Developmental innovation of inferior ovaries and flower sex orchestrated by KNOX1 in cucurbits.

In flowering plants, inferior ovaries are key morphological innovations that evolved multiple times from superior ovaries to protect female parts of the flower. However, the developmental mechanisms underlying inferior ovary formation remain largely unknown. Comparative spatial transcriptome mapping and cell lineage reconstructions in developing floral buds of cucumber and tomato, which have inferior and superior ovaries, respectively, revealed that inferior ovaries develop from accelerated receptacle growth resulting from the continuous activity of meristematic stems cells at the base of the cucumber floral organs. Genetic knockout of a receptacle-specific KNOX1 transcription factor in cucumber caused arrest in receptacle growth and yielded bisexual flowers with superior ovaries similar to those of tomato. Here we provide developmental and mechanistic insights into inferior ovary formation and sex determination in cucurbits.