| Summary: | 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 F<sub>1</sub> hybrid cultivars in many crops. A similar level of heterosis in all F<sub>1</sub> individuals is expected as they are genetically identical. However, we found variation in rosette size in individual F<sub>1</sub> plants from a cross between C24 and Columbia-0 accessions of <i>Arabidopsis thaliana</i>. Big-sized F<sub>1</sub> plants had 26.1% larger leaf area in the first and second leaves than medium-sized F<sub>1</sub> plants at 14 days after sowing in spite of the identical genetic background. We identified differentially expressed genes between big- and medium-sized F<sub>1</sub> 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, and some lines had increased plant size at 14 or 21 days after sowing but not at all time points during development. Change of expression levels in stress-responsive genes among individual F<sub>1</sub> plants could generate the variation in plant size of individual F<sub>1</sub> plants in <i>A. thaliana</i>.
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