Frost Resistance in Spruce [Picea abies(L.) Karst.]: IV, The Lipid Composition of Frost Resistant and Frost Sensitive Spruce Chloroplasts

Champions of winter survival: cold acclimation and molecular regulation of cold hardiness in evergreen conifers

Evergreen conifers are champions of winter survival, based on their remarkable ability to acclimate to cold and develop cold hardiness. Counterintuitively, autumn cold acclimation is triggered not only by exposure to low temperature, but also by a combination of decreasing temperature, decreasing photoperiod and changes in light quality. These environmental cues control a network of signaling pathways that coordinate cold acclimation and cold hardiness in overwintering conifers, leading to cessation of growth, bud dormancy, freezing tolerance and changes in energy metabolism. Advances in genomic, transcriptomic and metabolomic tools for conifers have improved our understanding of how trees sense and respond to changes in temperature and light during cold acclimation and the development of cold hardiness, but there remain considerable gaps deserving further research in conifers. In the first section of this review, we focus on the physiological mechanisms used by evergreen conifers to adjust metabolism seasonally and to protect overwintering tissues against winter stresses. In the second section, we review how perception of low temperature and photoperiod regulate the induction of cold acclimation. Finally, we explore the evolutionary context of cold acclimation in conifers and evaluate challenges imposed on them by changing climate and discuss emerging areas of research in the field.

Extreme low temperature tolerance in woody plants

Woody plants in boreal to arctic environments and high mountains survive prolonged exposure to temperatures below -40°C and minimum temperatures below -60°C, and laboratory tests show that many of these species can also survive immersion in liquid nitrogen at -196°C. Studies of biochemical changes that occur during acclimation, including recent proteomic and metabolomic studies, have identified changes in carbohydrate and compatible solute concentrations, membrane lipid composition, and proteins, notably dehydrins, that may have important roles in survival at extreme low temperature (ELT). Consideration of the biophysical mechanisms of membrane stress and strain lead to the following hypotheses for cellular and molecular mechanisms of survival at ELT: (1) Changes in lipid composition stabilize membranes at temperatures above the lipid phase transition temperature (-20 to -30°C), preventing phase changes that result in irreversible injury. (2) High concentrations of oligosaccharides promote vitrification or high viscosity in the cytoplasm in freeze-dehydrated cells, which would prevent deleterious interactions between membranes. (3) Dehydrins bind membranes and further promote vitrification or act stearically to prevent membrane-membrane interactions.

Metabolomic analysis of extreme freezing tolerance in Siberian spruce (Picea obovata)

Siberian spruce (Picea obovata) is one of several boreal conifer species that can survive at extremely low temperatures (ELTs). When fully acclimated, its tissues can survive immersion in liquid nitrogen. Relatively little is known about the biochemical and biophysical strategies of ELT survival. We profiled needle metabolites using gas chromatography coupled with mass spectrometry (GC-MS) to explore the metabolic changes that occur during cold acclimation caused by natural temperature fluctuations. In total, 223 metabolites accumulated and 52 were depleted in fully acclimated needles compared with pre-acclimation needles. The metabolite profiles were found to develop in four distinct phases, which are referred to as pre-acclimation, early acclimation, late acclimation and fully acclimated. Metabolite changes associated with carbohydrate and lipid metabolism were observed, including changes associated with increased raffinose family oligosaccharide synthesis and accumulation, accumulation of sugar acids and sugar alcohols, desaturation of fatty acids, and accumulation of digalactosylglycerol. We also observed the accumulation of protein and nonprotein amino acids and polyamines that may act as compatible solutes or cryoprotectants. These results provide new insight into the mechanisms of freezing tolerance development at the metabolite level and highlight their importance in rapid acclimation to ELT in P. obovata.

Physiological disturbances in plants caused by air pollutants

The physiological and biochemical features of plants that are responsible for determining susceptibility or tolerance to air pollutants have often proved hard to identify. In recent years, however, there has been new experimental evidence of responses which may be of critical importance. These include (a) changes in stomatal behaviour, affecting plant-water relations, (b) alterations in carbon and nitrogen assimilation and partitioning which can influence root growth, and (c) interference with the processes of winter hardening. Evidence of these changes to physiological processes, and the ways in which responses to pollutants may become more significant to plant survival in the presence of other environmental stresses, are discussed. The longer-term consequences, or secondary responses, such as effects on associations with other organisms, are also briefly reviewed. We have also examined the possibility that ecosystems may be overloaded with inputs of nitrogen from the atmosphere.

Cold-loving microbes, plants, and animals--fundamental and applied aspects

Microorganisms, plants, and animals have successfully colonized cold environments, which represent the majority of the biosphere on Earth. They have evolved special mechanisms to overcome the life-endangering influence of low temperature and to survive freezing. Cold adaptation includes a complex range of structural and functional adaptations at the level of all cellular constituents, such as membranes, proteins, metabolic activity, and mechanisms to avoid the destructive effect of intracellular ice formation. These strategies offer multiple biotechnological applications of cold-adapted organisms and/or their products in various fields. In this review, we describe the mechanisms of microorganisms, plants, and animals to cope with the cold and the resulting biotechnological perspectives.

Effects of summer ozone on membrane lipid composition during subsequent frost hardening in Norway spruce [Picea abies (L.) Karst]

Norway spruce trees [Picea abies (L.) Karst] were exposed to ozone during the summers of three consecutive years and the fatty acid composition of the chloroplast membrane lipid, monogalactosyl diglyceride (MCDG) was analysed over the last 14 months. Significant variations in the degree of unsaturation and the percentage of linolenic acid (18:3) were not found, either on a seasonal basis or in relation to ozone treatment. The proportion of octadecatetraenoic acid (18:4) and other Δ⁵ fatty acids increased during autumn in trees grown in filtered air, but this seasonal effect was less pronounced in ozone-treated trees. The ratio of two forms of linoleate, ^Δ5,9 18:2 and ^Δ9,12 18:2 also increased significantly during the frost hardening period but not in trees previously exposed to ozone. This implies that two separate biosynthetic pathways exist, one for the Δ¹² and one for the Δ¹² desaturated series, which operate at different relative rates, depending on environmental conditions. It is suggested that ozone may interfere with the biosynthesis of 18:4 during frost hardening by inhibiting the Δ⁵ desaturation of oleate.

INTERACTION BETWEEN OZONE AND COLD SENSITIVITY IN NORWAY SPRUCE: A FACTOR CONTRIBUTING TO THE FOREST DECLINE IN CENTRAL EUROPE?

Three-year-old clonal saplings of Picea abies L. Karst. were fumigated for 60 d during the summer of 1985 with four different levels of O₃ . Visible injury (in the form of severe, uniform brown necrosis and shedding of affected needles) occurred after a frost in November on the older needles of three of the clones which had received over 200 μg m^-3 of O₃ during the summer, 47 d previously. No visible injury occurred during the fumigation period or on the current (1985) year needles. It is suggested that the results provide preliminary evidence for an enhancement of frost sensitivity by O₃ which may be significant in the current wave of forest decline affecting high-altitude forests in central Europe.