Heterosis or hybrid vigour is a phenomenon in which hybrid progeny exhibit superior yield and biomass to parental lines and has been used to breed F1 hybrid cultivars in many crops. A similar level of heterosis in all F1 individuals is expected as they are genetically identical. However, we found variation of rosette size in individual F1 plants from a cross between C24 and Columbia-0 accessions of Arabidopsis thaliana. Big sized F1 plants had 26.1% larger leaf area in the 1st and 2nd leaves than medium sized F1 plants at 14 days after sowing in spite of the identical genetic background. We identified differentially expressed genes between big and medium sized F1 plants by microarray; genes involved in the category of stress response were overrepresented. We made transgenic plants overexpressing 21 genes, which were differentially expressed between the two size classes, some lines had increased plant size at 14 or 21 days after sowing but not at all time points. Change of expression levels in stress responsive genes among individual F1 plants, implying epigenetic changes, could generate the variation in plant size of individual F1 plants in A. thaliana.
