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Although many genes that regulate floral development have been identified in Arabidopsis thaliana, relatively few are known in the grasses. In normal maize (Zea mays), each spikelet produces an upper and lower floral meristem, which initiate floral organs in a defined phyllotaxy before being consumed in the production of an ovule. The bearded-ear (bde) mutation affects floral development differently in the upper and lower meristem. The upper floral meristem initiates extra floral organs that are often mosaic or fused, while the lower floral meristem initiates additional floral meristems. We cloned bde by positional cloning and found that it encodes zea agamous3 (zag3), a MADS box transcription factor in the conserved AGAMOUS-LIKE6 clade. Mutants in the maize homolog of AGAMOUS, zag1, have a subset of bde floral defects. bde zag1 double mutants have a severe ear phenotype, not observed in either single mutant, in which floral meristems are converted to branch-like meristems, indicating that bde and zag1 redundantly promote floral meristem identity. In addition, BDE and ZAG1 physically interact. We propose a model in which BDE functions in at least three distinct complexes to regulate floral development in the maize ear.
INTRODUCTION
Organogenesis in plants requires the ongoing activity of meristems. Meristems are groups of totipotent cells that give rise to lateral organs, stems, and roots, while maintaining a population of stem cells. Meristems can be indeterminate and give rise to an unlimited number of primordia, or they can be determinate and terminate in the production of primordia. Floral meristems (FMs) are determinate meristems: they produce a defined number of floral organs and terminate in the production of the ovule, which is contained in the carpel whorl. A typical eudicot flower contains four whorls of floral organs: sepals, petals, stamens, and carpels. Grass flowers contain stamens and carpels but also contain palea and lemma, organs unique to the grasses.
Floral development has been intensively studied in the model plant Arabidopsis thaliana. The molecular regulation of floral organ identity is described by the ABC model, which posits that floral organ identity is determined by the combinatorial action of the Class A, B, and C genes (Coen and Meyerowitz, 1991). Briefly, Class A genes alone specify whorl 1 organs (sepals), Class A and B genes together specify...