Thermal acclimation in Arabidopsis lyrata: genotypic costs and transcriptional changes

Frost and heat events can be challenging for sessile organisms that cannot escape thermal extremes. However, adverse effects of thermal stress on fitness may be reduced by pre‐exposure to cold or heat, a process known as acclimation. To understand the ecological and evolutionary implications of acclimation, we investigated (1) the reduction in performance due to stress pre‐exposure, (2) the magnitude of increased leaf resistance to subsequent stress, (3) the costs of acclimation and (4) the genes differing in expression due to stress pre‐exposure. Plants of Arabidopsis lyrata were raised under three treatments of pre‐exposure: bouts of frost, bouts of heat or constant temperature. Resistance of leaves to subsequent frost and heat stress was then measured by electrolyte leakage. RNA‐seq analysis was performed to examine the genes differentially expressed between stress‐pre‐exposed and control plants. Pre‐exposure to stress during growth decreased plant size and increased leaf resistance to subsequent stress independent of whether pre‐exposure was to frost or heat. But the highest increase in leaf resistance to frost was found after pre‐exposure to frost (as a trend) and in leaf resistance to heat after pre‐exposure to heat. No evidence for costs of acclimation was detected. RNA‐sequencing suggested that acclimation by frost and heat pre‐exposure was caused by distinct mechanisms: modification of the chloroplast membrane and modification of the cell wall and membrane, respectively. Our results suggest that thermal resistance is a labile complex of traits, strongly affected by the previously experienced stress environment, with undetermined costs.     

寒冷适应差异表达基因的研究

为寻找寒冷适应机体表达上调的相关基因,以Balb／C小鼠建立寒冷适应模型,分别提取肌肉和肝脏RNA,运用代表性差异分析(RDA)方法,寻找寒冷适应表达上调的相关基因片段,进行序列分析和同源性比较。结果显示,在肌肉和肝脏中有部分表达上调的基因片段,经狭线杂交验证,其中3条基因表达存在显著差异。这些新发现的机体寒冷适应相关基因为进一步理解寒冷适应的分子机制提供了帮助。     

Role of the methionine sulfoxide reductase MsrB3 in cold acclimation in Arabidopsis

Methionine residues of proteins are a major target for oxidation by reactive oxygen species (ROS), which are generated in response to a variety of stress conditions. Methionine sulfoxide (MetO) reductases are present in most organisms and play protective roles in the cellular response to oxidative stress, reducing oxidized MetO back to Met. Previously, an Arabidopsis MetO reductase, MsrB3, was identified as a cold-responsive protein. Here we report that MsrB3 functions in the process of cold acclimation, thus contributing to cold tolerance. In contrast to normal, wild-type plants, msrb3 mutant plants lost the ability to become tolerant to freezing temperatures following cold pre-treatment. Furthermore, when exposed to low temperature, msrb3 plants exhibited a larger increase in MetO and H(2)O(2) content and electrolyte leakage compared with wild-type and MsrB3 transgenic plants. It is also shown that MsrB3 is localized at the endoplasmic reticulum (ER). We propose that MsrB3 plays an important role in cold tolerance by eliminating MetO and ROS that accumulate at the ER during cold acclimation.     

Mass spectrometric approach for identifying putative plasma membrane proteins of Arabidopsis leaves associated with cold acclimation

Although enhancement of freezing tolerance in plants during cold acclimation is closely associated with an increase in the cryostability of plasma membrane, the molecular mechanism for the increased cryostability of plasma membrane is still to be elucidated. In Arabidopsis, enhanced freezing tolerance was detectable after cold acclimation at 2 degrees C for as short as 1 day, and maximum freezing tolerance was attained after 1 week. To identify the plasma membrane proteins that change in quantity in response to cold acclimation, a highly purified plasma membrane fraction was isolated from leaves before and during cold acclimation, and the proteins in the fraction were separated with gel electrophoresis. We found that there were substantial changes in the protein profiles after as short as 1 day of cold acclimation. Subsequently, using matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS), we identified 38 proteins that changed in quantity during cold acclimation. The proteins that changed in quantity during the first day of cold acclimation include those that are associated with membrane repair by membrane fusion, protection of the membrane against osmotic stress, enhancement of CO2 fixation, and proteolysis.     

Temporal heterogeneity of cold acclimation phenotypes in Arabidopsis leaves

To predict the effects of temperature changes on plant growth and performance, it is crucial to understand the impact of thermal history on leaf morphology, anatomy and physiology. Here, we document a comprehensive range of leaf phenotypes in 25/20 °C‐grown Arabidopsis thaliana plants that were shifted to 5 °C for up to 2 months. When warm‐grown, pre‐existing (PE) leaves were exposed to cold, leaf thickness increased due to an increase in mesophyll cell size. Leaves that were entirely cold‐developed (CD) were twice as thick (eight cell layers) as their warm‐developed (WD) counterparts (six layers), and also had higher epidermal and stomatal cell densities. After 4 d of cold, PE leaves accumulated high levels of total non‐structural carbohydrates (TNC). However, glucose and starch levels declined thereafter, and after 45 d in the cold, PE leaves exhibited similar TNC to CD leaves. A similar phenomenon was observed in δ¹³C and a range of photosynthetic parameters. In cold‐treated PE leaves, an increase in respiration (R_dark) with cold exposure time was evident when measured at 25 °C but not 5 °C. Cold acclimation was associated with a large increase in the ratio of leaf R_dark to photosynthesis. The data highlight the importance of understanding developmental thermal history in determining individual phenotypic traits.     

Fitness benefits and costs of cold acclimation in Arabidopsis thaliana

Abstract When resources are limited, there is a trade-off between growth/reproduction and stress defense in plants. Most temperate plant species, including Arabidopsis thaliana, can enhance freezing tolerance through cold acclimation at low but nonfreezing temperatures. Induction of the cold acclimation pathway should be beneficial in environments where plants frequently encounter freezing stress, but it might represent a cost in environments where freezing events are rare. In A. thaliana, induction of the cold acclimation pathway critically involves a small subfamily of genes known as the CBFs. Here we test for a cost of cold acclimation by utilizing (1) natural accessions of A. thaliana that originate from different regions of the species' native range and that have experienced different patterns of historical selection on their CBF genes and (2) transgenic CBF overexpression and T-DNA insertion (knockdown/knockout) lines. While benefits of cold acclimation in the presence of freezing stress were confirmed, no cost of cold acclimation was detected in the absence of freezing stress. These findings suggest that cold acclimation is unlikely to be selected against in warmer environments and that naturally occurring mutations disrupting CBF function in the southern part of the species range are likely to be selectively neutral. An unanticipated finding was that cold acclimation in the absence of a subsequent freezing stress resulted in increased fruit production, that is, fitness.     

Subcellular reprogramming of metabolism during cold acclimation in Arabidopsis thaliana

Metabolite changes in plant leaves during exposure to low temperatures involve re‐allocation of a large number of metabolites between sub‐cellular compartments. Therefore, metabolite determination at the whole cell level may be insufficient for interpretation of the functional significance of cellular compounds. To investigate the cold‐induced metabolite dynamics at the level of individual sub‐cellular compartments, an integrative platform was developed that combines quantitative metabolite profiling by gas chromatography coupled to mass spectrometry (GC‐MS) with the non‐aqueous fractionation technique allowing separation of cytosol, vacuole and the plastidial compartment. Two mutants of Arabidopsis thaliana representing antipodes in the diversion of carbohydrate metabolism between sucrose and starch were compared to Col‐0 wildtype before and after cold acclimation to investigate interactions of cold acclimation with subcellular re‐programming of metabolism. A multivariate analysis of the data set revealed dominant effects of compartmentation on metabolite concentrations that were modulated by environmental condition and genetic determinants. While for both, the starchless mutant of plastidial phospho‐gluco mutase (pgm) and a mutant defective in sucrose‐phosphate synthase A1, metabolic constraints, especially at low temperature, could be uncovered based on subcellularly resolved metabolite profiles, only pgm had lowered freezing tolerance. Metabolic profiles of pgm point to redox imbalance as a possible reason for reduced cold acclimation capacity.