Overexpression of the apple SEP1/2-like gene MdMADS8 promotes floral determinacy and enhances fruit flesh tissue and ripening

The over-expression of the apple SEP1/2 like gene (MADS8) supports its role as a key regulator of fleshy fruit patterning and ripening in pome fruit. Fruit flesh patterning in accessory fruits such as apple and strawberry has been associated with the SEPALLATA (SEP) class of genes. Antisense suppression of multiple SEP homologs greatly reduced fruit fleshy tissue growth and ripening in strawberries and apple, suggesting a co-evolution of fleshy fruit growth and its ability to ripen. In the present study, transgenic apple trees were generated that overexpressed the apple SEP1/2-like gene MADS8 (MADS8ox). These lines produced precocious flowers in the second year of growth, but subsequent flower numbers were not increased. Overexpression MdMADS8 in Arabidopsis promoted flowering and caused curled leaves and a conversion of sepals to petals. In apple, MADS8ox fruit were larger than wild type 'Royal Gala' fruit with a higher proportion of flesh tissue compared to core tissue. MADS8ox fruit ripened earlier and faster with higher expression of ripening-related genes such as ACS1 and ACO1. Together, these results add further insights into the role that SEP-like genes have in controlling flowering and fruit patterning and provide further evidence that MdMADS8 is one of the key regulators of fruit flesh development and ripening in apples.