No abstract

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 ability to control extracellular ice formation during freezing is critical to the survival of freezing-tolerant plants. Antifreeze proteins, which are proteins that have the ability to retard ice crystal growth, were recently identified as the most abundant apoplastic proteins in cold-acclimated winter rye (Secale cereale 1.) leaves. In the experiments reported here, amino-terminal sequence comparisons, immuno-cross-reactions, and enzyme activity assays all indicated that these antifreeze proteins are similar to members of three classes of pathogenesis-related proteins, namely, endochitinases, endo-p-l,3-glucanases, and thaumatin-like proteins. Apoplastic endochitinases and endo-p-l,3-glucanases that were induced by pathogens in freezing-sensitive tobacco did not exhibit antifreeze activity. Our findings suggest that subtle structural differences may have evolved in the pathogenesis-related proteins that accumulate at cold temperatures in winter rye to confer upon these proteins the ability to bind to ice.

Dehydrins are glycine-rich, hydrophilic, heat-stable proteins isoelectric focusing followed by size exclusion. Purified PCA60, as well as crude protein extract, preserved the in 7itro and are generally induced in response to a wide array of enzymatic activity of lactate dehydrogenase after several environmental stresses. In previous research (Artlip et al.freeze-thaw cycles in liquid nitrogen. PCA also exhibited 1997, Plant Molecular Biology 33: 61-70), a full-length dehydrin gene, ppdhn1, was isolated from peach, and its distinct antifreeze activity as evidenced by ice crystal morexpression was associated with qualitative and quantitative phology and thermal hysteresis. This is the first time antifreeze activity has been demonstrated for dehydrins. differences in cold hardiness in sibling genotypes of evergreen Immunomicroscopy, utilizing an affinity-purified, polyclonal and deciduous peach. Similar results were obtained for levels of the corresponding 60 kDa peach dehydrin protein (PCA60). antibody developed against a synthetic peptide of the lysinerich consensus portion of dehydrins, indicated that PCA60 The objective of the present study was to purify the PCA60, test the purified protein for cryoprotective and/or antifreeze was freely distributed in the cytoplasm, plastids, and nucleus activity, and to determine the cellular localization of PCA60 of bark cells and xylem parenchyma cells. Although the using immunomicroscopy. PCA60 was extracted from winter functional role of dehydrins remains speculative, the data support the hypothesis that it plays a role in preventing bark tissues of peach (Prunus persica [L.] Batsch) and purified in a two-step process. Separation was based on free-solution denaturation of proteins exposed to dehydrative stresses.Abbre6iations -AFP: antifreeze protein; BSA: bovine serum albumin; LDH: lactate dehydrogenase; PBS: phosphate buffered saline.

Thellungiella, an Arabidopsis (Arabidopsis thaliana)-related halophyte, is an emerging model species for studies designed to elucidate molecular mechanisms of abiotic stress tolerance. Using a cDNA microarray containing 3,628 unique sequences derived from previously described libraries of stress-induced cDNAs of the Yukon ecotype of Thellungiella salsuginea, we obtained transcript profiles of its response to cold, salinity, simulated drought, and rewatering after simulated drought. A total of 154 transcripts were differentially regulated under the conditions studied. Only six of these genes responded to all three stresses of drought, cold, and salinity, indicating a divergence among the end responses triggered by each of these stresses. Unlike in Arabidopsis, there were relatively few transcript changes in response to high salinity in this halophyte. Furthermore, the gene products represented among drought-responsive transcripts in Thellungiella associate a down-regulation of defenserelated transcripts with exposure to water deficits. This antagonistic interaction between drought and biotic stress response may demonstrate Thellungiella's ability to respond precisely to environmental stresses, thereby conserving energy and resources and maximizing its survival potential. Intriguingly, changes of transcript abundance in response to cold implicate the involvement of jasmonic acid. While transcripts associated with photosynthetic processes were repressed by cold, physiological responses in plants developed at low temperature suggest a novel mechanism for photosynthetic acclimation. Taken together, our results provide useful starting points for more in-depth analyses of Thellungiella's extreme stress tolerance.