Rice hybrid mimics have stable yields equivalent to those of the F1 hybrid and suggest a basis for hybrid vigour.

MAIN CONCLUSION: We have developed long term stable high yielding rice lines, Hybrid Mimics, from commercial hybrids. The vigour of the Mimic and the hybrid are developmental changes. These Mimics could substitute for hybrid seed for planting. We have used two pre-existing high-yielding hybrid systems (FLY1 and DY527) to develop Hybrid Mimics. In the FLY1 hybrid system we selected, under field conditions, F6 lines which have high grain yields and biomass equivalent to the F1 hybrids, stable over subsequent F7, F8 and later generations. We have termed these lines Hybrid Mimics. The mimics are mostly homozygous as a consequence of selfing in each generation. We have repeated this selection procedure in the second independent hybrid system DY527, producing Mimics with similar characteristics to the F1 hybrid. In both hybrid systems the selection criterion, based on the phenotype of the F1 hybrid, results in the Mimics having grain yield and biomass similar to that of the F1 hybrid. In each generation of the breeding program the plant population has increased phenotypic homogeneity. The genomes of the Mimic plants do not contain any common heterozygous segments negating claims that the vigour of hybrids depends upon heterozygosity of particular loci. Both hybrids and Mimics have early germination and commence photosynthesis before the parents, providing enhanced growth which is maintained throughout the life cycle. The biochemical parameters of photosynthesis in the hybrids and Mimics do not differ from those of the parents. Grain quality and resistance to the two major diseases, bacterial blight and rice blast are similar in the Mimics and hybrids. The Mimics overcome the major disadvantage of hybrids where F2 phenotypic segregation prevents their use as a crop beyond the F1 generation.

Early Establishment of Photosynthesis and Auxin Biosynthesis Plays a Key Role in Early Biomass Heterosis in Brassica napus (Canola) Hybrids.

Heterosis or hybrid vigor has been used widely for more than a decade in Canola (Brassica napus) production. Canola hybrids show heterosis in a variety of traits compared to parents, including increased biomass at the early stages of seedling establishment, which is a critical developmental step that impacts future plant growth and seed yield. In this study, we examined transcriptomes of two parental lines, Garnet (Gar) and NX0052 (0052), and their reciprocal hybrids, Gar/0052, at 4 and 8 days after sowing (DAS). In hybrids, early seedling biomass heterosis is correlated with earlier expression of genes in photosynthesis pathways relative to parents. The hybrids also showed early expression of genes in the auxin biosynthesis pathway, consistent with the higher auxin concentrations detected in hybrid seedlings at 4 DAS. Auxin is a key phytohormone that regulates plant development promoting cell expansion and cell proliferation. Consistent with the increased levels of auxin, hybrids have larger and more palisade cells than the parents at the same time point. We propose a possible mechanism of early biomass heterosis through the early establishment of photosynthesis and auxin biosynthesis, providing insights into how transcriptional changes in hybrids are translated into phenotypical heterosis. This finding could be utilized in future Canola breeding to identify hybrid combinations with the superior early seedling establishment and strong levels of hybrid vigor in later plant development.

Arabidopsis Col/Ler and Ws/Ler hybrids and Hybrid Mimics produce seed yield heterosis through increased height, inflorescence branch and silique number.

MAIN CONCLUSION: The seed yield increase of the hybrids and their derived Mimics compared to parents is associated with increased plant height and inflorescence branch number which are correlated with decreased expression of FT, SOC1 and FUL. In Arabidopsis, plant size has been extensively investigated, but few studies have been carried out on seed yield heterosis. In hybrids between Columbia (Col) and Landsberg erecta (Ler), and Wassilewskija (Ws) and Ler, there was significant seed yield heterosis. F6/F7 Hybrid Mimics derived from hybrids of each of the two systems had seed yield increases similar to that of the F1 hybrid (approximately 50-70% greater than the average of the parents). Increased seed yield of the Hybrid Mimics was accompanied by changes of plant architecture with increased plant height and increased inflorescence branch number relative to the parents. Three of the Hybrid Mimic lines derived from the Ws/Ler system had 20% increase in seed yield relative to the F1 hybrid. Genes which repress flowering were up-regulated and the expression levels of flowering -promoting genes including FLOWERING LOCUS T (FT), SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1) and FRUITFULL (FUL) were negatively correlated with the increase in seed yield in both hybrids and F7 Mimics of both systems.

Senescence and Defense Pathways Contribute to Heterosis.

Hybrids are used extensively in agriculture due to their superior performance in seed yield and plant growth, yet the molecular mechanisms underpinning hybrid performance are not well understood. Recent evidence has suggested that a decrease in basal defense response gene expression regulated by reduced levels of salicylic acid (SA) may be important for vigor in certain hybrid combinations. Decreasing levels of SA in the Arabidopsis (Arabidopsis thaliana) accession C24 through the introduction of the SA catabolic enzyme salicylate1 hydroxylase (NahG) increases plant size, phenocopying the large-sized C24/Landsberg erecta (Ler) F1 hybrids. C24♀ × Ler♂ F1 hybrids and C24 NahG lines shared differentially expressed genes and pathways associated with plant defense and leaf senescence including decreased expression of SA biosynthetic genes and SA response genes. The expression of TL1 BINDING TRANSCRIPTION FACTOR1, a key regulator in resource allocation between growth and defense, was decreased in both the F1 hybrid and the C24 NahG lines, which may promote growth. Both C24 NahG lines and the F1 hybrids showed decreased expression of the key senescence-associated transcription factors WRKY53, NAC-CONTAINING PROTEIN29, and ORESARA1 with a delayed onset of senescence compared to C24 plants. The delay in senescence resulted in an extension of the photosynthetic period in the leaves of F1 hybrids compared to the parental lines, potentially allowing each leaf to contribute more resources toward growth.

Leaf growth in early development is key to biomass heterosis in Arabidopsis.

Arabidopsis thaliana hybrids have similar properties to hybrid crops, with greater biomass relative to the parents. We asked whether the greater biomass was due to increased photosynthetic efficiency per unit leaf area or to overall increased leaf area and increased total photosynthate per plant. We found that photosynthetic parameters (electron transport rate, CO2 assimilation rate, chlorophyll content, and chloroplast number) were unchanged on a leaf unit area and unit fresh weight basis between parents and hybrids, indicating that heterosis is not a result of increased photosynthetic efficiency. To investigate the possibility of increased leaf area producing more photosynthate per plant, we studied C24×Landsberg erecta (Ler) hybrids in detail. These hybrids have earlier germination and leaf growth than the parents, leading to a larger leaf area at any point in development of the plant. The developing leaves of the hybrids are significantly larger than those of the parents, with consequent greater production of photosynthate and an increased contribution to heterosis. The set of leaves contributing to heterosis changes as the plant develops; the four most recently emerged leaves make the greatest contribution. As a leaf matures, its contribution to heterosis attenuates. While photosynthesis per unit leaf area is unchanged at any stage of development in the hybrid, leaf area is greater and the amount of photosynthate per plant is increased.

PIF4-controlled auxin pathway contributes to hybrid vigor in Arabidopsis thaliana.

F1 hybrids in Arabidopsis and crop species are uniform and high yielding. The F2 generation loses much of the yield advantage and the plants have heterogeneous phenotypes. We generated pure breeding hybrid mimic lines by recurrent selection and also selected a pure breeding small phenotype line. The hybrid mimics are almost completely homozygous with chromosome segments from each parent. Four particular chromosomal segments from C24 and 8 from Ler were present in all of the hybrid mimic lines, whereas in the F6 small phenotype line, the 12 segments were each derived from the alternative parent. Loci critical for promoting hybrid vigor may be contained in each of these 12 conserved segments. We have identified genes with similar altered expression in hybrid mimics and F1 plants but not in the small phenotype line. These genes may be critical for the generation of hybrid vigor. Analysis of transcriptomes indicated that increased expression of the transcription factor PHYTOCHROME-INTERACTING FACTOR (PIF4) may contribute to hybrid vigor by targeting the auxin biosynthesis gene YUCCA8 and the auxin signaling gene IAA29 A number of auxin responsive genes promoting leaf growth were up-regulated in the F1 hybrids and hybrid mimics, suggesting that increased auxin biosynthesis and signaling contribute to the hybrid phenotype. The hybrid mimic seeds had earlier germination as did the seeds of the F1 hybrids, indicating cosegregation of the genes for rosette size and the germination trait. Early germination may be an indicator of vigorous hybrids.

Cotyledons contribute to plant growth and hybrid vigor in Arabidopsis.

MAIN CONCLUSION: In hybrids of Arabidopsis, cotyledons influence the amount and proportion of hybrid vigor in total plant growth. We found Arabidopsis cotyledons are essential for plant growth and in some hybrids for hybrid vigor. In hybrids between C24 and Landsberg erecta (Ler), biomass vigor (heterosis) occurs in the first few days after sowing (DAS), with hybrid cotyledons being larger than those of their parents. C24xLer hybrids are ahead of their parents in activating photosynthesis and auxin pathway genes in cotyledons at 3-4 DAS. "Earliness" is also present in newly emerged C24xLer hybrid leaves. We showed cotyledon removal at 4 DAS caused significant biomass reduction in later growth in hybrids and parental lines. The biomass decrease caused by cotyledon removal can be partially rescued by exogenous sucrose or auxin with different genotypes responding to sucrose and/or auxin differently. Cotyledon removal has different effects on heterosis in different hybrids. After cotyledon removal, in C24xLer hybrids, both growth and heterosis were reduced in similar proportions, but the level of hybrid vigor was reduced as a proportion of growth in C24xColumbia (Col) and ColxLer hybrids. The removal of cotyledons at 4 DAS markedly decreased the level of growth and eliminated the heterotic phenotype of Wassilewskija (Ws)/Ler hybrids. In mutant Ws/Ler hybrids which had a reduced level of photosynthesis in the cotyledons, there was a reduction in plant growth and loss of heterosis. The variation in contribution of cotyledons to heterosis in different hybrids indicates there are multiple pathways to achieve heterotic phenotypes.

Twenty-four-nucleotide siRNAs produce heritable trans-chromosomal methylation in F1 Arabidopsis hybrids.

Hybrid Arabidopsis plants undergo epigenetic reprogramming producing decreased levels of 24-nt siRNAs and altered patterns of DNA methylation that can affect gene expression. Driving the changes in methylation are the processes trans-chromosomal methylation (TCM) and trans-chromosomal demethylation (TCdM). In TCM/TCdM the methylation state of one allele is altered to resemble the other allele. We show that Pol IV-dependent sRNAs are required to establish TCM events. The changes in DNA methylation and the associated changes in sRNA levels in the F1 hybrid can be maintained in subsequent generations and affect hundreds of regions in the F2 epigenome. The inheritance of these altered epigenetic states varies in F2 individuals, resulting in individuals with genetically identical loci displaying different epigenetic states and gene expression profiles. The change in methylation at these regions is associated with the presence of sRNAs. Loci without any sRNA activity can have altered methylation states, suggesting that a sRNA-independent mechanism may also contribute to the altered methylation state of the F1 and F2 generations.

Role of DNA methylation in hybrid vigor in Arabidopsis thaliana.

Hybrid vigor or heterosis refers to the superior performance of F1 hybrid plants over their parents. Heterosis is particularly important in the production systems of major crops. Recent studies have suggested that epigenetic regulation such as DNA methylation is involved in heterosis, but the molecular mechanism of heterosis is still unclear. To address the epigenetic contribution to heterosis in Arabidopsis thaliana, we used mutant genes that have roles in DNA methylation. Hybrids between C24 and Columbia-0 (Col) without RNA polymerase IV (Pol IV) or methyltransferase I (MET1) function did not reduce the level of biomass heterosis (as evaluated by rosette diameter). Hybrids with a mutation in decrease in dna methylation 1 (ddm1) showed a decreased heterosis level. Vegetative heterosis in the ddm1 mutant hybrid was reduced but not eliminated; a complete reduction could result if there was a change in methylation at all loci critical for generating the level of heterosis, whereas if only a proportion of the loci have methylation changes there may only be a partial reduction in heterosis.

Hybrid mimics and hybrid vigor in Arabidopsis.

F1 hybrids can outperform their parents in yield and vegetative biomass, features of hybrid vigor that form the basis of the hybrid seed industry. The yield advantage of the F1 is lost in the F2 and subsequent generations. In Arabidopsis, from F2 plants that have a F1-like phenotype, we have by recurrent selection produced pure breeding F5/F6 lines, hybrid mimics, in which the characteristics of the F1 hybrid are stabilized. These hybrid mimic lines, like the F1 hybrid, have larger leaves than the parent plant, and the leaves have increased photosynthetic cell numbers, and in some lines, increased size of cells, suggesting an increased supply of photosynthate. A comparison of the differentially expressed genes in the F1 hybrid with those of eight hybrid mimic lines identified metabolic pathways altered in both; these pathways include down-regulation of defense response pathways and altered abiotic response pathways. F6 hybrid mimic lines are mostly homozygous at each locus in the genome and yet retain the large F1-like phenotype. Many alleles in the F6 plants, when they are homozygous, have expression levels different to the level in the parent. We consider this altered expression to be a consequence of transregulation of genes from one parent by genes from the other parent. Transregulation could also arise from epigenetic modifications in the F1. The pure breeding hybrid mimics have been valuable in probing the mechanisms of hybrid vigor and may also prove to be useful hybrid vigor equivalents in agriculture.

Hormone-regulated defense and stress response networks contribute to heterosis in Arabidopsis F1 hybrids.

Plant hybrids are extensively used in agriculture to deliver increases in yields, yet the molecular basis of their superior performance (heterosis) is not well understood. Our transcriptome analysis of a number of Arabidopsis F1 hybrids identified changes to defense and stress response gene expression consistent with a reduction in basal defense levels. Given the reported antagonism between plant immunity and growth, we suggest that these altered patterns of expression contribute to the greater growth of the hybrids. The altered patterns of expression in the hybrids indicate decreases to the salicylic acid (SA) biosynthesis pathway and increases in the auxin [indole-3-acetic acid (IAA)] biosynthesis pathway. SA and IAA are hormones known to control stress and defense responses as well as plant growth. We found that IAA-targeted gene activity is frequently increased in hybrids, correlating with a common heterotic phenotype of greater leaf cell numbers. Reduced SA concentration and target gene responses occur in the larger hybrids and promote increased leaf cell size. We demonstrated the importance of SA action to the hybrid phenotype by manipulating endogenous SA concentrations. Increasing SA diminished heterosis in SA-reduced hybrids, whereas decreasing SA promoted growth in some hybrids and phenocopied aspects of hybrid vigor in parental lines. Pseudomonas syringae infection of hybrids demonstrated that the reductions in basal defense gene activity in these hybrids does not necessarily compromise their ability to mount a defense response comparable to the parents.

Recent research on the mechanism of heterosis is important for crop and vegetable breeding systems.

Heterosis or hybrid vigor is a phenomenon where hybrid progeny have superior performance compared to their parental inbred lines. This is important in the use of F1 hybrid cultivars in many crops and vegetables. However, the molecular mechanism of heterosis is not clearly understood. Gene interactions between the two genomes such as dominance, overdominance, and epistasis have been suggested to explain the increased biomass and yield. Genetic analyses of F1 hybrids in maize, rice, and canola have defined a large number of quantitative trait loci, which may contribute to heterosis. Recent molecular analyses of transcriptomes together with reference to the epigenome of the parents and hybrids have begun to uncover new facts about the generation of heterosis. These include the identification of gene expression changes in hybrids, which may be important for heterosis, the role of epigenetic processes in heterosis, and the development of stable high yielding lines.

Early changes of gene activity in developing seedlings of Arabidopsis hybrids relative to parents may contribute to hybrid vigour.

Hybrid vigour (heterosis) has been used for decades in crop industries, especially in the production of maize and rice. Hybrid varieties usually exceed their parents in plant biomass and seed yield. But the molecular basis of hybrid vigour is not fully understood. In this project, we studied heterosis at early stages of seedling development in Arabidopsis hybrids derived from crossing Ler and C24 accessions. We found that early heterosis is associated with non-additive gene expression that resulted from earlier changes in gene expression in the hybrids relative to the parents. The non-additively expressed genes are involved in metabolic pathways, including photosynthesis, critical for plant growth. The early increased expression levels of genes involved in energy production in hybrids is associated with heterosis in the young seedlings that could be essential for biomass heterosis at later developmental stages of the plant.

Genome-wide analyses of four major histone modifications in Arabidopsis hybrids at the germinating seed stage.

BACKGROUND: Hybrid vigour (heterosis) has been used for decades in cropping agriculture, especially in the production of maize and rice, because hybrid varieties exceed their parents in plant biomass and seed yield. The molecular basis of hybrid vigour is not fully understood. Previous studies have suggested that epigenetic systems could play a role in heterosis. RESULTS: In this project, we investigated genome-wide patterns of four histone modifications in Arabidopsis hybrids in germinating seeds. We found that although hybrids have similar histone modification patterns to the parents in most regions of the genome, they have altered patterns at specific loci. A small subset of genes show changes in histone modifications in the hybrids that correlate with changes in gene expression. Our results also show that genome-wide patterns of histone modifications in geminating seeds parallel those at later developmental stages of seedlings. CONCLUSION: Ler/C24 hybrids showed similar genome-wide patterns of histone modifications as the parents at an early germination stage. However, a small subset of genes, such as FLC, showed correlated changes in histone modification and in gene expression in the hybrids. The altered patterns of histone modifications for those genes in hybrids could be related to some heterotic traits in Arabidopsis, such as flowering time, and could play a role in hybrid vigour establishment.

Inheritance of Trans Chromosomal Methylation patterns from Arabidopsis F1 hybrids.

Hybridization in plants leads to transinteractions between the parental genomes and epigenomes that can result in changes to both 24 nt siRNA and cytosine methylation ((m)C) levels in the hybrid. In Arabidopsis the principle processes altering the hybrid methylome are Trans Chromosomal Methylation (TCM) and Trans Chromosomal deMethylation (TCdM) in which the (m)C pattern of a genomic segment attains the same (m)C pattern of the corresponding segment on the other parental chromosome. We examined two loci that undergo TCM/TCdM in the Arabidopsis C24/Landsberg erecta (Ler) F1 hybrids, which show patterns of inheritance dependent on the properties of the particular donor and recipient chromosomal segments. At At1g64790 the TCM- and TCdM-derived (m)C patterns are maintained in the F2 generation but are transmitted in outcrosses or backcrosses only by the C24 genomic segment. At a region between and adjacent to At3g43340 and At3g43350, the originally unmethylated Ler genomic segment receives the C24 (m)C pattern in the F1, which is then maintained in backcross plants independent of the presence of the parental C24 segment. In backcrosses to an unmethylated Ler allele, the newly methylated F1 Ler segment may act as a TCM source in a process comparable to paramutation in maize. TCM-derived (m)C patterns are associated with reduced expression of both At3g43340 and At3g43350 in F1 and F2 plants, providing support for such events influencing the transcriptome. The inheritance of the F1 (m)C patterns and the segregation of other genetic and epigenetic determinants may contribute to the reduced hybrid vigor in the F2 and subsequent generations.

The role of epigenetics in hybrid vigour.

Hybrid vigour, or heterosis, refers to the increased yield and biomass of hybrid offspring relative to the parents. Although this has been exploited in plants for agriculture and horticulture, the molecular and cellular mechanisms underlying hybrid vigour are largely unknown. Genetic analyses show that there are a large number of quantitative trait loci (QTLs) that contribute to the heterotic phenotype, indicating that it is a complex phenomenon. Gene expression in hybrids is regulated by the interactions of the two parental epigenetic systems and the underlying genomes. Increasing understanding of the interplay of small RNA (sRNA) molecules, DNA methylation, and histone marks provides new opportunities to define the basis of hybrid vigour and to understand why F1 heterosis is not passed on to subsequent generations. We discuss recent findings that suggest the existence of several pathways that alter DNA methylation patterns, which may lead to transcriptional changes resulting in the heterotic phenotype.

Molecular and cellular characteristics of hybrid vigour in a commercial hybrid of Chinese cabbage.

BACKGROUND: Heterosis or hybrid vigour is a phenomenon in which hybrid progeny exhibit superior performance compared to their parental inbred lines. Most commercial Chinese cabbage cultivars are F1 hybrids and their level of hybrid vigour is of critical importance and is a key selection criterion in the breeding system. RESULTS: We have characterized the heterotic phenotype of one F1 hybrid cultivar of Chinese cabbage and its parental lines from early- to late-developmental stages of the plants. Hybrid cotyledons are larger than those of the parents at 4 days after sowing and biomass in the hybrid, determined by the fresh weight of leaves, is greater than that of the larger parent line by approximately 20% at 14 days after sowing. The final yield of the hybrid harvested at 63 days after sowing is 25% greater than the yield of the better parent. The larger leaves of the hybrid are a consequence of increased cell size and number of the photosynthetic palisade mesophyll cells and other leaf cells. The accumulation of plant hormones in the F1 was within the range of the parental levels at both 2 and 10 days after sowing. Two days after sowing, the expression levels of chloroplast-targeted genes in the cotyledon cells were upregulated in the F1 hybrid relative to their mid parent values. Shutdown of chlorophyll biosynthesis in the cotyledon by norflurazon prevented the increased leaf area in the F1 hybrid. CONCLUSIONS: In the cotyledons of F1 hybrids, chloroplast-targeted genes were upregulated at 2 days after sowing. The increased activity levels of this group of genes suggested that their differential transcription levels could be important for establishing early heterosis but the increased transcription levels were transient. Inhibition of the photosynthetic process in the cotyledon reduced heterosis in later seedling stages. These observations suggest early developmental events in the germinating seedling of the hybrid may be important for later developmental vigour and yield advantage.

Epigenetic Changes in Hybrids.

Genome-wide approaches to the study of hybrid vigor have identified epigenetic changes in the hybrid nucleus in Arabidopsis (Arabidopsis thaliana), maize (Zea mays), and rice (Oryza sativa). DNA methylation associated with 24-nucleotide small interfering RNAs exhibits transallelic effects in hybrids of Arabidopsis and other species. Some of the transmethylation changes are inherited and some affect gene expression. Hybrids have larger leaves than those of the parents and have increases in cell size and number. The increased leaf size results in a greater photosynthetic capacity, which may support the increased vegetative and reproductive yields of the F1 hybrids. Genes and metabolic pathways that have altered expression relative to the parents include loci involved in responses to hormones and to biotic and abiotic stress. Whereas epigenetically induced changes in gene expression may contribute to hybrid vigor, the link between the transcriptional changes and the hybrid phenotype is not confirmed. Recurrent selection of high yielding F1 lines from the F2/F3 of a number of crops has fixed heterosis yields in pure breeding lines. These hybrid-like lines may have valuable applications in crop systems.

Intraspecific Arabidopsis hybrids show different patterns of heterosis despite the close relatedness of the parental genomes.

Heterosis is important for agriculture; however, little is known about the mechanisms driving hybrid vigor. Ultimately, heterosis depends on the interactions of specific alleles and epialleles provided by the parents, which is why hybrids can exhibit different levels of heterosis, even within the same species. We characterize the development of several intraspecific Arabidopsis (Arabidopsis thaliana) F1 hybrids that show different levels of heterosis at maturity. We identify several phases of heterosis beginning during embryogenesis and culminating in a final phase of vegetative maturity and seed production. During each phase, the hybrids show different levels and patterns of growth, despite the close relatedness of the parents. For instance, during the vegetative phases, the hybrids develop larger leaves than the parents to varied extents, and they do so by exploiting increases in cell size and cell numbers in different ratios. Consistent with this finding, we observed changes in the expression of genes known to regulate leaf size in developing rosettes of the hybrids, with the patterns of altered expression differing between combinations. The data show that heterosis is dependent on changes in development throughout the growth cycle of the hybrid, with the traits of mature vegetative biomass and reproductive yield as cumulative outcomes of heterosis at different levels, tissues, and times of development.

Heterosis of Arabidopsis hybrids between C24 and Col is associated with increased photosynthesis capacity.

Arabidopsis thaliana shows hybrid vigor (heterosis) in progeny of crosses between Columbia-0 and C24 accessions. Hybrid vigor was evident as early as mature seeds and in seedlings 3 d after sowing (DAS). At 3 DAS, genes encoding chloroplast-located proteins were significantly overrepresented (187) among the 724 genes that have greater than midparent values of expression in the hybrid. Many of these genes are involved in chlorophyll biosynthesis and photosynthesis. The rate of photosynthesis was constant per unit leaf area in parents and hybrids. Larger cell sizes in the hybrids were associated with more chloroplasts per cell, more total chlorophyll, and more photosynthesis. The increased transcription of the chloroplast-targeted genes was restricted to the 3-7 DAS period. At 10 DAS, only 118 genes had expression levels different from the expected midparent value in the hybrid, and only 12 of these genes were differentially expressed at 3 DAS. The early increase in activity of genes involved in photosynthesis and the associated phenomena of increases in cell size and number through development, leading to larger leaf areas of all leaves in the hybrid, suggest a central role for increased photosynthesis in the production of the heterotic biomass. In support of this correlation, we found that an inhibitor of photosynthesis eliminated heterosis and that higher light intensities enhanced both photosynthesis and heterosis. In hybrids with low-level heterosis (Landsberg erecta x Columbia-0), chloroplast-targeted genes were not up-regulated and leaf areas were only marginally increased.