An unusual group 2 LEA gene family in citrus responsive to low temperature

The unknown soldier in citrus plants: polyamines-based defensive mechanisms against biotic and abiotic stresses and their relationship with other stress-associated metabolites

Citrus plants are challenged by a broad diversity of abiotic and biotic stresses, which definitely alter their growth, development, and productivity. In order to survive the various stressful conditions, citrus plants relay on multi-layered adaptive strategies, among which is the accumulation of stress-associated metabolites that play vital and complex roles in citrus defensive responses. These metabolites included amino acids, organic acids, fatty acids, phytohormones, polyamines (PAs), and other secondary metabolites. However, the contribution of PAs pathways in citrus defense responses is poorly understood. In this review article, we will discuss the recent metabolic, genetic, and molecular evidence illustrating the potential roles of PAs in citrus defensive responses against biotic and abiotic stressors. We believe that PAs-based defensive role, against biotic and abiotic stress in citrus, is involving the interaction with other stress-associated metabolites, particularly phytohormones. The knowledge gained so far about PAs-based defensive responses in citrus underpins our need for further genetic manipulation of PAs biosynthetic genes to produce transgenic citrus plants with modulated PAs content that may enhance the tolerance of citrus plants against stressful conditions. In addition, it provides valuable information for the potential use of PAs or their synthetic analogs and their emergence as a promising approach to practical applications in citriculture to enhance stress tolerance in citrus plants.

Novel dehydrins lacking complete K-segments in Pinaceae. The exception rather than the rule

Dehydrins are thought to play an essential role in the plant response, acclimation and tolerance to different abiotic stresses, such as cold and drought. These proteins contain conserved and repeated segments in their amino acid sequence, used for their classification. Thus, dehydrins from angiosperms present different repetitions of the segments Y, S, and K, while gymnosperm dehydrins show A, E, S, and K segments. The only fragment present in all the dehydrins described to date is the K-segment. Different works suggest the K-segment is involved in key protective functions during dehydration stress, mainly stabilizing membranes. In this work, we describe for the first time two Pinus pinaster proteins with truncated K-segments and a third one completely lacking K-segments, but whose sequence homology leads us to consider them still as dehydrins. qRT-PCR expression analysis show a significant induction of these dehydrins during a severe and prolonged drought stress. By in silico analysis we confirmed the presence of these dehydrins in other Pinaceae species, breaking the convention regarding the compulsory presence of K-segments in these proteins. The way of action of these unusual dehydrins remains unrevealed.

Identification and expression analysis of early cold-induced genes from cold-hardy Citrus relative Poncirus trifoliata (L.) Raf

Citrus is one of the most economically important fruit crops growing in subtropical and tropical regions. Most commercially important Citrus varieties are susceptible to cold; therefore, low and freezing temperatures are the main limiting factors for citrus production in subtropical areas. Since Poncirus trifoliata (L.) Raf. is a cold-hardy, interfertile Citrus relative, it serves as a genetic resource for improving cold tolerance in cold sensitive commercial Citrus species. While gene induced in response to long-term cold acclimation was previously identified in Poncirus, early response of Poncirus to cold has not been explored in detail. To identify early cold-responsive genes, a subtractive cDNA library was constructed using 4-h cold-treated and untreated control Poncirus seedlings in this study. A total of 210 randomly picked clones from the subtracted library with cold-induced genes were sequenced. The sequences obtained from the majority of these clones shared homology with previously identified cold-induced and/or environmental stress-regulated genes in other plants. Reverse northern blot analysis of the expression of these cDNAs with cold-treated and untreated control probes revealed that expression of 64 cDNAs was increased two to 11 fold in response to 4-h cold treatment. While the majority of these genes were related with cell rescue, defense, cell death and aging, transcription, metabolism, protein fate, energy, cellular communication and signal transduction, transport facilitation and development, some of them did not show homology with genes with known functions. Individual expression analysis of nine selected genes by semi-quantitative RT-PCR using mRNA from cold-treated and untreated control plants confirmed that the expression of selected cDNAs was all induced in response to cold. The results demonstrated that although a few genes were commonly induced in response to both short and long-term cold acclimation in Poncirus, majority of early cold-responsive genes were different from previously identified late cold-responsive genes in Poncirus.

Ptcorp gene induced by cold stress was identified by proteomic analysis in leaves of Poncirus trifoliata (L.) Raf

A proteomic approach was employed to investigate the cold stress-responsive proteins in trifoliate orange (Poncirus trifoliata (L.) Raf.), which is a well-known cold tolerant citrus relative and widely used as rootstock in China. Two-year-old potted seedlings were exposed to freezing temperature (-6°C) for 50 min (nonlethal) and 80 min (lethal), and the total proteins were isolated from leaves of the treated plants. Nine differentially accumulated proteins over 2-fold changes in abundance were identified by two-dimensional gel electrophoresis and mass spectrometry. Among these proteins, a resistance protein induced by the nonlethal cold treatment (protein spot #2 from P. trifoliata) was selected as target sequence for degenerated primer design. By using the designed primers, a PCR product of about 700 bp size was amplified from P. trifoliata genomic DNA, which was further cloned and sequenced. A nucleotide sequence of 676 bp was obtained and named Ptcorp. Blast retrieval showed that Ptcorp shared 88% homology with an EST of cold acclimated Bluecrop (Vaccinium corymbosum) library (Accession number: CF811080), indicating that Ptcorp had association with cold acclimation. Semiquantitative RT-PCR analysis demonstrated that Ptcorp gene was up-regulated by cold stress which was consistent with the former result of protein expression profile. As the resistance protein (NBS-LRR disease resistance protein family) gene was up-regulated by cold stress in trifoliate orange and satsuma mandarin, it may imply that NBS-LRR genes might be associated with cold resistance in citrus.

Comparative expression and transcript initiation of three peach dehydrin genes

Dehydrin genes encode proteins with demonstrated cryoprotective and antifreeze activity, and they respond to a variety of abiotic stress conditions that have dehydration as a common component. Two dehydrins from peach (Prunus persica L. [Batsch.]) have been previously characterized; here, we describe the characterization of a third dehydrin from peach bark, PpDhn3, isolated by its response to low temperature. The expression of all three dehydrin genes was profiled by semi-quantitative reverse transcription PCR, and transcript initiation was mapped for all three genes using the RNA ligase-mediated 5' rapid amplification of cDNA ends technique. PpDhn3 transcripts from bark collected in December or July, as well as transcripts from developing fruit, initiated at a single site. Although most of the PpDhn1 transcripts initiated at a similar position, those from young fruit initiated much further upstream of the consensus TATA box. Bark and fruit transcripts encoding PpDhn2 initiated ca. 30 bases downstream of a consensus TATA box; however, transcripts from ripe fruit initiated further upstream. Ripe fruit transcripts of PpDhn2 contain a 5' leader intron which is predicted to add some 34 amino acids to the N-terminal methionine of the cognate protein when properly processed. Secondary structure prediction of sequences surrounding the TATA box suggests that conformational transitions associated with decreasing temperature contribute to the regulation of expression of the cold-responsive dehydrin genes. Taken together these results reveal new, unexpected levels of gene regulation contributing to the overall expression pattern of peach dehydrins.

FG. Citrus genomics

Citrus is one of the most widespread fruit crops globally, with great economic and health value. It is among the most difficult plants to improve through traditional breeding approaches. Currently, there is risk of devastation by diseases threatening to limit production and future availability to the human population. As technologies rapidly advance in genomic science, they are quickly adapted to address the biological challenges of the citrus plant system and the world's industries. The historical developments of linkage mapping, markers and breeding, EST projects, physical mapping, an international citrus genome sequencing project, and critical functional analysis are described. Despite the challenges of working with citrus, there has been substantial progress. Citrus researchers engaged in international collaborations provide optimism about future productivity and contributions to the benefit of citrus industries worldwide and to the human population who can rely on future widespread availability of this health-promoting and aesthetically pleasing fruit crop.

Two AP2 domain containing genes isolated from the cold-hardy Citrus relative Poncirus trifoliata are induced in response to cold

Poncirus trifoliata (L.) Raf. is a cold-hardy, interfertile Citrus relative able to tolerate temperatures as low as -26°C when cold acclimated. Therefore, it has been used for improving cold tolerance in cold-sensitive commercial citrus varieties. A cold-induced cDNA library was constructed by subtractive hybridisation of non-acclimated and 2-d cold-acclimated P. trifoliata seedlings and many genes induced in response to cold were identified. Two of these cDNAs, PI-B05 and PI-C10, were selected from this library for further characterisation. Full-length cDNA sequences of these genes were obtained by 5' and 3' rapid amplification of cDNA ends (RACE). Sequence analysis revealed that PI-B05 contained an apetala2 / ethylene response factor (AP2 / ERF) domain and showed homology with ERF proteins from other plants, some of which are involved in environmental stress-induced gene expression. PI-C10 contained both AP2 / ERF and B3 DNA binding domains and showed homology with other plant proteins in the RAV subfamily of the AP2 / ERF transcription factors, some of which are induced in response to cold and other environmental stresses. Expression patterns of these genes in cold-tolerant P. trifoliata and cold-sensitive pummelo [Citrus grandis (L.) Osb.] in response to cold and drought at different time points were analysed by northern blots. Expression analysis showed that both genes were induced in response to cold, but not under drought conditions in cold-hardy P. trifoliata. However, little or no expression of these genes was detected by northern analysis in cold-sensitive pummelo under cold or drought conditions. The sequence analysis and expression data indicated that these genes may play a role in cold-responsive gene expression in cold-hardy P. trifoliata and could possibly be used for improving cold tolerance in cold-sensitive citrus cultivars.

Identification and expression analysis of cold-regulated genes from the cold-hardy Citrus relative Poncirus trifoliata (L.) Raf

Citrus is a cold-sensitive genus and most commercially important varieties of citrus are susceptible to freezes. On the other hand, Poncirus trifoliata (L.) Raf. is an interfertile Citrus relative that can tolerate temperatures as low as -26 degrees C when fully cold acclimated. Therefore, it has been used for improving cold tolerance in cold-sensitive commercial citrus rootstock varieties and in attempts to improve scion varieties. In this study, cDNA libraries were constructed from both 2-day cold-acclimated and from non-acclimated Poncirus seedlings using a subtractive hybridization method with the objective of identifying cold-regulated genes. A total of 192 randomly picked clones, 136 from the cold-induced library and 56 from the cold-repressed library, were sequenced. The majority of these clones showed sequence homology to previously identified cold-induced and/or environmental stress-regulated genes in Arabidopsis. In addition, some of them shared homology with cold and/or environmental stress-induced genes previously identified in other herbaceous and woody perennial plants and some showed no homology with sequences in GenBank. When these 192 cDNAs were analyzed by reverse northern blot with cold-acclimated and non-acclimated probes, 92 of the cDNAs displayed significantly increased expression, ranging from 2 to 49-fold, during cold acclimation; all 92 were from the cold-induced library. Surprisingly no clones displayed significantly repressed expression in response to cold. Analysis of a number of selected genes individually in northern blots of mRNA from cold-acclimated and non-acclimated plants largely confirmed the reverse northern analysis, verifying induction of expression of selected cDNAs in response to cold. The results showed that subtractive hybridization is an efficient method for identification of cold-induced genes in plants with limited sequence information available. This study also revealed that genes induced during cold acclimation of the cold-hardy citrus relative P. trifoliata are similar to those in Arabidopsis, indicating that similar pathways may be present and activated during cold acclimation in woody perennial plants.

Expression analysis of a cold responsive transcript from trifoliate orange by real-time PCR and RT-PCR

CLTa and CLTb are two transcripts produced by the citrus CLT gene. CLTb is constitutively expressed, while CLTa is a low temperature responsive transcript expressed in temperatures below 10 degrees C during the acclimation process of some species of citrus. Real-time PCR was used to study CLTa expression pattern in trifoliate orange during acclimation, gradual deacclimation and abrupt deacclimation. A standard curve of nine dilution series from 10 x 10(6) to 10 x 10(-3) fg was constructed, using part of the CLTa transcript, which enabled us to determine the concentration of the transcript at different temperatures and conditions. It was found that during acclimation, CLTa expression is first detected at 10 degrees C, reaching a maximum after 24 h of exposure at -1 degrees C. During gradual deacclimation, the transcript accumulated at 5 degrees C and after this point it degrades, reaching a low level at 10 degrees C. When the plants were abruptly transferred to room temperature after being acclimated to -1 degrees C, the transcript reduced from 11,303 fg to levels below the threshold 3 h later, indicating a rapid degradation and how the expression of CLTa is dependent on low temperature. Exposure of trifoliate plants to abscisic acid (ABA) and salts indicated that CLTa is not induced by these treatments at the concentrations used. CLTa expression was also verified in other citrus species and was not detected in the acclimated cold-sensitive species "Mexican lime" (Citrus aurantifolia Swing.) and in the cold hardy "Satsuma mandarin (C. unchiu Marc.).

Identification of cold acclimated genes in leaves of Citrus unshiu by mRNA differential display

Citrus unshiu is freeze tolerant to -10 degrees C when fully acclimated after exposure to cold, nonfreezing temperatures. To gain an understanding of its cold tolerance mechanism, mRNA differential display reverse transcriptase polymerase chain reaction (DDRT-PCR) and quantitative relative RT-PCR were used to study gene expression under a gradual cold-acclimation temperature regime. Six up-regulated and two down regulated genes were identified based on their amino acid sequences. The identified proteins encoded by the up-regulated genes were: 14-3-3 protein, 40S ribosomal protein S23, putative 60S ribosomal protein L15, nucleoside diphosphate kinase III protein, regulator of chromosome condensation-like protein, and amino acid permease 6. The proteins encoded by the two down-regulated genes were: miraculin-like protein and beta-galactosidase. Their individual function has been briefly reviewed based on published information. In addition to the findings in this study, we compared the function of cold responsive genes of Poncirus trifoliata, a very cold hardy relative of Citrus species that is freeze tolerant to -30 degrees C when fully acclimated, to the function of genes in the current study.

Cold acclimation induced genes of trifoliate orange (Poncirus trifoliata)

Commercial citrus varieties are sensitive to low temperature. Poncirus trifoliata is a close relative of Citrus species and has been widely used as a cold-hardy rootstock for citrus production in low-temperature environments. mRNA differential display-reverse transcription (DDRT)-PCR and quantitative relative-RT-PCR were used to study gene expression of P. trifoliata under a gradual cold-acclimation temperature regime. Eight up-regulated cDNA fragments were isolated and sequenced. These fragments showed high similarities at the amino acid level to the following genes with known functions: betaine/proline transporter, water channel protein, aldo-keto reductase, early light-induced protein, nitrate transporter, tetratricopeptide-repeat protein, F-box protein, and ribosomal protein L15. These cold-acclimation up-regulated genes in P. trifoliata are also regulated by osmotic and photo-oxidative signals in other plants.

Short-day potentiation of low temperature-induced gene expression of a C-repeat-binding factor-controlled gene during cold acclimation in silver birch

Development of winter hardiness in trees is a two-stage process involving sequential perception of distinct environmental cues, short-day (SD) photoperiod and low temperature (LT). We have shown that both SD and LT are recognized by leaves of silver birch (Betula pendula cv Roth) leading to increased freezing tolerance, and thus leaves can be used as an experimental model to study the physiological and molecular events taking place during cold acclimation. To obtain a molecular marker for the acclimation process in birch we cloned a gene, designated Bplti36, encoding a 36-kD acidic SK2 type of dehydrin. The gene was responsive to LT, drought, salt, and exogenous abscisic acid. This responsiveness to abiotic stresses and abscisic acid was retained when Bplti36 was introduced to Arabidopsis (Arabidopsis thaliana). The LT induction of the gene appeared to be under the control of the C-repeat-binding factor pathway as suggested by the presence of several C-repeat/dehydration-responsive element/LT-responsive elements in the Bplti36 promoter and its constitutive expression in C-repeat-binding factor overproducing Arabidopsis. In birch SD photoperiod at normal-growth temperature did not result in significant induction of Bplti36. However, preexposure to SD followed by LT treatment resulted in a remarkable increase in Bplti36 transcript accumulation as compared to LT-treated plants grown at long-day photoperiod. This suggests that SD photoperiod potentiates the LT response by conditioning the leaf tissue to be more responsive to the LT stimulus.

Dehydrin variability among rhododendron species: a 25-kDa dehydrin is conserved and associated with cold acclimation across diverse species

•  Here we examine the accumulation pattern of dehydrins in non- vs cold-acclimated leaves of 21 species comprising two divergent groups of Rhododendron, Subgenus Hymenanthes and Subgenus Rhododendron. Individuals from five other Ericaceous genera were also evaluated in the same way. Quantitative comparisons of cold-inducibility of a 25-kDa dehydrin and cold acclimation ability in six Rhododendron species were also performed. •  Leaf freezing tolerance assay and dehydrin detection and quantification were performed as previously described. •  Eleven dehydrins, ranging from 25- to 73-kDa, were observed among the 21 species, and most were more abundant in winter-collected leaves than in summer-collected leaves. One dehydrin, a 25-kDa protein, was uniquely conserved across most (95%) of the species surveyed, and was absent only in R. brookeanum, a tropical species that may not be capable of cold acclimation. The 25-kDa dehydrin was also identified in Kalmia, a genus closely related to Rhododendron but not in four other less related Ericaceous genera. Comparison of dehydrin profiles in non- and cold-acclimated leaf tissue from six species (three very hardy, and three less hardy, species) indicated a close association (R²  = 0.95) between relative changes in leaf freezing tolerance and 25-kDa dehydrin accumulation. •  The taxonomic and physiological comparisons suggest a central, but as yet unknown, function for the 25-kDa dehydrin in protecting rhododendron leaves from freezing injury.

Isolation of a dehydrin cDNA from orange and grapefruit citrus fruit that is specifically induced by the combination of heat followed by chilling temperatures

Dehydrins (DHNs; late embryogenesis abundant D-11) are a family of plant proteins induced in response to environmental stresses such as water stress, salinity and freezing or which occur during the late stages of embryogenesis. Previously, it was reported that citrus contains a small gene family encoding a unique class of dehydrins that differs from most other plant dehydrins in various respects, such as having an unusual K-segment similar to that of gymnosperms. In the present study, we identified by cDNA differential display analysis a 'Navel' orange 202-bp polymerase chain reaction (PCR) fragment, which encoded the typical plant angiosperm-type K-segment consensus sequence, and of which the expression was down-regulated by exposure to low oxygen levels. The full-length cDNA sequence of the orange DHN, designated csDHN (for Citrus sinensis DHN), was further isolated by 5'-and 3'-RACE; it had a total length of 933 bp and encoded a predicted polypeptide of 235 amino acids. In addition, the same 202-bp 'Navel' dehydrin PCR fragment was used to screen a 'Star Ruby' grapefruit flavedo cDNA library, and its full-length grapefruit homologue, designated cpDHN (for C. paradisi DHN) was isolated and found to have a total length of 1024 bp and to encode a predicted polypeptide of 234 amino acids. The defined orange and grapefruit DHN proteins were completely identical in the 196 amino acids of their N-terminus but differed in their C-terminus region. Overall, the csDHN and cpDHN proteins share 84% identity and contain the conserved dehydrin serine cluster (S-segment) and a putative nuclear localization signal, but csDHN has one conserved dehydrin K-segment consensus sequence, whereas cpDHN contains two dehydrin K-segments. Both csDHN and cpDHN represent single copy genes, in 'Navel' orange and 'Star Ruby' grapefruit genomes, respectively. We found that the cpDHN gene was consistently expressed in the fruit peel tissue at harvest, but that its message levels dramatically decreased during storage at either ambient or low temperatures. However, a pre-storage hot water treatment, given to enhance fruit-chilling tolerance, increased cpDHN mRNA levels during the first 3 weeks of cold storage at 2 degrees C, and enabled the message levels to be retained for up to a further 8 weeks of cold storage at 2 degrees C. The hot water treatment by itself had no inductive effect on cpDHN gene expression when the fruits were held at non-chilling temperatures. Other stresses applied to the fruit, such as wounding, UV irradiation, water stress, low oxygen and exposure to the stress hormone ethylene decreased DHN mRNA levels, whereas abscisic acid had no effect at all.

Identification of a grapefruit cDNA belonging to a unique class of citrus dehydrins and characterization of its expression patterns under temperature stress conditions

Citrus fruits are sensitive to low temperatures and this often results in the development of chilling injuries during postharvest storage. In order to gain more insight into the molecular mechanisms involved in the acquisition of fruit chilling tolerance, we initiated a grapefruit (Citrus paradisi, cv. Marsh Seedless) flavedo cDNA sequencing project and used it to identify a cDNA similar to other Poncirus trifoliata and Citrus unshiu dehydrin genes reported to be responsive to low temperatures. The grapefruit dehydrin cDNA, designated cor15, encodes a predicted polypeptide of 15.1 kDa, that is almost completely identical with other reported citrus dehydrin proteins, except that it contains two large amino acid repeats, whereas P. trifoliata COR11 has only one such repeat and P. trifoliata COR19 and C. unshiu COR19 have three repeats. Together, the various grapefruit, P. trifoliata and C. unshiu dehydrins form a closely related and unique dehydrin gene family that differs from most other plant dehydrins in having an unusual K-segment similar to that of gymnosperms and in having a serine cluster (S-segment) at an unusual position at the carboxy-terminus. The grapefruit cor15 gene is consistently expressed in the fruit peel tissue at harvest, but its message levels dramatically decrease during storage at 2 degrees C. However, a pre-storage hot water treatment, which enhances fruit chilling tolerance, elicited retention of the constant level of cor15 gene expression during cold storage and eliminated its decline. The hot water treatment had no inductive effect on cor15 gene expression when the fruit were held at non-chilling temperatures. The effects of other stresses, such as exposure to ethylene, UV irradiation and wounding, on cor15 gene expression, were temporary and persisted for 1-2 days after the treatments.

Differential screening indicates a dramatic change in mRNA profiles during grape berry ripening. Cloning and characterization of cDNAs encoding putative cell wall and stress response proteins

We used differential screening to isolate ripening-associated cDNAs from a Shiraz grape (Vitis vinifera L.) berry cDNA library. A rapid increase in the mRNA levels of a number of cDNAs not present in unripe fruit occurred in grape berries at the onset of ripening. The putative translation products of some of these clones had homologs in other species that are involved in cell wall structure. These included four proline-rich proteins, a small protein that is similar to the non-catalytic, N-terminal domain of some pectin methylesterases, and two other glutamate-rich proteins. The remainder of the clones encoded putative stress response proteins. These included two thaumatin-like proteins, a metallothionein, a transcription factor, a cytochrome P450 enzyme, and proteins induced by water, sugar, and/or cold stress in other species. Many of the homologs of the grape cDNAs thought to be involved in cell wall structure or stress-related responses also accumulate in a developmental manner in other plants. This may indicate that the grape mRNAs accumulate in response to stresses such as the storage of high concentrations of sugars and rapid cell expansion, or they may accumulate as part of the ripening developmental program.

Anoxia-induced gene expression in turtle heart. Upregulation of mitochondrial genes for NADH-ubiquinone oxidoreductase subunit 5 and cytochrome c oxidase subunit 1

A cDNA library constructed from heart of anoxia-exposed adult turtles (Trachemys scripta elegans) was differentially screened with 32P-labeled single-stranded cDNA probes from heart of control versus anoxic animals to clone genes induced by anoxia stress. Four cDNA clones, pBTaR20, pBTaR34, pBTaR63 and pBTaR914 were obtained and confirmed to be upregulated in response to anoxic submergence (20 h in N2-bubbled water at 7 degrees C). Two clones, pBTaR20 and pBTaR63, were characterized by sequence analysis and in vivo expression. The clone pBTaR20 had a 1597-bp cDNA sequence and pBTaR63 contained a 1837-bp sequence. The pBTaR20 sequence contained a single open reading frame that was very close to full length and could potentially encode a polypeptide with 508 amino acids. The deduced polypeptide sequence showed approximately 83% of the residues identical with the sequence of cytochrome c oxidase subunit 1 (CO1) that is encoded by a mtDNA gene Cox1. The clone pBTaR63 contained a single potentially full-length open reading frame that could encode a polypeptide of 591 residues. This was similar to another mitochondrial protein, NADH-ubiquinone oxidoreductase subunit 5 (ND5), which is encoded by mtDNA gene Nad5. Analysis of the time course of expression of Cox1 and Nad5 by northern hybridization analysis showed that mRNA transcripts for both accumulated rapidly (within 1 h) in response to anoxia exposure. Both showed similar increases in their transcript content after 1 h of anoxia but with longer anoxia exposures (5 or 20 h) Nad5 mRNA levels remained high whereas Cox1 mRNA content declined somewhat. Northern-blot hybridization also revealed differential expression of these two genes in five other organs of T. s. elegans during anoxia exposure (brain, kidney, liver, red and white skeletal muscle), with a particularly large increase in mRNA transcript levels of both genes in anoxic red muscle. Organ-specific analysis of these genes in a freeze-tolerant turtle species (Chrysemys picta marginata) also showed that differential expression of these genes occurred in response to the ischemia induced by plasma freezing.