Fleshy fruit development: characterization of apple MADS8

Abstract

Fruits are an important part of the human diet as they provide the vitamins, minerals and phytochemicals that are essential for human health. Commercialization of the fruit has driven new frontiers for crop development. The study of model plant systems such as Arabidopsis and tomato has uncovered highly conserved complex regulatory networks involving MADS-box transcription factors for controlling critical aspects of flower and fruit development. In apple, the effects of suppressing the apple SEP1/2-like gene, MADS8, have shown that the function of MADS8 is likely involved in fruit development. Fruits of MADS8-suppressed apple lines had reduced fruit size due to a loss of flesh tissues and severely inhibited ripening. In this research, five independent MADS8-overexpressing (MADS8ox) apple lines were analyzed. It was found that changes in tree architecture when compared to wild-type were strongly correlated with MADS8-overexpression. Early in tree development, the overexpression of MADS8 was found to result in reprioritization of flowering over vegetative growth, thereby changing the overall architecture of the tree with detrimental effects observed in the following year. At the fruit level, MADS8 overexpression was found to increase fruit flesh development and accelerate ripening particularly as seen from an increased rate of degreening in MADS8ox fruits. The analysis of possible protein interactions with MADS8 showed the ability of MADS8 to homodimerize which suggests that this may be the stable or activating form of the MADS8 protein. Transformation of Arabidopsis thaliana for novel expression of MADS8 in floral tissues did not produce any significant change in floral phenotypes. Overall, the results from this research have provided clear support for MADS8 as a key genetic determinant in a both tree and fruit development in apple. Elucidation of the functions of MADS8 provides new opportunities for the development of apple lines with improved yield and fruit quality. The results from this research also adds to the growing body of work that shows the diversification of MADS-box protein functions in fruit development arising from a conserved genetic framework at floral development.