaves. Exposure of fully expanded leaves of winter cereals to short-term, low temperature shiftsinhibits whereas low temperature growthstimulates electron transport capacity and carbon assimilation. However, the photosynthetic response to low temperature is clearly species and cultivar dependent. Winter annuals and algae which actively grow and develop at low temperature and moderate irradiance acquire a resistance to irradiance 5- to 6-fold higher than their growth irradiance. Resistance to short-term photoinhibition (hours) in winter cereals is a reflection of the increased capacity to keep QA oxidized under high light conditions and low temperature. This is due to an increased capacity for photosynthesis. These characteristics reflect photosynthetic acclimation to low growth temperature and can be used to predict the freezing tolerance of cereals. It is proposed that the enhanced photosynthetic capacity reflects an increased flux of fixed carbon through to sucrose in source tissue as a consequence of the combined effects of increased storage of carbohydrate as fructans in the vacuole of leaf mesophyll cells and an enhanced export to the crown due to its increased sink activity. Long-term exposure (months) of cereals to low temperature photoinhibition indicates that this reduction of photochemical efficiency of PS II represents a stable, long-term down regulation of PS II to match the energy requirements for CO2 fixation. Thus, photoinhibition in vivo should be viewed as the capacity of plants to adjust photosynthetically to the prevailing environmental conditions rather than a process which necessarily results in damage or injury to plants. Not all cold tolerant, herbaceous annuals use the same mechanism to acquire resistance to photoinhibition. In contrast to annuals and algae, overwintering evergreens become dormant during the cold hardening period and generally remain susceptible to photoinhibition. It is concluded that the photosynthetic response to low temperatures and susceptibility to photoinhibition are consequences of the overwintering strategy of the plant species.

The hot and the cold: unravelling the variable response of plant respiration to temperature

When predicting the effects of climate change, global carbon circulation models that include a positive feedback effect of climate warming on the carbon cycle often assume that (1) plant respiration increases exponentially with temperature (with a constant Q10) and (2) that there is no acclimation of respiration to long-term changes in temperature. In this review, we show that these two assumptions are incorrect. While Q10 does not respond systematically to elevated atmospheric CO2 concentrations, other factors such as temperature, light, and water availability all have the potential to influence the temperature sensitivity of respiratory CO2 efflux. Roots and leaves can also differ in their Q10 values, as can upper and lower canopy leaves. The consequences of such variable Q10 values need to be fully explored in carbon modelling. Here, we consider the extent of variability in the degree of thermal acclimation of respiration, and discuss in detail the biochemical mechanisms underpinning this variability; the response of respiration to long-term changes in temperature is highly dependent on the effect of temperature on plant development, and on interactive effects of temperature and other abiotic factors (e.g. irradiance, drought and nutrient availability). Rather than acclimating to the daily mean temperature, recent studies suggest that other components of the daily temperature regime can be important (e.g. daily minimum and / or night temperature). In some cases, acclimation may simply reflect a passive response to changes in respiratory substrate availability, whereas in others acclimation may be critical in helping plants grow and survive at contrasting temperatures. We also consider the impact of acclimation on the balance between respiration and photosynthesis; although environmental factors such as water availability can alter the balance between these two processes, the available data suggests that temperature-mediated differences in dark leaf respiration are closely linked to concomitant differences in leaf photosynthesis. We conclude by highlighting the need for a greater process-based understanding of thermal acclimation of respiration if we are to successfully predict future ecosystem CO2 fluxes and potential feedbacks on atmospheric CO2 concentrations.

A plant for all seasons : alterations in photosynthetic carbon metabolism during cold acclimation in Arabidopsis

Low temperatures lead to the inhibition of sucrose synthesis and photosynthesis. The biochemical and physiological adaptations of plants to low temperatures include the post-translational activation and increased expression of enzymes of the sucrose synthesis pathway, the changed expression of Calvin cycle enzymes, and changes in the leaf protein content. Recent progress has been made in understanding both the signals that trigger these processes and how the regulation of photosynthetic carbon metabolism interacts with other processes during cold acclimation.

Cold hardening of spring and winter wheat and rape results in differential effects on growth, carbon metabolism, and carbohydrate content

The effect of long-term (months) exposure to low temperature (5[deg]C) on growth, photosynthesis, and carbon metabolism was studied in spring and winter cultivars of wheat (Triticum aestivum) and rape (Brassica napus). Cold-grown winter rape and winter wheat maintained higher net assimilation rates and higher in situ CO2 exchange rates than the respective cold-grown spring cultivars. In particular, the relative growth rate of spring rape declined over time at low temperature, and this was associated with a 92% loss in in situ CO2 exchange rates. Associated with the high photosynthetic rates of cold-grown winter cultivars was a 2-fold increase per unit of protein in both stromal and cytosolic fructose-1,6-bisphosphatase activity and a 1.5- to 2-fold increase in sucrose-phosphate synthase activity. Neither spring cultivar increased enzyme activity on a per unit of protein basis. We suggest that the recovery of photosynthetic capacity at low temperature and the regulation of enzymatic activity represent acclimation in winter cultivars. This allow these overwintering herbaceous annuals to maximize the production of sugars with possible cryoprotective function and to accumulate sufficient carbohydrate storage reserve to support basal metabolism and regrowth in the spring.

Cold signalling and cold acclimation in plants

Exposure to low temperatures is one of the most important plant abiotic stress factors. In this review we describe the damages that chilling and/or freezing temperatures can cause to plant cells. Confronted to these damages, some plants are able to adapt through mechanisms based on protein synthesis, membrane composition changes, and activation of active oxygen scavenging systems. These adaptive mechanisms rely in part on gene induction. The best understood genetic pathway leading to gene induction upon a temperature downshift is based on C-repeat-binding factors (CBF) activating promoters through the C-repeat (CRT) cis-element. Such activation of transcription factors suggests that cold, as a signal, has been transduced into the cells. Calcium entry is a major signalling event occurring immediately after a temperature downshift. The increase in cytosolic calcium will activate many enzymes, such as phospholipases and calcium dependent-protein kinases. A MAP-kinase module has been shown to be involved in the cold response. Ultimately, the activation of those signalling pathways leads to changes to the transcriptome. In this review we have focused on the genetic and signalling pathways activated early after cold exposure. Much of the data cited is from the model plant Arabidopsis but when possible evidence from other plants is presented.

Development of Arabidopsis thaliana leaves at low temperatures releases the suppression of photosynthesis and photosynthetic gene expression despite the accumulation of soluble carbohydrates

Arabidopsis thaliana plants were grown at 23 degrees C and changes in carbohydrate metabolism, photosynthesis and photosynthetic gene expression were studied after the plants were shifted to 5 degrees C. The responses of leaves shifted to 5 degrees C after development at 23 degrees C are compared to leaves that developed at 5 degrees C. Shifting warm developed leaves to 5 degrees C lead to a severe suppression of photosynthesis that correlated with a rapid and sustained accumulation of hexose phosphates and soluble sugars. Associated with the suppression of photosynthesis and the accumulation of soluble sugars was a reduction in the amount of transcript for genes encoding photosynthetic proteins (cab and rbcS). In contrast, leaves that developed at 5 degrees C showed an increase in photosynthesis and control levels of photosynthetic gene expression. This recovery occurred even though leaves that developed at 5 degrees C maintained large pools of soluble sugars. Leaves that developed at 5 degrees C also showed a strong upregulation of the cytosolic pathway for soluble sugar synthesis but not of the chloroplastic pathway for starch synthesis. This was shown at the level of both enzyme activity and the amount of transcript. Thus, development of Arabidopsis leaves at 5 degrees C resulted in metabolic changes that enabled them to produce and accumulate large soluble sugar pools without any associated suppression of photosynthesis or photosynthetic gene expression. These changes were also associated with enhanced freezing tolerance. We suggest that this reprogramming of carbohydrate metabolism associated with development at low temperature is essential to the development of full freezing tolerance and for winter survival of over-wintering herbaceous annuals.

Effects of a Short-Term Shift to Low Temperature and of Long-Term Cold Hardening on Photosynthesis and Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase and Sucrose Phosphate Synthase Activity in Leaves of Winter Rye (Secale cereale L.)

The effect of a short-term (hours) shift to low temperature (5[deg]C) and long-term (months) cold hardening on photosynthesis and carbon metabolism was studied in winter rye (Secale cereale L. cv Musketeer). Cold-hardened plants grown at 5[deg]C exhibited 25% higher in situ CO2 exchange rates than nonhardened plants grown at 24[deg]C. Cold-hardened plants maintained these high rates throughout the day, in contrast to nonhardened plants, which showed a gradual decline in photosynthesis after 3 h. Associated with the increase in photosynthetic capacity following cold hardening was an increase in ribulose-1,5-bisphosphate carboxylase/oxygenase and sucrose phosphate synthase activity and 3- to 4-fold increases in the pools of associated metabolites. Leaves of nonhardened plants shifted overnight to 5[deg]C required 9 h in the light at 5[deg]C before maximum rates of photosynthesis were reached. The gradual increase in photosynthesis in leaves shifted to 5[deg]C was correlated with a sharp decline in the 3-phosphoglycerate/triose phosphate ratio and by an increase in the ribulose bisphosphate/3-phosphoglycerate ratio, indicating the gradual easing of aninorganic phosphate-mediated feedback inhibition on photo-synthesis. We suggest that the strong recovery of photosynthesis in winter rye following cold hardening indicates that the buildup of photosynthetic enzymes, as well as those involved in sucrose synthesis, is an adaptive response that enables these plants to maximize the production of sugars that have both cryoprotective and storage functions that are critical to the performance of these cultivars during over-wintering.

Sucrose-feeding leads to increased rates of nitrate assimilation, increased rates of alpha-oxoglutarate synthesis, and increased synthesis of a wide spectrum of amino acids in tobacco leaves

Abstract. To investigate the importance of the sugar supply for the regulation of nitrogen and organic acid metabolism, various sugars and nitrogenous compounds were supplied for 8 h to detached tobacco leaves in low light. (i) In control leaves supplied with water, there was a large decrease of the Nia transcript level, a 50% decline of nitrate reductase (NR) activity, starch increased and sugars remained low, nitrate decreased by 50%, and amino acids increased only slightly during the 8 h incubation. About half of the nitrogen accumulating in amino acids was present in glutamine (Gln). (ii) When 25 mM sucrose was supplied, the in-vivo rate of nitrate assimilation (estimated from the accumulation of ammonium and amino acids) increased 2-fold. The Nia transcript level still decreased, but the decline of NR activity was less pronounced and NR activation was increased. The in-vivo net rate of ammonium assimilation (estimated from the accumulation of amino acids) also doubled after feeding sucrose. Ammonium and glutamate (Glu) decreased and Gln rose markedly, showing that in-vivo activity of glutamine synthetase had been stimulated. Glutamine still accounted for about half of the nitrogen, indicating that sucrose does not selectively stimulate glutamine synthase. Glutamate and aspartate decreased and all the minor amino acids increased, showing that the amino acid biosynthesis pathways are activated by sucrose. There was a decrease of 3-phosphoglycerate (3PGA) and phosphoenolpyruvate (PEP) and a large increase of α-oxoglutarate, showing that the flow of carbon from glycolysis into organic acids has been stimulated by sucrose. (iii) The changes of 3PGA, PEP, α-oxoglutarate, Glu, aspartate and the minor amino acids were smaller when 50 mM glucose was supplied, even though the internal levels of sugars at the end of the incubation resembled those found after feeding 25 mM sucrose. This indicates that the signals that regulate nitrogen and respiratory metabolism are derived from the uptake or metabolism of sucrose, rather than glucose. (iv) A different spectrum of changes was found when 20 mM nitrate was supplied. The estimated rate of nitrate assimilation increased 2-fold, and this was accompanied by an increase of NR activity but not of NR activation. Nitrate-feeding did not lead to a decrease of Glu, and the increase of minor amino acids was slightly smaller than with sucrose. There was a decrease of sugars, starch, and hexose phosphates, but 3PGA and PEP were not significantly decreased and isocitrate increased instead of α-oxoglutarate. (v) A different spectrum of changes was also found when 10 mM Gln was supplied. The estimated rate of nitrate assimilation decreased, and this was accompanied by a decrease of NR activity and NR activation. Glutamate did not decrease, and the increase of minor amino acids was smaller than with sucrose. Starch and sugars remained high and, although hexose phosphates decreased, 3PGA and PEP were not significantly decreased. Isocitrate remained unaltered and the increase of α-oxoglutarate was smaller than after supplying sucrose. (vi) When 25 mM sucrose was added together with 20 mM nitrate or 10 mM Gln, the effect on NR activity, NR activation and the estimated rate of nitrate assimilation was additive to the effect of nitrate, and antagonistic to the effect of Gln. Sucrose still led to a decrease of Glu, an increase of the minor amino acids, a decrease of 3PGA and PEP, and an increase of α-oxoglutarate when it was supplied together with nitrate or Gln. (vii) It is concluded that sucrose initiates a co-ordinate activation of nitrate assimilation, ammonium assimilation, amino acid biosynthesis, and α-oxoglutarate synthesis. Sucrose acts in concert with nitrate and antagonistically to Gln to increase NR activity and nitrate assimilation, and complements the action of nitrate and Gln to increase the flow of nitrogen from ammonium into amino acids, and to increase α-oxoglutarate formation.

Phosphate status affects the gene expression, protein content and enzymatic activity of UDP-glucose pyrophosphorylase in wild-type and pho mutants of Arabidopsis.

Abstract. The effects of inorganic phosphate (Pi) deficiency on the expression of the UDP-glucose pyrophosphorylase (UGPase) gene (Ugp), involved in sucrose synthesis/metabolism, and on carbohydrate status were investigated in different tissues of Arabidopsis thaliana (L.) Heynh. For leaves, a decrease in internal Pi status caused by growth of plants on a medium lacking Pi (−P conditions) led to an increase in the overall content of glucose and starch, but had little effect on sucrose content. The Pi deficiency also led to an increased carbohydrate content in stems/flowers, but not in roots. The expression of Ugp was upregulated in both leaves and roots, but not in stems/flowers. The effects of Pi status on Ugp expression were confirmed using leaves of both pho1-2 and pho2-1 mutants of Arabidopsis (Pi-deficient and Pi-accumulating, respectively) and by feeding the leaves with d-mannose, which acts as a sink for Pi. The Pi-status-dependent changes in Ugp expression followed the same patterns as those of ApS, a gene encoding the small subunit of ADP-glucose pyrophosphorylase, a key enzyme of starch synthesis. The changes in Ugp mRNA levels, depending on internal Pi status, were generally correlated with changes in UGPase protein content and enzymatic activity. This was demonstrated both for wild-type plants grown under Pi-deficiency and for Pi mutants. The data suggest that, under Pi-deficiency, UGPase represents a transcriptionally regulated step in sucrose synthesis/metabolism, involved in homeostatic mechanisms readjusting the nutritional status of a plant under Pi-stress conditions.

Low-Temperature Effects on Photosynthesis and Correlation with Freezing Tolerance in Spring and Winter Cultivars of Wheat and Rye'

Winter cultivars of rye (Secale cereale L., cv Musketeer) and wheat (Triticum aestivum L. cvs Kharkov and Monopol), but not a spring cultivar of wheat (Glenlea), grown at cold-hardening temperatures showed, at high irradiances, a higher proportion of oxidized to reduced primary, stable quinone receptor (QA) than did the same cultivars grown under nonhardening conditions. In addition, there was a positive correlation between the effects of low-growth temperature on this increased proportion of oxidized QA, and a concomitant increase in the capacity for photosynthesis, and LT50, the temperature at which 50% of the seedlings are killed, in cultivars showing different freezing tolerances. This suggests that low-temperature modulation of the photosynthetic apparatus may be an important factor during the induction of freezing resistance in cereals. Finally, the control of photosystem II photochemistry by nonphotochemical quenching of excitation energy was identical for nonhardened and cold-hardened winter rye. However, examination of measuring temperature effects per se revealed that, irrespective of growth temperature, nonphotochemical quenching exerted a stronger control on photosystem II photochemistry at 10[deg] C rather than at 20[deg] C.