Box 1 | Monocot flowers

The ABC model for flower development describes typical higher EUDICOT flowers that consist of sepals, petals, stamens (st) and carpels (ca). Although MONOCOT flowers contain stamens and carpels, they differ from eudicot flowers in the type of organs that are present in the outer whorls. Liliaceae family members often have two outer whorls of showy petal-like tepal (te) organs (panel a), whereas grass flowers have paleas (pa), lemmas (le) and lodicules (lo) in place of sepals and petals (panel b). A modified ABC model in which B function is present in whorls 1, 2 and 3 has been proposed to explain the presence of tepals in Liliaceae flowers (panel a)¹⁰¹. This is supported by the observation of class B AP3/DEF-like (APETALA 3 or DEFICIENS-like) and PI/GLO-like (PISTILLATA or GLOBOSA-like) gene expression in the outer three whorls of tulip flowers²⁵, and the absence or low expression of AP3/DEF-like genes in the outermost whorl of other monocots that produce distinct sepals and petals¹⁰².

Studies in maize and rice indicate that class B genes have similar roles in grass and eudicot flowers²⁹. Loss of the single AP3/DEF-like gene in maize (SILKY 1) and rice (SUPERWOMAN 1), results in replacement of lodicules by paleas or lemmas, or palea-like organs, respectively, and the replacement of stamens by carpels^{28, 103}. These homeotic transformations are similar to those observed in Arabidopsis thaliana and Antirrhinum majus class B mutants, indicating that paleas and lemmas are homologous to sepals, and lodicules are homologous to petals (panel b). Maize contains two potential class C AG/PLE-like (AGAMOUS or PLENA-like) genes (ZAG1 and ZMM2), but redundancy makes it difficult to determine their exact roles¹⁰⁴. Mutations in ZAG1 affect floral determinacy but not organ identity. Carpels are still produced in rice plants with reduced expression of the AG/PLE-like gene MADS3 (Ref. 105), indicating that other factors are required for carpel specification in grasses. The YABBY gene DROOPING LEAF (DL) is one such factor (panel b), as mutations in DL result in complete homeotic transformations of carpels into stamens^{28, 106}. Although class A AP1/SQUA-like (APETALA 1 or SQUAMOSA-like) genes have been identified in maize and rice, their roles in flower development are not well defined.