Abstract:
A method for providing thermotolerance of a plant and the genetic engineering applications thereof are disclosed. DNA fragment containing a gene encoding EXPORTIN1A is transferred into a plant cell to provide or enhance thermotolerance of the plant. The method can be applied in genetic engineering to select transgenic plant.

Description:
RELATED APPLICATIONS 
     This application claims priority to Taiwan Application Serial Number 99130236, filed Sep. 7, 2010, which is herein incorporated by reference. 
     SEQUENCE LISTING 
     The sequence listing submitted via EFS, in compliance with 37 CFR §1.52(e)(5), is incorporated herein by reference. The sequence listing text file submitted via EFS contains the file “TWT01669US-rsequencelisting”, created on Jul. 4, 2013, which is 40,976 bytes in size. 
     BACKGROUND 
     1. Field of Invention 
     The present disclosure relates to a method for providing thermotolerance of a plant. More particularly, the present disclosure relates to a method for providing basal thermotolerance of a transgenic plant and genetic engineering applications thereof. 
     2. Description of Related Art 
     In the suitable environment, the growth and reproduction of plants are improved. On the other hand, the environmental variation may affect and be harmful to the plant growth. The environmental stresses, such as water deprivation, flood, heat, cold, chemical, heavy metal, peroxides poisonings, etc., may be harmful to the plants. Since those environmental stresses affect the growth or even survival of plants, finding out the mechanism of stress responses of plants can improve crop varieties and increase the yield and quality of crop. 
     The current global climate change is unusually warming or cooling. For instance, the greenhouse effect creates a heat stress for plant growth, and results in ecological calamity and food shortage problems. Over the past years, the research of heat stress response of plants was focused on heat shock proteins (HSPs), a class of functionally related proteins whose expression increase when cells are exposed to elevated temperatures or other stress, to investigate the short-term heat stress response of the plants. Heat shock proteins have a chaperone function in protein refolding. If a plant is placed at a higher, but not a lethal temperature, the heat shock proteins play an important role in protein-protein interactions to assist the refolding of the heat-denatured proteins to their proper conformation or prevent undesired protein misfolding and aggregation. These mechanisms are essential for plant survival after undergoing the short-term and lethally heat stress. 
     However, there is no further investigation of long-term, continuously heat stress response of plants. The long-term stresses affect the growth of plants; hence finding out the mechanism of long-term heat stress responses of plants can contribute to improving crop varieties, and increasing the yield and quality of crop. 
     In general, there are two ways to give the thermotolerance of a plant to resist the heat stress. One is to screen a mutant plant with a naturally occurring thermotolerance mutation. The other is to express one or more thermotolerance-related protein in a plant by using molecular biotechnology. 
     SUMMARY 
     In one aspect of the present disclosure, a plant, plant cell, plant material or seed of a plant is disclosed. A recombinant vector construct is introduced into the plant, the plant cell, the plant material or the seed of the plant. The recombinant vector construct comprises a promoter and an isolated nucleic acid sequence operably linked to the promoter for expressing an exogenous XPO1A protein in the plant, the plant cell, the plant material or the seed of the plant. The isolated nucleic acid sequence encodes a XPO1A protein of SEQ ID NO: 11. The plant, the plant cell, the plant material or the seed of the plant that expresses the exogenous XPO1A protein has basal thermotolerance under long-term heat stress and sudden heat shock condition. 
     In accordance with one embodiment of the present disclosure, the promoter is a constitutive promoter or an inducible promoter. 
     In accordance with one or more embodiments of the present disclosure, wherein the isolated nucleic acid sequence comprises a polynucleotide sequence of SEQ ID NO: 1, a polynucleotide sequence of SEQ ID NO: 2 or a polynucleotide sequence of SEQ ID NO: 3. 
     In another aspect of the present disclosure, a method of producing a transformed plant cell is disclosed. The method includes following steps. A polynucleotide that encodes an exogenous XPO1A protein of SEQ ID NO: 11 is introduced into a plant cell. The exogenous XPO1A protein is expressed in the plant cell. The plant cell that expresses the exogenous XPO1A protein has basal thermotolerance. 
     In another aspect of the present disclosure, a method of producing a so transformed plant is provided. The method includes following steps. A polynucleotide that encodes an exogenous protein and an exogenous XPO1A protein of SEQ ID NO: 11 is introduced into a plant cell. The exogenous protein and the XPO1A protein are expressed in the plant cell. A whole plant from the plant cell is regenerated at a survivable temperature. The transgenic plants are subjected to a sustained high temperature or a sudden heat shock treatment, wherein a sustained high temperature or a heat shock temperature is equal to or higher than a lethal temperature of a non-transgenic plant. After the treatment, the transgenic plant survived after sustained high temperature or sudden heat shock treatment is selected. The survived transgenic plant expresses the exogenous protein and the exogenous XPO1A protein under sustained high temperature or sudden heat shock condition. 
     In accordance with one embodiment of the present disclosure, the polynucleotide comprises a polynucleotide sequence of SEQ ID NO: 1. 
     In accordance with one or more embodiments of the present disclosure, the polynucleotide sequence of SEQ ID NO: 1 is operably linked to a constitutive promoter or an inducible promoter. 
     In accordance with one or more embodiments of the present disclosure, the polynucleotide comprises a polynucleotide sequence of SEQ ID NO: 2 or a polynucleotide sequence of SEQ ID NO: 3. 
     In accordance with one or more embodiments of the present disclosure, the plant cell belongs to Brassicaceae family or tomato. 
     In accordance with one or more embodiments of the present disclosure, the plant cell belongs to  Arabidopsis  genus or tomato. 
     In accordance with one or more embodiments of the present disclosure, recombinant vector is introduced into the plant cell by using a plasmid or a viral vehicle. 
     In accordance with one or more embodiments of the present disclosure, the plasmid is a Ti-plasmid. 
     It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the disclosure as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be illustrated with respect to the accompanying figures and examples, which serve to illustrate this disclosure but are not binding thereon, wherein: 
         FIGS. 1(A) to 1(G)  illustrate the recombination mapping of the heat-intolerant 2 (hit2) locus; 
         FIGS. 2(A) to 2(B)  are photographs showing the sensitivity of  Arabidopsis  heat-intolerant 2 (hit2) mutant to sustained high temperature and sudden heat shock; 
         FIGS. 3(A) to 3(D)  are photographs showing the viability of wild-type and hit2 seedlings after different heat stress treatments; 
         FIGS. 4(A) to 4(D)  are diagrams illustrating the seed germination and seedling development of wild-type and hit2 on medium that contained various concentrations of methyl viologen (MV); 
         FIG. 5  is a bar diagram illustrating the survival rate of  Arabidopsis  wild-type and hit2 seedlings after heat treatment in the light or dark; 
         FIGS. 6(A) to 6(B)  are agarose gel electrophoresis diagrams illustrating the expression of XPO1A in wild-type plants; and 
         FIG. 7  illustrates a schematic diagram of the plasmid of pCAMBIA1300-XPO1A. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, for purposes of explanation, two parts are set forth in order to provide a thorough understanding of the disclosed embodiments and their theoretical basis. 
     Part 1 provides numerous physiological experiments of an  Arabidopsis  heat-intolerant 2 (hit2) mutant to build the theoretical basis of relationship of XPO1A protein and the basal thermotolerance of plant. 
     In accordance with the evidences of Part 1, Part 2 further discloses numerous examples for confirming effects of a transgenic plant with an exogenous polynucleotide encoding a XPO1A protein. Part 2 also provides some specific details that are essential for inducing the phenotype of thermotolerance of the transgenic plant. It will be apparent, however, that one or more embodiments may be practiced without these specific details. 
     The “basal thermotolerance” of plants herein is referred to survival without pretreatment. The plants with basal thermotolerance can survive under long-term heat stress (37° C. for 3-4 days) or sudden heat shock (44° C. for 30 min). 
     The XPO1A protein of the present disclosure is introduced into a plant cell for enhancing the “basal thermotolerance” but not the “acquired thermotolerance” because “acquired thermotolerance” is referred to the thermotolerance induced after moderate and non-lethal heat stress. For example, incubation at 44° C. for 1 h is lethal for the plants. But after 1 h of pretreatment at 37° C., the plants are able to withstand heat shock at 44° C. for 1 h. 
     Part 1: Functional Analysis of XPO1A 
     (1) Obtaining of the  Arabidopsis  Heat-Intolerant 2 (hit2) Mutant 
     The genome of  Arabidopsis thaliana , with five chromosomes containing about 114.5 million base pairs, had been completely sequenced in 2000 by  Arabidopsis  Genome Initiative. Since the relative shorter life cycle, smaller body and larger amount of offspring than those of most of plants,  Arabidopsis  is broadly used in genetics and molecular biology approaches. Moreover, numerous  Arabidopsis  mutants and genomic data are accumulated from the routine work of transferring DNA to  Arabidopsis  by utilizing  Agrobacterium tumefaciens  with Ti-plasmid conjugal transfer system. The information of ecotypes of  Arabidopsis , such as Columbia (Col), Landsberg erecta (Ler), Wassilewsdiaja (Ws), etc., or the polymorphism of the ecotypes can be obtained from library and internet resources. 
     For purpose of improving the thermotolerance of a plant, a forward genetics approach was used to find out a gene related essentially to the thermotolerance of plant. A heat intolerant mutant, hit2, exhibited the phenotype of heat intolerance was isolated from  Arabidopsis thaliana  ecotype Columbia-0 (Col-0). A gene encoding XPO1A (XPO1A, At5g17020) was mutated and therefore exhibited the heat intolerance phenotype. The codon for tryptophan 364 (TGG) of the XPO1A gene changed to a premature TGA stop codon is recognized by a map-based cloning approach. That is, the G to A mutation of the XPO1A gene of  Arabidopsis  creates the heat intolerance phenotype of the mutant. 
       Arabidopsis  contains two copies of the XPO1 genes, XPO1A (At5g17020) and XPO1B (At3g03110). In general, it was believed that XPO1A and XPO1B might have the same function and could produce complementary or synergistic effects. 
     In view of the gene conservation, the gene product of XPO1A is a nuclear transport receptor. The nuclear transport receptor may be employed to transfer substrates from nuclear to cytoplasm in a plant cell. However, the real mechanism of the nuclear transport receptor in the plant cell has not been fully understood yet. Also, there was no finding about the relationships between XPO1A protein and thermotolerance of plants. 
     According to the studies of nuclear transport receptors, a nuclear transport receptor transports a target protein by recognizing a specific amino acid sequence of the target protein. A nuclear transport receptor can recognize and transport one or more proteins with a specific amino acid sequence. Similarly, a protein having the specific amino acid sequence can be recognized and transported by one or more nuclear transport receptors. For the foregoing reasons, if a gene mutation occurred in a nuclear transport receptor of an organism, the physiological function of the nuclear transport receptor could be accomplished by another nuclear transport receptor. Hence, the phenotype of the organism may not be changed. On the other hand, a mutation occurred in a single nuclear transport receptor is also likely to affect transportation of one or more proteins and therefore changes the phenotypes of the mutant. 
     The  Arabidopsis  heat-intolerant 2 (hit2) mutant was analyzed by following experiments to identify the function of XPO1A, including genetic characteristics of the hit2 mutant, physiological experiments of thermotolerance of the hit2 mutant, and expression of XPO1A gene in plant tissues. 
     (2) Characterizing the Single Recessive Nuclear Mutation of the hit2 Mutant 
     The genetic characteristics of the hit2 mutant can be analyzed by interbreeding (crossing) the mutant (hit2) with a wild-type plant to identify whether the mutant is recessive or dominant. Furthermore, map-based cloning is used to localize the locus of the mutation. 
     To identify whether hit2 is a recessive or a dominant and a single or a multiple mutation, wild-type  Arabidopsis  plants in the Columbia gl-1 background (Col-5) is employed as female gametophytes to interbreed with the mutant (hit2, in Col-0 background) that is employed as male gametophytes. Crosses between mutant and wild-type are accomplished by transferring pollen from the mutant to the stigma of emasculated wild-type flowers. The leaf trichomes will present in the F 1  plants if the crossing succeeds. F 1  plants are self-pollinated to produce the F 2  generation. 
     Table 1 is a summary of genetic analysis results of the  Arabidopsis  heat-intolerant 2 (hit2) mutant and exportin1b (xpo1b-1) mutants. 143 wild-type Col-0, 140 hit2, 57 F 1  from a cross between hit2 and wild type, 440 F 2  from self-pollinated F 1 , and 45 xpo1b-1 plants were analyzed. These plants were tested for their heat sensitivity by planting seeds on MS agar plates at 22° C. for 10 days followed by treatment at 37° C. for 4 days. This treatment was shown to be lethal for the hit2 mutant but not for wild-type (Col-0) seedlings. As shown in table 1, all of the 143 wild type seedlings survived and all of the 140 seedlings of heat-intolerant 2 (hit2) mutant were bleached and dead (Tolerance=0). The 57 heterozygous seedlings (hit2×Col-5 F 1 ) also survived after the treatment. In 440 F 2  seedlings (hit2×Col-5 F 2 ), 327 seedlings of them survived and 133 seedlings of them exhibited heat sensitivity and were bleached completely after the heat treatment. Mutation at xpo1b did not affect the plant heat sensitivity as all of the 45 xpo1b-1 seedlings tested survived after the heat treatment. Furthermore, analysis of the self-pollinated F 2  plants showed a 2.89:1 segregation ratio of wild-type to hit2 plants. Hence, the heat intolerance phenotype was caused by a single recessive nuclear mutation. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Genetic analysis of the Arabidopsis heat-intolerant 2 (hit2) and exportin 1b-1 (xpo 1b-1) mutants 
               
             
          
           
               
                 Strains or crosses 
                   
                   
                   
                   
                   
               
               
                 (♂ × ♀) 
                 Generation 
                 Total 
                 Tolerant a   
                 Sensitive a   
                 χ 2b   
               
               
                   
               
             
          
           
               
                 Wild-type (Col-0) 
                   
                 143 
                 143 
                 0 
                   
               
               
                 hit2/hit2 
                   
                 140 
                 0 
                 140 
                   
               
               
                 Wild-type × hit2/hit2 
                 F 1   
                 57 
                 57 
                 0 
                   
               
               
                   
                 F 2   
                 440 
                 327 
                 113 (34.6%) 
                 0.109 b   
               
               
                 xpo 1b-1/xpo 1b-1 
                   
                 45 
                 45 
                 0 
                   
               
               
                   
               
               
                   a Tolerance or sensitivity was determined in the survivability assay as described in the Materials and Methods section. 
               
               
                   b The calculated v2 value was based on the expected ratio of 3:1 for tolerant to sensitive individuals, assuming that hit2 was a single recessive mutation (P &gt; 0.05). 
               
             
          
         
       
     
     The present disclosure demonstrates that the XPO1A protein plays an important role of assisting plant to resist long-term heat stress. Moreover, the heat intolerance phenotype can be created by XPO1A gene single mutation. 
     (3) Characterizing the Single Recessive Nuclear Mutation of the hit2 Mutant 
     Genetic analysis revealed that heat-intolerant 2 (hit2) mutant is a single recessive mutation. To determine the genetic determinant that underlines the mutant (hit2) thermosensitive phenotype, map-based cloning, also called positional cloning, was employed to identify the hit2 locus. The method of map-based cloning relies on the process of chromosomal crossing-over between the paired chromosomes that occurs during meiosis. Crossing-over of the chromosomes during meiosis leads to genetic recombination, and the recombination frequency between different genes on a chromosome can be used to estimate their order and distances apart. In general, the closer two genes are on a chromosome, the more co-segregation is, that is, the less likely it is that crossing-over will occur between them. Therefore, the map-based cloning is to identify molecular markers on a known gene, and calculate the recombination frequency to estimate locus of the mutant. 
     Ecotype-specific genetic markers were used for mapping of the  Arabidopsis  heat-intolerant 2 (hit2) locus. Initially, identify two sets of genetic markers on each chromosome. For rough mapping, the marker near the mutated locus (hit2) was recognized. New markers located in the vicinity of the previous marker were then found to locate the hit2 mutation. 
     Useful methods to analyze the different ecotypes in species include, but are not limited to simple sequence length polymorphism (SSLP) and cleaved amplified polymorphic sequence (CAPS). SSLPs are used as genetic markers that detect differences in the length of a genomic region. Typically the differences are due to small insertions or deletions (InDels) such as those caused by differences in the number of simple sequence repeats. For SSLP, primers complementary to a given genomic region are used to amplify the region from genomic DNA by polymerase chain reaction (PCR) and the resulting PCR products are separated on an agarose gel. Variances in the length of SSLPs are used to understand genetic variance between two individuals in a certain species (Ponce M R 1999, Lukowitz W et al. 2000, Peters et al. 2003). SSLP and CAPS are by far the most commonly used genetic markers. Since these markers are co-dominant, both alleles can be identified to provide sufficient information. Furthermore, it is convenient and low-cost to perform PCR-based markers analysis, and the resulting products are analyzed directly by agarose gel electrophoresis (Lukowitz W. et al. 2000). Publicly available genetic markers which are specific to  Arabidopsis  ecotypes can be searched on TAIR. Markers that are based on insertions/deletions (INDELs) and single nucleotide polymorphisms (SNPs) can also be identified from Cereon  Arabidopsis  Polymorphism Collection by comparing the genomes of  Arabidopsis  ecotype Columbia and Lansberg. Primers are then designed for SSLP and CAPS. 
     Initially, rough mapping located the mutation on the upper arm of chromosome 5 near the molecular marker so262 with 17.8% of recombination frequency. Further mapping localized the hit2 locus to a c. 1.3-Mb region that was delimited by the genetic markers nga51 and Cer456932 (Wang, 2006). The genetic markers Cer483173 and Cer483157 were selected for fine mapping that localized the hit2 locus further to a c. 60 kb region. Genomic DNA from hit2 plants was sequenced in this region and compared with that from wild-type plants. The mutated locus was mapped to the EXPORT1N1A (XPO1A) gene, which encodes the nuclear export receptor XPO1A. A person having ordinary skill in the art can accomplish the experiments described above using conventional techniques. On the other hand, the person having ordinary skill in the art can readily appreciate how to make and use the same by referring the following published paper “Isolation and characterization of the  Arabidopsis  heat-intolerant 2 (hit2) mutant reveal the essential role of the nuclear export receptor EXPORTIN1A (XPO1A) in plant heat tolerance” (New Phytologist (2010) 186:833-842, www.newphytologist.com), and the details of the paper are incorporated in the present disclose. 
       FIGS. 1(A) to 1(G)  illustrate the recombination mapping of the HEAT-INTOLERANT 2 (HIT2) locus.  FIG. 1(A)  indicates  Arabidopsis  chromosome 5 with simple sequence length polymorphism (SSLP) marker nga151, which is adjacent to the HIT2 locus.  FIG. 1(B)  shows expansion of the region that encompasses the HIT2 locus. Five overlapping bacterial artificial chromosomes within the region are shown. The relative positions of the molecular markers (vertical lines) are indicated. The number of recombinants versus the total number of chromosomes tested for a given marker is shown in parentheses.  FIG. 1(C)  is the chromosomal region between markers CER483173 and CER483157, which was sequenced and compared with that of the wild type. The gene shown in black represents At5g17020, in which a single base substitution was found in the hit2 mutant plants. Exon-intron structure of the HIT2 gene is shown as  FIG. 1(D) . The hit2 mutation is a G-to-A mutation within the 13th exon of At5g17020. This gene encodes the nuclear export receptor XPO1A. Additional T-DNA insertion alleles of At5g17020, exportin1a-1 (xpo1a-1) and exportin1a-3 (xpo1a-3), are also indicated.  FIG. 1(E)  indicates a nucleotide change found in hit2 (SEQ ID NO: 12) corresponded to a nonsense mutation that truncated the HIT2 protein (SEQ ID NO: 13) at amino acid 364. 
       FIG. 1(F)  shows phenotypes of 10-day-old wild-type, hit2, xpo1a-1, and xpo1a-3 plants after incubation at 37° C. for 4 days. Phenotypes of wild-type, hit2, xpo1a-1, and xpo1a-3  Arabidopsis  seedlings after heat shock at 44° C. for 20 min are shown as  FIG. 1(G) . In the heat survivability assay or after heat shock treatment, wild-type seedlings could survive, but xpo1a-1 and xpo1a-3 homozygous seedlings exhibited the same thermosensitive phenotype as that of hit2. 
     (4) Physiological Analysis of Heat-Intolerant 2 (hit2) Mutant 
     To evaluate the role of XPO1A gene in defense against heat stress, the physiological experiments were performed, including the effects of heat shock, the sensitivity to heat or oxidative stress and the influence of heat-induced oxidative stress and light conditions. The hit2 mutant and wild-type seedlings were compared to identify the functions of XPO1A. 
     1. The Effects of Heat Shock on the Phenotypes of Wild-Type and hit2 Seedlings 
     The hit2 mutant was isolated on the basis of its impaired ability to withstand sustained high temperature. Hence, the response of hit2 to heat shock was investigated to understand the relationships between the mechanisms by which plants cope with heat shock and prolonged heat stress. 
       FIGS. 2(A) to 2(B)  are photographs showing the sensitivity of  Arabidopsis  heat-intolerant 2 (hit2) mutant to sustained high temperature and sudden heat shock.  FIG. 2(A)  is photographs taken before and after heat exposure at 37° C. for 4 days. Progression of phenotypes of wild-type and hit2 seedlings after heat shock treatment is shown as  FIG. 2(B) . Seven-day-old plants grown on medium, with opened cotyledons and an emerging first pair of leaves, were subjected to heat shock at 44° C. for 20 min. After treatment, plants were returned to room temperature for recovery and photography. 
     There is no difference between wild-type and hit2 plants before 37° C. treatment. However, incubation at 37° C. for 4 days was lethal for hit2 but not wild-type plants ( FIG. 2(A) ).  FIG. 2(B)  shows the development of the seedlings after they were exposed to heat shock for 20 min. No difference in appearance between the mutant and wild-type seedlings was observed immediately after treatment. However, after 6 days at 23° C., the leaves of wild-type seedlings remained green in color and exhibited noticeable growth and expansion, whereas the leaves of the hit2 seedlings were bleached completely and showed no sign of growth. 
     2. The Influence of Sub-Lethal Pre-Acclimation on the Survival Rates of Wild-Type and hit2 Seedlings to Otherwise Lethal Heat Shock Treatment 
       FIGS. 3(A) to 3(D)  are photographs showing the viability of wild-type and hit2 seedlings after different heat stress treatments. Seven-day-old seedlings grown on medium were subjected to the different heating regimens indicated above each photograph. Survival rates were calculated from the number of seedlings that showed continuous growth of green leaves after 6 days. 
     Treatment (A) was that plants were exposed to 44° C. for 30 min and then incubated at 23° C. Plants were exposed to 44° C. for 45 min and incubated at 23° C. in treatment (B). Treatment (C) was that plants were pre-acclimated at 37° C. for 60 min and incubated at 23° C. for 60 min, followed by treated at 44° C. for 45 min. After pre-acclimation at 37° C. for 60 min, the plants were incubated at 23° C. for 2 days and exposed to 44° C. for 90 min in treatment (D). Apart from heat treatment, plants were incubated at 23° C. Plates were photographed at 6-8 days after the final heat treatment. 
     The survival rate of wild-type seedlings in treatment (A) was 95%, whereas that of hit2 seedlings was 0%. Neither wild-type nor hit2 seedlings in treatment (B) could survive after 6 days. However, after 1 h of pre-acclimation at 37, hit2 plants were able to survive heat shock that was otherwise lethal, as were wild-type (WT) plants (C,D). These results implied that the hit2 mutants were defective in basal but not acquired thermotolerance. 
     3. The Effects of Oxidative Stress on the Phenotypes of Wild-Type and hit2 Seedlings 
       FIGS. 4(A) to 4(D)  are diagrams illustrating the seed germination and seedling development of wild-type and hit2 on medium that contained various concentrations of methyl viologen (MV). Seeds were sown on agar plates that contained various concentrations of methyl viologen (MV) and allowed to germinate at 23° C. with continuous illumination. Methyl viologen is a common inducer of oxidative stress.  FIGS. 4(A) to 4(C)  indicate the percentage of seeds that germinated or matured. The open circles represent  Arabidopsis  heat-intolerant 2 (hit2) seedlings, and the closed circles are wild-type seedlings. Data for plots were obtained from a sample size of c. 60 seeds on each plate, and at least three replicated plates were used for each treatment. Error bars represent SD for all experiments.  FIG. 4(D)  shows the phenotypes of representative wild-type and mutant seedlings grown at 0.5 μg MV for 14 days. Seedlings from the same plate were removed and reorganized for the photograph. 
       FIG. 4(A)  is the percentage of seeds that germinated. It was determined by counting the seeds with visible protruding radicals after 10 days. The percentage of seedlings that matured in  FIG. 4(B)  was calculated from the number of seeds that showed green opened cotyledons after 14 days.  FIG. 4(C)  is the progression of seedling maturation of wild-type and hit2 seeds sown on agar supplemented with 0.5 μM MV. 
     As shown in  FIG. 4(A) , wild-type and hit2 mutant seeds exhibited similar germination rates at various concentrations of MV. Seed germination of the hit2 mutant was relatively unaffected by MV inhibition. However, the development of hit2 seedlings was more sensitive to inhibition by treatment with MV than that of wild-type seedlings ( FIG. 4(B) ). Furthermore, although some of the hit2 seeds were able to reach the seedling stage, their cotyledons soon became pale and their growth ceased ( FIGS. 4(C) and 4(D) ). The decline in maturation rate of hit2 seedlings after day 7 might have been caused by fading of the green color of the cotyledons of hit2 seedlings that had developed. 
     4. The Effects of Heat-Induced Oxidative Stress and Light Conditions on Survival of Wild-Type and hit2 Seedlings 
     Heat-induced oxidative damage is known to be exacerbated by light, and the light conditions to which plants are exposed have been shown to influence plant survival after heat stress (Larkindale and Knight, 2002; Larkindale et al., 2005). As shown in  FIG. 4 , hit2 is sensitive to MV-induced photo-oxidative stress, and therefore it is likely that light conditions will affect the sensitivity of hit2 to heat. To confirm the relationships between light conditions and the heat sensitivity of hit2, hit2 seedlings were subjected to the heat survivability assay as before, but the plants were protected from exposure to light. 
       FIG. 5  is a bar diagram illustrating the survival rate of  Arabidopsis  wild-type and hit2 seedlings after heat treatment in the light or dark. Ten-day-old plants grown on medium were heated to 37° C. in the light or dark for 4 days. Seven-day-old seedlings were heated to 44° C. for 20 min and allowed to recover at 23° C. in the light or dark for 6 days. The numbers of plants that were alive before and after treatment were counted to determine the survival rate. Data shown are the averages of six replicates; error bars represent SD. * represents zero survival rates. 
       FIG. 5  shows that the viability of hit2 seedlings after long-term heat stress (37° C. for 4 days) was increased markedly, from zero survival in the light to near total survival in the dark. In the case of heat shock stress, hit2 seedlings were treated at 44° C. for 20 min as before, but were allowed to recover in the dark. Again, the survival of hit2 seedlings was increased, but to a much lesser extent; only 10% of the hit2 seedlings were able to recover from the stress treatment. This phenomenon might have been caused by other forms of damage, which were more severe than heat-induced oxidative injury and which occurred rapidly in heat-shocked hit2 seedlings, and masked the influence of light during later recovery. Collectively, these results indicate that the hit2 phenotype is attributable, at least in part, to the lack of a sufficient response to oxidative injury, which strongly affects hit2 viability under sustained high temperature condition, but is less significant for protection against heat shock. Heat stress will cause overproduction of reactive oxygen species, and light make heat-induced oxidative damage worse. Hence, the results shown as  FIG. 5  indicated that XPO1A might have the ability to protect the plants from heat-induced oxidative damage. 
     5. The Effect of Heat Stress on XPO1A Gene Expression 
     To evaluate the expression of XPO1A gene, total RNA was extracted from different tissues of wild-type seedlings. The 3′ region of the XPO1A cDNA was amplified by semiquantitative RT-PCR using primers SEQ ID NO 4 and SEQ ID NO 5 and analyzed by agarose gel electrophoresis. 
       FIG. 6(A)  is an agarose gel electrophoresis diagram illustrating the expression of XPO1A in hit2 plants. Reverse transcription-polymerase chain reaction (RT-PCR) was performed on first-strand cDNA generated from different  Arabidopsis  plant tissues grown at 23° C. ( FIG. 6(A) ). Tissue designations were as follows: R, root; RL, rosette leaf; CL, cauline leaf; ST, stem; FL, flower. Poly-ubiquitin (UBQ10) RNA served as an internal control for RT-PCR. The XPO1A transcript was detected in the root, rosette leaves, cauline leaves, stem, and flowers, which suggested that XPO1A was expressed ubiquitously in sporophytic tissues. 
       FIG. 6(B)  shows the effect of heat stress on XPO1A expression. Total RNA for RT-PCR analysis were extracted from 10-day-old seedlings that had been incubated at 37° C. for 0, 6, 12, 24, 36, or 48 h, respectively. Heat shock protein 17.6A (At-HSP17.6A) RNA served as a positive control. Although expression of the heat-responsive small HSP gene AtHSP17.6A was induced within 6 h, XPO1A was expressed at a relatively constant level throughout the treatment. 
     XPO1A has a different function from that of XPO1B under heat stress conditions. The results demonstrated that  Arabidopsis  XPO1A is dispensable for normal plant growth and development but is essential for thermotolerance. The importance of XPO1A in plants against heat stress provides new insights into its role in heat tolerance. 
     Part 2: Applications and Embodiments 
     One embodiment of this disclosure is to transfect an exogenous polynucleotide encoding a XPO1A protein into plants. Therefore, the transgenic plants can express exogenous XPO1A and be able to survive under heat stress. 
     The gene encoding XPO1A is XPO1A gene or a sequence which shares homology and analogy with XPO1A gene. The sequence described above can be obtained by isolation from appropriate seedlings or be synthesized artificially. Plants having the gene encoding XPO1A include, but are not limited to Brassicaceae plants, such as  Arabidopsis . A person having ordinary skill in the art can obtain the sequence encoding XPO1A by conventional procedures. 
     The gene described above, which is XPO1A gene or a sequence sharing homology and analogy with XPO1A gene, includes the sequence of SEQ ID NO 1, its complementary sequences, and conservative analogs. For example, a homologous sequence having degenerative codon substitutions. Degenerative codon is that more than one triplet sequence of codons can specify the insertion of the same amino acid into a polypeptide chain. 
     In the embodiments of this disclosure, XPO1A gene or the sequence sharing homology and analogy with XPO1A gene were used to construct the plasmid that encoded XPO1A. The plasmid was then transfected into plants to generate transgenic plants that could withstand high temperature conditions. The plant tissues or cells that are appropriate for the disclosure include, but are not limited to roots, stems or leaves of Brassicaceae or tomato. 
     Suitable transfection methods for the disclosure include, but are not limited to calcium phosphate, gene gun, microinjection, electroporation, liposome, etc. The gene that encodes XPO1A was transfected into plant tissues or cells. 
     The vectors that can be used in the disclosure include, but are not limited to the plasmid that encodes XPO1A or viral vectors that infects plants to transfect plant tissues or cells. 
     For example, plasmids can be transfected into plant cells by bacteria-mediated transformation. The bacteria may include but are not limited to  Agrobacterium tumefaciens . For gene transfer, Ti-plasmids comprising a polynucleotide sequence which encodes XPO1A are transfected into plant cells, or viral vectors are used to infect cells for introducing XPO1A gene. In one embodiment, the vectors comprise a polynucleotide sequence of SEQ ID NO 1. 
     The polynucleotide encoding XPO1A protein comprises a promoter that is linked to the upstream region of XPO1A gene or a polynucleotide sequence of SEQ ID NO: 1. The promoter is a constitutive promoter or an inducible promoter, such as 35S promoter or AlcA promoter (enthanol-inducible system). In one embodiment, the polynucleotide encoding a XPO1A protein comprises a constitutive promoter and a polynucleotide sequence of SEQ ID NO: 1. The polynucleotide comprising polynucleotide sequence of SEQ ID NO: 1 is a polynucleotide sequence of SEQ ID NO: 2. In other embodiment, the polynucleotide encoding a XPO1A protein comprises an inducible promoter and a polynucleotide sequence of SEQ ID NO: 1. The polynucleotide comprising polynucleotide sequence of SEQ ID NO: 1 is a polynucleotide sequence of SEQ ID NO: 3. 
     Unless otherwise indicated, promoter regions described herein comprise sequences recognized by RNA polymerase, regulatory sequences, constitutive promoter or inducible promoter sequences and transcription initiation site. 
     In addition, for each plant, the codon or promoter preferred by host cells are used to promote the expression of exogenous XPO1A protein. 
     According to another embodiment of this disclosure, the polynucleotide encoding XPO1A is utilized as a selection marker of transgenic plants. The recombinant plasmid comprising target gene and the selection marker was transfected into plant. By long-term high temperature stress or heat shock treatment, the transgenic plant with basal thermotolerance phenotype can be isolated. 
     The following embodiments are provided to describe methods of the present disclosure and the effects of XPO1A gene on the ability of plants to withstand heat stress. 
     EMBODIMENTS 
     1. Construction of pCAMBIA1300-XPO1A 
       FIG. 7  illustrates a schematic diagram of the plasmid of pCAMBIA1300-XPO1A. In the embodiment, the chromosome of  Arabidopsis thaliana  was served as the template and primers SEQ ID NO 6 and SEQ ID NO 7 were used to amplify the XPO1A gene which is shown as SEQ ID NO 2 by PCR. 
     The amplified fragments and the vector pLOLA were digested by restriction enzymes NcoI and BamHI. The sequences of XPO1A gene were cloned into the vector via restriction sites for NcoI and BamHI using T 4  DNA ligase to obtain pLOLA-35S-XPO1A-nos. 
     Digestion of pLOLA-35S-XPO1A-nos by KpnI generated the sequences which contain promoter (SEQ ID NO 8), XPO1A gene and terminator (SEQ ID NO 9). The sequences which encoded XPO1A protein were ligated into the vector pCAMBIA1300 digested by KpnI using T 4  DNA ligase to obtain pCAMBIA1300-XPO1A (SEQ ID NO 10). 
     To construct the plasmids, mRNA was extracted from plant tissues by commercial kit (Genemark Plant Total RNA Miniprep Purification Kit) or traditional extraction methods. The purified RNA was stored at −70° C. Reversed transcription was performed using SuperScript™ II Reverse Transcriptase (Invitrogen). For cDNA synthesis, 5 μl (1 ng-5 μg) RNA solution was first incubated with 1 μl of Oligo dT (500 μg/μl), 1 μl of dNTP (10 mM) and 5 μl of DEPC-treated water. After incubation for 5 min at 65° C. to denature RNA secondary structure, the mixture was then quickly chill on ice to let the primer anneal the RNA. Four μl of 5× first-stand buffer, 1 μl of RNase inhibitor (40 U/μl) and 2 μl of DTT (0.1M) was then added. The mixture was incubated for 2 min at 42° C. Add 1 μl (200 U) of SuperscriptII Reverse Transcriptase and incubate at 42° C. for 50 min. The reaction was then terminated at 70° C. for 15 min and stored at 4° C. For removing unreacted RNA, 1 μl (2 U) of RNasH was added and incubated at 37° C. for 20 min. The resulting cDNA was then stored at −20° C. for further use. 
     XPO1A gene was amplified by PCR. The PCR reactions contained 0.5 μl of cDNA, 0.4 μl of dNTPs (10 mM), 1 μl of 10×Taq DNA polymerase buffer, 0.4 μl of each primer (SEQ ID NO 6 and SEQ ID NO 7), 0.1 μl of Taq DNA polymerase, and 6.7 μl of sterilized distilled water in a total volume of 9.5 μl. The conditions of PCR were as follows: 10 min at 94° C. followed by 40 cycles of denaturation, annealing and elongation at 94° C. for 30 sec, 65° C. for 30 sec and 68° C. for 90 sec, respectively, then additional extension at 68° C. for 7 min. The resulting PCR products were visualized on agarose gels stained with EtBr. 
     The PCR products can be purified by commercial kit or traditional purification methods for digestion and ligation. 
     DNA was digested with restriction enzymes following the manufacturers&#39; recommendations. Briefly, DNA, buffer, distilled water and restriction enzyme were added and mixed well, and the reaction was performed at the proper temperature. 
     Ligation of DNA was performed using commercial kit (pGEM®-T and pGEM®-T Easy Vector Systems) according to the manufacturer&#39;s protocol. The pGEM®-T vector DNA, inserted DNA fragments, distilled water and ligation buffer were added and mixed well. Subsequently, T 4  ligase was added to the mixture, which was then incubated at the adequate temperature. The reaction was then terminated and the resulting plasmids were transformed into  E. coli  DH5α. 
     2. Transformation of  Agrobacterium tumefaciens    
     The method described by Hofgen and Willmitzer (1988) was utilized for transformation of pCAMBIA1300-XPO1A into  Agrobacterium.    
     A. Preparation of Competent Cells 
     Grow 200 ml culture of  Agrobacterium  strain GV3101 PMP90 overnight at 28° C. in LB medium with appropriate antibiotic. The logarithmically growing cells (OD 550  0.5-0.8) were centrifuged for 10 min. The pellet was washed twice by sterilized Tris-EDTA buffer and resuspended in 20 ml fresh LB medium. Aliquots of 500 μl were frozen in liquid nitrogen and stored at −80° C. 
     B. Transformation 
     Stored competent cells were thawed on ice. Competent cells were mixed with 10 μl plasmid DNA. The cells were incubated 5 min on ice, 5 min in liquid nitrogen and 5 min at 37° C. After addition of 1 ml LB medium, the cells were shaken for 2-4 hours at 28° C. The cells were centrifuged and plated on LB-plates and incubated for 2 days at 28° C. 
     3. Transfection of Plant Cells 
     A.  Arabidopsis  was given as an example of transfection of plant cells. The seeds of  Arabidopsis  were grown in organic soil under 16 h:8 h light:dark cycles at 22° C. with light and 18° C. under dark conditions. 
     B. For transfection of pCAMBIA1300-XPO1A from  Agrobacterium  into  Arabidopsis, Agrobacterium -mediated vacuum infiltration transformation was performed. Single colony of  Agrobacterium  was picked and grown in 5 ml of YEP medium containing appropriate antibiotics at 28° C. overnight. The small scale cultures were then diluted 50-fold into medium with appropriate antibiotics for large scale cultures. The cells were then harvested by centrifugation and resuspend in fresh AIM medium to an OD 600  of 0.6-1. AIM medium consists of half-strength MS salts, B5 vitamins, 0.01 mg/l benzylaminopurine (BA), 500 mg/l 2-(N-morpholino)ethanesulfonic acid (MES), 5% sucrose and 0.02% silwet-77. 
     The pots of  Arabidopsis  with 15-cm long rachis were selected and the siliques and bloomed flowers were removed. The plants were inverted and placed into the  Agrobacterium  suspension. Draw vacuum for 15 min using an oil pump. The pots were then removed from the  Agrobacterium  suspension and placed on their sides into a tray and incubated in dark. The next day, the pots were turned upright. The plants were grown under 16 h:8 h light:dark cycles at 22° C. with light and 18° C. under dark conditions. The seeds were harvested after approximately 45 days. 
     Although the present disclosure has been described in considerable detail with reference to certain embodiments thereof, other embodiments are possible. Therefore, their spirit and scope of the appended claims should not be limited to the description of the embodiments container herein. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present disclosure without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the present disclosure cover modifications and variations of this disclosure provided they fall within the scope of the following claim.