Box 2 | Regulation of flower size

Flower size can vary greatly between closely related species. Although the size of an organ reflects the number and size of the cells contained in it, altering cell number or cell size through genetic manipulation is often not sufficient to change the final size of an organ. Compensatory mechanisms seem to function within developing organs to maintain overall organ size¹⁰⁷, but some genes work outside such mechanisms. ARGOS (an auxin-regulated gene that is involved in organ size) and AINTEGUMENTA (ANT) in Arabidopsis thaliana, might function in a single pathway that is downstream of the plant hormone auxin¹⁰⁸. Mutations in either gene result in smaller floral organs (compare a wild-type flower in panel a with an ant mutant in panel b), whereas constitutive expression of either gene results in larger floral organs (panel c shows a 35S::ANT flower)^{108, 109, 110, 111, 112}. ARGOS and ANT are thought to promote growth within developing organs by maintaining cells in a division-competent state¹¹². Members of the YABBY gene family, including FILAMENTOUS FLOWER (FIL) and YABBY 3 (YAB3) in A. thaliana and GRAMINIFOLIA (GRAM) in Antirrhinum majus, promote abaxial identity and lateral growth of leaves and floral organs. The juxtaposition of cells with adaxial and abaxial identities has been proposed to promote lamina expansion¹¹³. Mutations in GRAM result in smaller leaves and petals¹¹⁴, whereas fil yab3 double mutants produce small leaves and radialized floral organs¹¹⁵. Furthermore, expression of YAB3 under the control of an abaxial domain promoter results in the formation of large leaves and floral organs (Y. Eshed, personal communication). Another important regulator of growth is the A. majus CINCINNATA (CIN) gene, a member of the TCP domain transcription-factor family. Interestingly, although CIN promotes petal growth, it restricts growth within developing leaves¹¹⁶, indicating that different growth mechanisms might operate in leaves and floral organs. A recent QTL analysis of A. thaliana leaf and floral-organ size supports this hypothesis: of the eight QTLs identified that affect floral organs, only two also affect leaf formation¹¹⁷.