Summary It is becoming increasingly vital to improve the yield of seed crops to feed an expanding population and, more recently, for biofuel production. One strategy to increase the yield is to increase the seed size, provided that there is not a concomitant decrease in seed number. In a previous study, we described a mutant in the AUXIN RESPONSE FACTOR 2 (ARF2) gene which produced extra cells in the seed coat and, subsequently, enlarged seeds. However, arf2 mutant plants also show severely reduced self‐fertility caused, in part, by over‐elongated sepals that prevent flower opening. As a low seed set increases individual seed size, a meaningful comparison of the yield in arf2 and wild‐type plants could not be conducted. In this study, we show that targeted expression of wild‐type ARF2 in the sepals and petals of arf2‐9 mutant flowers restores flower opening and dramatically increases seed set. The restored plants retain both enlarged integuments and increased seed size, reinforcing previous evidence that arf2 mutations increase seed weight through their effect on integuments and not only via reduced fertility. We also show that the measurement of the harvest index in Arabidopsis is useful in assessing the impact of introduced traits on the yield.

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