Validation of A integumenta as a gene to modify floral size in ornamental plants

Summary The gene AINTEGUMENTA ( At ANT ) is an APETALA 2 transcription factor in A rabidopsis activating growth downstream of auxin signalling. Lateral organ size is positively correlated with ANT expression in Arabidopsis. We tested the use of A t ANT as a tool to modify floral size in two different plants used as model organisms and ornamental crops, P etunia × hybrida and A ntirrhinum majus . Petunia plants expressing P h ANT RNA i showed a decrease in P h ANT expression correlated with smaller petal limbs. In contrast Petunia plants overexpressing A t ANT had larger petal limbs. Petal tube length was less affected in down‐regulation of P h ANT or overexpression of A t ANT . Overexpression of A t ANT in Antirrhinum caused increased flower size via increased petal limb width and tube length. Down‐regulation of P h ANT showed an effect on cell size while overexpression of A t ANT in P etunia and A ntirrhinum caused significant increases in cell expansion that could explain the differences in floral organ size. The endogenous expression levels of P h ANT and Am ANT tended to be higher in the limb than in the tube in both A ntirrhinum and P etunia. At ANT overexpression caused significant A m ANT up‐regulation in A ntirrhinum limbs but not of P h ANT in P etunia, indicating differences in the regulatory network. The differential effect of A t ANT on limb and tube in P etunia and A ntirrhinum correspond to phenotypic differences observed in natural variation in the corresponding genus indicating a relation between the phenotypic space of a genus and the effect of modified ANT levels, validating ANT as a gene to modify floral size.