Heterosis of biomass production in Arabidopsis. Establishment during early development

Towards systems biology of heterosis: a hypothesis about molecular network structure applied for the Arabidopsis metabolome

We propose a network structure-based model for heterosis, and investigate it relying on metabolite profiles from Arabidopsis. A simple feed-forward two-layer network model (the Steinbuch matrix) is used in our conceptual approach. It allows for directly relating structural network properties with biological function. Interpreting heterosis as increased adaptability, our model predicts that the biological networks involved show increasing connectivity of regulatory interactions. A detailed analysis of metabolite profile data reveals that the increasing-connectivity prediction is true for graphical Gaussian models in our data from early development. This mirrors properties of observed heterotic Arabidopsis phenotypes. Furthermore, the model predicts a limit for increasing hybrid vigor with increasing heterozygosity--a known phenomenon in the literature.

Heterosis in the freezing tolerance, and sugar and flavonoid contents of crosses between Arabidopsis thaliana accessions of widely varying freezing tolerance

Heterosis is defined as the increased vigour of hybrids in comparison to their parents. We investigated 24 F(1) hybrid lines of Arabidopsis thaliana generated by reciprocally crossing either C24 or Col with six other parental accessions (Can, Co, Cvi, Ler, Rsch, Te) that differ widely in their freezing tolerance. The crosses differed in the degree of heterosis for freezing tolerance, both in the non-acclimated state and after a 14 d cold acclimation period. Crosses with C24 showed more heterosis than crosses with Col, and heterosis was stronger in acclimated than in non-acclimated plants. Leaf content of soluble sugars and proline showed more deviation from mid-parent values in crosses involving C24 than in those involving Col, and deviations were larger in acclimated than in non-acclimated plants. There were significant correlations between the content of different sugars and leaf freezing tolerance, as well as between heterosis effects in freezing tolerance and sugar content. Flavonoid content and composition varied between accessions, and between non-acclimated and acclimated plants. In the crosses, large deviations from the mid-parent values in the contents of different flavonols occurred, and there were strikingly strong correlations between both flavonol content and freezing tolerance, and between heterosis effects in freezing tolerance and flavonol content.

Construction and analysis of 2 reciprocal Arabidopsis introgression line populations

Two new large reciprocal sets of introgression lines (ILs) were created between the Arabidopsis accessions Col-0 and C24. In both sets (78 ILs with Col-0 background and 62 ILs with C24 background), the donor segments cover almost the entire genome with an average substitution size of 18.3 cM. In addition to the basic sets of ILs, further subILs were developed for 2 genomic regions allowing better mapping resolution. SubILs carrying donor segments with candidate genes for flowering time and reduced fertility were used to demonstrate the usefulness of the reciprocal ILs for quantitative trait loci detection and fine mapping. For subIL development at high resolution around the reduced fertility locus, we used modified CelI-based assays in one-well format for both marker development and genotyping. This serves as a very flexible and cost-effective approach.

Heterosis for biomass-related traits in Arabidopsis investigated by quantitative trait loci analysis of the triple testcross design with recombinant inbred lines

Arabidopsis thaliana has emerged as a leading model species in plant genetics and functional genomics including research on the genetic causes of heterosis. We applied a triple testcross (TTC) design and a novel biometrical approach to identify and characterize quantitative trait loci (QTL) for heterosis of five biomass-related traits by (i) estimating the number, genomic positions, and genetic effects of heterotic QTL, (ii) characterizing their mode of gene action, and (iii) testing for presence of epistatic effects by a genomewide scan and marker x marker interactions. In total, 234 recombinant inbred lines (RILs) of Arabidopsis hybrid C24 x Col-0 were crossed to both parental lines and their F1 and analyzed with 110 single-nucleotide polymorphism (SNP) markers. QTL analyses were conducted using linear transformations Z1, Z2, and Z3 calculated from the adjusted entry means of TTC progenies. With Z1, we detected 12 QTL displaying augmented additive effects. With Z2, we mapped six QTL for augmented dominance effects. A one-dimensional genome scan with Z3 revealed two genomic regions with significantly negative dominance x additive epistatic effects. Two-way analyses of variance between marker pairs revealed nine digenic epistatic interactions: six reflecting dominance x dominance effects with variable sign and three reflecting additive x additive effects with positive sign. We conclude that heterosis for biomass-related traits in Arabidopsis has a polygenic basis with overdominance and/or epistasis being presumably the main types of gene action.

Genetic basis of heterosis for growth-related traits in Arabidopsis investigated by testcross progenies of near-isogenic lines reveals a significant role of epistasis

Epistasis seems to play a significant role in the manifestation of heterosis. However, the power of detecting epistatic interactions among quantitative trait loci (QTL) in segregating populations is low. We studied heterosis in Arabidopsis thaliana hybrid C24 x Col-0 by testing near-isogenic lines (NILs) and their triple testcross (TTC) progenies. Our objectives were to (i) provide the theoretical basis for estimating different types of genetic effects with this experimental design, (ii) determine the extent of heterosis for seven growth-related traits, (iii) map the underlying QTL, and (iv) determine their gene action. Two substitution libraries, each consisting of 28 NILs and covering approximately 61 and 39% of the Arabidopsis genome, were assayed by 110 single-nucleotide polymorphism (SNP) markers. With our novel generation means approach 38 QTL were detected, many of which confirmed heterotic QTL detected previously in the same cross with TTC progenies of recombinant inbred lines. Furthermore, many of the QTL were common for different traits and in common with the 58 QTL detected by a method that compares triplets consisting of a NIL, its recurrent parent, and their F(1) cross. While the latter approach revealed mostly (75%) overdominant QTL, the former approach allowed separation of dominance and epistasis by analyzing all materials simultaneously and yielded substantial positive additive x additive effects besides directional dominance. Positive epistatic effects reduced heterosis for growth-related traits in our materials.

Morphological and physiological traits of three major Arabidopsis thaliana accessions

Arabidopsis is currently the most studied organism in plant biology. Its short life cycle and small genome size have rendered it one of the principal model systems. Additionally, numerous large T-DNA insertion mutant collections are available. The advent of molecular biology and the completion of the Arabidopsis genome sequence have contributed to helping researchers discover a large variety of mutants identified for their phenotypes. Yet, it is important to consider that natural phenotypic variations exist and appear in natural ecotypes, differing greatly in several traits. Although there are a vast number of ecotypes available, only a few have been extensively studied, and some have been created in laboratories. In order to identify new phenotypic differences, we chose to study the differences observed between three ecotypes: Columbia (Col-0), Landsberg erecta (Laer-0) and Wassilewskija (Ws-0). Our research focuses on observable morphological traits throughout plant growth and development along the entire plant life cycle. We then attempted to shed some light on phenotypic discrepancies through the study of the class III peroxidase protein family, which is involved in many aspects of plant growth and tissue differentiation. Both morphological and molecular aspects reveal that there are major variations between ecotypes, hence indicating a possibly interesting heterotic effect in the F1 from crosses between different Arabidopsis ecotypes.

Classical genetic and quantitative trait loci analyses of heterosis in a maize hybrid between two elite inbred lines

The exploitation of heterosis is one of the most outstanding advancements in plant breeding, although its genetic basis is not well understood yet. This research was conducted on the materials arising from the maize single cross B73 x H99 to study heterosis by procedures of classical genetic and quantitative trait loci (QTL) analyses. Materials were the basic generations, the derived 142 recombinant inbred lines (RILs), and the three testcross populations obtained by crossing the 142 RILs to each parent and their F(1). For seedling weight (SW), number of kernels per plant (NK), and grain yield (GY), heterosis was >100% and the average degree of dominance was >1. Epistasis was significant for SW and NK but not for GY. Several QTL were identified and in most cases they were in the additive-dominance range for traits with low heterosis and mostly in the dominance-overdominance range for plant height (PH), SW, NK, and GY. Only a few QTL with digenic epistasis were identified. The importance of dominance effects was confirmed by highly significant correlations between heterozygosity level and phenotypic performance, especially for GY. Some chromosome regions presented overlaps of overdominant QTL for SW, PH, NK, and GY, suggesting pleiotropic effects on overall plant vigor.

The metabolic signature related to high plant growth rate in Arabidopsis thaliana

The decline of available fossil fuel reserves has triggered world-wide efforts to develop alternative energy sources based on plant biomass. Detailed knowledge of the relations of metabolism and biomass accumulation can be expected to yield powerful novel tools to accelerate and enhance energy plant breeding programs. We used metabolic profiling in the model Arabidopsis to study the relation between biomass and metabolic composition using a recombinant inbred line (RIL) population. A highly significant canonical correlation (0.73) was observed, revealing a close link between biomass and a specific combination of metabolites. Dividing the entire data set into training and test sets resulted in a median correlation between predicted and true biomass of 0.58. The demonstrated high predictive power of metabolic composition for biomass features this composite measure as an excellent biomarker and opens new opportunities to enhance plant breeding specifically in the context of renewable resources.

Analysis of a triple testcross design with recombinant inbred lines reveals a significant role of epistasis in heterosis for biomass-related traits in Arabidopsis

Primary causes of heterosis are still unknown. Our goal was to investigate the extent and underlying genetic causes of heterosis for five biomass-related traits in Arabidopsis thaliana. We (i) investigated the relative contribution of dominance and epistatic effects to heterosis in the hybrid C24 x Col-0 by generation means analysis and estimates of variance components based on a triple testcross (TTC) design with recombinant inbred lines (RILs), (ii) estimated the average degree of dominance, and (iii) examined the importance of reciprocal and maternal effects in this cross. In total, 234 RILs were crossed to parental lines and their F1's. Midparent heterosis (MPH) was high for rosette diameter at 22 days after sowing (DAS) and 29 DAS, growth rate (GR), and biomass yield (BY). Using the F2-metric, directional dominance prevailed for the majority of traits studied but reciprocal and maternal effects were not significant. Additive and dominance variances were significant for all traits. Additive x additive and dominance x dominance variances were significant for all traits but GR. We conclude that dominance as well as digenic and possibly higher-order epistatic effects play an important role in heterosis for biomass-related traits. Our results encourage the use of Arabidopsis hybrid C24 x Col-0 for identification and description of quantitative trait loci (QTL) for heterosis for biomass-related traits and further genomic studies.

Heterosis associated gene expression in maize embryos 6 days after fertilization exhibits additive, dominant and overdominant pattern

Heterosis is important for conventional plant breeding and is intensively used to increase the productivity of crop plants. Genetic processes shortly after fertilization might be of particular importance with respect to heterosis, because coordination of the diverse genomes establishes a basis for future performance of the sporophyte. Here we demonstrate a strong crossbreeding advantage of hybrid maize embryos as early as 6 days after fertilization in a modern maize hybrid and provide the first embryo specific analysis of associated gene expression pattern at this early stage of development. We identified differentially expressed genes between hybrid embryos and the parental genotypes by a combined approach of suppression subtractive hybridization and differential screening by microarray hybridizations. Association of heterosis in embryos with genes related to signal transduction and other regulatory processes was implied by the enrichment of these functional classes among the identified gene set. Quantitative RT-PCR analysis validated the expression pattern of 7 of 12 genes analysed and revealed predominantly additive, but also dominant and overdominant expression patterns in hybrid embryos. These patterns indicate that gene regulatory interactions among parental alleles act at this early developmental stage and the genes identified provide entry points for the exploration of gene regulatory networks associated with the specification of the phenomenon heterosis in the plant life cycle.

Allelic variation and heterosis in maize: how do two halves make more than a whole?

In this review, we discuss the recent research on allelic variation in maize and possible implications of this work toward our understanding of heterosis. Heterosis, or hybrid vigor, is the increased performance of a hybrid relative to the parents, and is a result of the variation that is present within a species. Intraspecific comparisons of sequence and expression levels in maize have documented a surprisingly high level of allelic variation, which includes variation for the content of genic fragments, variation in repetitive elements surrounding genes, and variation in gene expression levels. There is evidence that transposons and repetitive DNA play a major role in the generation of this allelic diversity. The combination of allelic variants provides a more comprehensive suite of alleles in the hybrid that may be involved in novel allelic interactions. A major unresolved question is how the combined allelic variation and interactions in a hybrid give rise to heterotic phenotypes. An understanding of allelic variation present in maize provides an opportunity to speculate on mechanisms that might lead to heterosis. Variation for the presence of genes, the presence of novel beneficial alleles, and modified levels of gene expression in hybrids may all contribute to the heterotic phenotypes.

Dynamics of seedling growth acclimation towards altered light conditions can be quantified via GROWSCREEN: a setup and procedure designed for rapid optical phenotyping of different plant species

Using a novel setup, we assessed how fast growth of Nicotiana tabacum seedlings responds to alterations in the light regime and investigated whether starch-free mutants of Arabidopsis thaliana show decreased growth potential at an early developmental stage. Leaf area and relative growth rate were measured based on pictures from a camera automatically placed above an array of 120 seedlings. Detection of total seedling leaf area was performed via global segmentation of colour images for preset thresholds of the parameters hue, saturation and value. Dynamic acclimation of relative growth rate towards altered light conditions occurred within 1 d in N. tabacum exposed to high nutrient availability, but not in plants exposed to low nutrient availability. Increased leaf area was correlated with an increase in shoot fresh and dry weight as well as root growth in N. tabacum. Relative growth rate was shown to be a more appropriate parameter than leaf area for detection of dynamic growth acclimation. Clear differences in leaf growth activity were also observed for A. thaliana. As growth responses are generally most flexible in early developmental stages, the procedure described here is an important step towards standardized protocols for rapid detection of the effects of changes in internal (genetic) and external (environmental) parameters regulating plant growth.

Comparative expression profiling in meristems of inbred-hybrid triplets of maize based on morphological investigations of heterosis for plant height

Heterosis, the superior performance of hybrids as compared to their parental mean is an agronomically important phenomenon well-described morphologically. However, little is known about its molecular basis. We investigated four genetically unrelated maize (Zea mays L.) inbred lines and their F(1) crosses both at the phenotype and transcriptome level, focusing on plant height (PHT) component traits. Substantial mid-parent heterosis (MPH) was found for all parent-hybrid triplets for PHT in the range of 37.9-56.4% in the field and 11.1-39.5% under controlled greenhouse conditions. Analyses of heterosis for number and length of internodes showed two to three times higher MPH in the field as compared to the greenhouse. All three traits exhibited high heritabilities, highest for PHT 95-98%. Two methods for gene expression quantification were applied. High-density cDNA uni-gene microarrays containing 11,827 ESTs were utilized for the selection of differentially expressed genes related to heterosis for PHT. For the four triplets with eight possible parent-hybrid comparisons we identified 434 consistently differentially expressed genes with a p < or = 0.05. Microarray results were used to verify the dominance/overdominance hypothesis. In our study, more than 50% genes showed overdominance, 26% partial dominance, 12.6% complete dominance and 10.2% additive gene action. Moreover, more consistently differentially expressed genes were detected in related triplets, sharing one parent, than in unrelated triplets. Quantitative RT-PCR was applied in order to validate microarray results. The role of the differentially expressed genes in relation to heterosis for PHT is discussed.

Heterosis: revisiting the magic

Heterosis results in the phenotypic superiority of a hybrid over its parents with respect to traits such as growth rate, reproductive success and yield. This hybrid vigor is determined by non-mutually exclusive mechanisms, including dominance complementation, overdominance and epistasis. Heterotic genes responsible for elevating crop yields are now being sought using genomics, particularly transcriptomics, but with contradictory results. Because heterosis is an environmentally modified quantitative phenotype, genomic analyses alone will not suffice. Future research should focus on integrating genomic tools in a framework of comprehensive quantitative trait locus (QTL)-based phenotyping, followed by map-based cloning. This 'phenomics' approach should identify loci controlling heterotic phenotypes, and improve understanding of the role of heterosis in evolution and the domestication of crop plants.

Heterosis and polymorphisms of gene expression in an elite rice hybrid as revealed by a microarray analysis of 9198 unique ESTs

Despite the significant contributions of utilizing heterosis to crop productivity worldwide, the biological mechanisms of heterosis remained largely uncharacterized. In this study, we analyzed gene expression profiles of an elite rice hybrid and the parents at three stages of young panicle development, using a cDNA microarray consisting of 9198 expressed sequence tags (ESTs), with the objective to reveal patterns of gene expression that may be associated with heterosis in yield. A total of 8422 sequences showed hybridization signals in all three genotypes in at least one stage and 5771 sequences produced detectable signals in all slides. Significant differences in expression level were detected for 438 sequences among the three genotypes in at least one of the three stages, as determined by ANOVA validated with 100 permutations at P < 0.05. Significant mid-parent heterosis was detected for 141 sequences, which demonstrated the following features: a much larger number of sequences showed negative heterosis than ones showing positive heterosis; genes functioning in DNA replication and repair tended to show positive heterosis; genes functioning in carbohydrate metabolism, lipid metabolism, energy metabolism, translation, protein degradation, and cellular information processing showed negative heterosis; both positive and negative heterosis were observed for genes in amino acid metabolism, transcription, signal transduction, plant defense and transportation. The results are indicative of the biochemical and physiological activities taking place in the hybrid relative to the parents. Identification of genes showing expression polymorphisms among different genotypes and heterotic expression in the hybrid may provide new avenues for exploring the biological mechanisms underlying heterosis.

Heterosis in root development and differential gene expression between hybrids and their parental inbreds in wheat (Triticum aestivum L.)

In spite of commercial use of heterosis in agriculture, the molecular basis of heterosis is poorly understood. In this study, heterosis was estimated for eight root traits in 20 wheat hybrids derived from a NC Design II mating scheme. Positive mid-parent heterosis was detected in 96 of 160 hybrid-trait combinations, and positive high-parent heterosis was detected in 79 of 160 hybrid-trait combinations. Improved differential display was used to analyze alterations in gene expression between hybrids and their parents in roots at the jointing stage. More than 990 fragments were repeatedly displayed, among which 27.52% were differentially expressed between hybrids and their parents. Four differential expression patterns were observed. Thirty differentially expressed cDNA fragments and three genes with open reading frames were cloned, and their expression patterns were confirmed by reverse-northern blot and semi-quantitative RT-PCR analysis, respectively. We concluded that these differentially expressed genes, though mostly with unknown function, could play important roles for hybrids to demonstrate heterosis in root system traits.

Genetic analysis of variation in gene expression in Arabidopsis thaliana

In Arabidopsis thaliana, significant efforts to determine the extent of genomic variation between phenotypically divergent accessions are under way, but virtually nothing is known about variation at the transcription level. We used microarrays to examine variation in transcript abundance among three inbred lines and two pairs of reciprocal F1 hybrids of the highly self-fertilizing species Arabidopsis. Composite additive genetic effects for gene expression were estimated from pairwise comparisons of the three accessions Columbia (Col), Landsberg erecta (Ler), and Cape Verde Islands (Cvi). For the pair Col and Ler, 27.0% of the 4876 genes exhibited additive genetic effects in their expression (alpha = 0.001) vs. 32.2 and 37.5% for Cvi with Ler and Col, respectively. Significant differential expression ranged from 32.45 down to 1.10 in fold change and typically differed by a factor of 1.56. Maternal or paternal transmission affected only a few genes, suggesting that the reciprocal effects observed in the two crosses analyzed were minimal. Dominance effects were estimated from the comparisons of hybrids with the corresponding midparent value. The percentage of genes showing dominance at the expression level in the F1 hybrids ranged from 6.4 to 21.1% (alpha = 0.001). Breakdown of these numbers of genes according to the magnitude of the dominance ratio revealed heterosis for expression for on average 9% of the genes. Further advances in the genetic analysis of gene expression variation may contribute to a better understanding of its role in affecting quantitative trait variation at the phenotypic level.

Distant sequences determine 5' end formation of cox3 transcripts in Arabidopsis thaliana ecotype C24

The genomic environments and the transcripts of the mitochondrial cox3 gene are investigated in three Arabidopsis thaliana ecotypes. While the proximate 5′ sequences up to nucleotide position −584, the coding regions and the 3′ flanking regions are identical in Columbia (Col), C24 and Landsberg erecta (Ler), genomic variation is detected in regions further upstream. In the mitochondrial DNA of Col, a 1790 bp fragment flanked by a nonanucleotide direct repeat is present beyond position −584 with respect to the ATG. While in Ler only part of this insertion is conserved, this sequence is completely absent in C24, except for a single copy of the nonanucleotide direct repeat. Northern hybridization reveals identical major transcripts in the three ecotypes, but identifies an additional abundant 60 nt larger mRNA species in C24. The extremities of the most abundant mRNA species are identical in the three ecotypes. In C24, an extra major 5′ end is abundant. This terminus and the other major 5′ ends are located in identical sequence regions. Inspection of Atcox3 transcripts in C24/Col hybrids revealed a female inheritance of the mRNA species with the extra 5′ terminus. Thus, a mitochondrially encoded factor determines the generation of an extra 5′ mRNA end.

Identification of differentially expressed genes in leaf and root between wheat hybrid and its parental inbreds using PCR-based cDNA subtraction

Heterosis was defined as the advantage of hybrid performance over its parents in terms of growth and productivity. Previous studies showed that differential gene expression between hybrids and their parents is responsible for the heterosis; however, information on systematic identification and characterization of the differentially expressed genes are limited. In this study, an interspecific hybrid between common wheat (Triticum aestivum. L., 2n = 6x = 42, AABBDD) line 3338 and spelt (Triticum spelta L. 2n = 6x = 42, AABBDD) line 2463 was found to be highly heterotic in both aerial growth and root related traits, and was then used for expression assay. A modified suppression subtractive hybridization (SSH) was used to generate four subtracted cDNA libraries, and 748 nonreduandant cDNAs were obtained, among which 465 had high sequence similarity to the GenBank entries and represent diverse of functional categories, such as metabolism, cell growth and maintenance, signal transduction, photosynthesis, response to stress, transcription regulation and others. The expression patterns of 68.2% SSH-derived cDNAs were confirmed by reverse Northern blot, and semi-quantitative RT-PCR exhibited the similar results (72.2%). And it was concluded that the genes differentially expressed between hybrids and their parents involved in diverse physiological process pathway, which might be responsible for the observed heterosis.

Contribution of transcriptional regulation to natural variations in Arabidopsis

BACKGROUND

Genetic control of gene transcription is a key component in genome evolution. To understand the transcriptional basis of natural variation, we have studied genome-wide variations in transcription and characterized the genetic variations in regulatory elements among Arabidopsis accessions.

RESULTS

Among five accessions (Col-0, C24, Ler, WS-2, and NO-0) 7,508 probe sets with no detectable genomic sequence variations were identified on the basis of the comparative genomic hybridization to the Arabidopsis GeneChip microarray, and used for accession-specific transcriptome analysis. Two-way ANOVA analysis has identified 60 genes whose mRNA levels differed in different accession backgrounds in an organ-dependent manner. Most of these genes were involved in stress responses and late stages of plant development, such as seed development. Correlation analysis of expression patterns of these 7,508 genes between pairs of accessions identified a group of 65 highly plastic genes with distinct expression patterns in each accession.

CONCLUSION

Genes that show substantial genetic variation in mRNA level are those with functions in signal transduction, transcription and stress response, suggesting the existence of variations in the regulatory mechanisms for these genes among different accessions. This is in contrast to those genes with significant polymorphisms in the coding regions identified by genomic hybridization, which include genes encoding transposon-related proteins, kinases and disease-resistance proteins. While relatively fewer sequence variations were detected on average in the coding regions of these genes, a number of differences were identified from the upstream regions, several of which alter potential cis-regulatory elements. Our results suggest that nucleotide polymorphisms in regulatory elements of genes encoding controlling factors could be primary targets of natural selection and a driving force behind the evolution of Arabidopsis accessions.

Contribution of transcriptional regulation to natural variations in Arabidopsis

BACKGROUND

Genetic control of gene transcription is a key component in genome evolution. To understand the transcriptional basis of natural variation, we have studied genome-wide variations in transcription and characterized the genetic variations in regulatory elements among Arabidopsis accessions.

RESULTS

Among five accessions (Col-0, C24, Ler, WS-2, and NO-0) 7,508 probe sets with no detectable genomic sequence variations were identified on the basis of the comparative genomic hybridization to the Arabidopsis GeneChip microarray, and used for accession-specific transcriptome analysis. Two-way ANOVA analysis has identified 60 genes whose mRNA levels differed in different accession backgrounds in an organ-dependent manner. Most of these genes were involved in stress responses and late stages of plant development, such as seed development. Correlation analysis of expression patterns of these 7,508 genes between pairs of accessions identified a group of 65 highly plastic genes with distinct expression patterns in each accession.

CONCLUSION

Genes that show substantial genetic variation in mRNA level are those with functions in signal transduction, transcription and stress response, suggesting the existence of variations in the regulatory mechanisms for these genes among different accessions. This is in contrast to those genes with significant polymorphisms in the coding regions identified by genomic hybridization, which include genes encoding transposon-related proteins, kinases and disease-resistance proteins. While relatively fewer sequence variations were detected on average in the coding regions of these genes, a number of differences were identified from the upstream regions, several of which alter potential cis-regulatory elements. Our results suggest that nucleotide polymorphisms in regulatory elements of genes encoding controlling factors could be primary targets of natural selection and a driving force behind the evolution of Arabidopsis accessions.