Large Flower Size: Molecular Basis and Role of Cytokinin

Large flower size is an important trait that influences the economic value of floricultural plants. Large flower size is conferred by two morphological changes i.e. increased petal number and individual petal size. Increased petal number is induced by enlargement of the floral meristem or homeotic conversion of stamens and carpels into petals. Genes regulating meristematic competence mediate enlargement of the floral meristem, while floral homeotic genes mediate the homeotic conversion of floral organs. Increased individual petal size is induced by increased cell size or cell number. Polyploidization of the nuclear genome increases petal cell size, while increased petal cell number is induced by enhanced expression of the genes involved in auxin signal transduction and cytokinin biosynthesis. In this review, the molecular mechanisms affecting flower size, elucidated mainly using model plants such as Arabidopsis, are summarized. Further, our research on the roles of cytokinin biosynthesis and signal transduction in increased flower size in petunia, induced by the single major gene Grandiflora, is discussed. Based on these results, a possible systematic breeding method for increasing flower size is discussed.