RESUMO
In monocots and eudicots, B class function specifies second and third whorl floral organ identity as described in the classic ABCE model. Grass B class APETALA3/DEFICIENS orthologs have been functionally characterized; here, we describe the positional cloning and characterization of a maize (Zea mays) PISTILLATA/GLOBOSA ortholog Zea mays mads16 (Zmm16)/sterile tassel silky ear1 (sts1). We show that, similar to many eudicots, all the maize B class proteins bind DNA as obligate heterodimers and positively regulate their own expression. However, sts1 mutants have novel phenotypes that provide insight into two derived aspects of maize flower development: carpel abortion and floral asymmetry. Specifically, we show that carpel abortion acts downstream of organ identity and requires the growth-promoting factor grassy tillers1 and that the maize B class genes are expressed asymmetrically, likely in response to zygomorphy of grass floral primordia. Further investigation reveals that floral phyllotactic patterning is also zygomorphic, suggesting significant mechanistic differences with the well-characterized models of floral polarity. These unexpected results show that despite extensive study of B class gene functions in diverse flowering plants, novel insights can be gained from careful investigation of homeotic mutants outside the core eudicot model species.
Assuntos
Flores/crescimento & desenvolvimento , Flores/metabolismo , Proteínas de Plantas/metabolismo , Zea mays/crescimento & desenvolvimento , Zea mays/metabolismo , Clonagem Molecular , DNA de Plantas/metabolismo , Flores/ultraestrutura , Regulação da Expressão Gênica de Plantas , Técnicas de Silenciamento de Genes , Genes de Plantas , Mutação/genética , Fenótipo , Folhas de Planta/fisiologia , Proteínas de Plantas/genética , Ligação Proteica , Multimerização Proteica , Transporte Proteico , Interferência de RNA , Homologia de Sequência de Aminoácidos , Zea mays/genética , Zea mays/ultraestruturaRESUMO
Suppression of inflorescence leaf, or bract, growth has evolved multiple times in diverse angiosperm lineages, including the Poaceae and Brassicaceae. Studies of Arabidopsis thaliana mutants have revealed several genes involved in bract suppression, but it is not known if these genes play a similar role in other plants with suppressed bracts. We identified maize (Zea mays) tassel sheath (tsh) mutants, characterized by the loss of bract suppression, that comprise five loci (tsh1-tsh5). We used map-based cloning to identify Tsh1 and found that it encodes a GATA zinc-finger protein, a close homolog of HANABA TARANU (HAN) of Arabidopsis. The bract suppression function of Tsh1 is conserved throughout the grass family, as we demonstrate that the rice (Oryza sativa) NECK LEAF1 (NL1) and barley (Hordeum vulgare) THIRD OUTER GLUME (TRD) genes are orthologous with Tsh1. Interestingly, NL1/Tsh1/TRD expression and function are not conserved with HAN. The existence of paralogous NL1/Tsh1/TRD-like genes in the grasses indicates that the NL1/Tsh1/TRD lineage was created by recent duplications that may have facilitated its neofunctionalization. A comparison with the Arabidopsis genes regulating bract suppression further supports the hypothesis that the convergent evolution of bract suppression in the Poaceae involved recruitment of a distinct genetic pathway.
Assuntos
Evolução Molecular , Folhas de Planta/crescimento & desenvolvimento , Proteínas de Plantas/genética , Zea mays/genética , Sequência de Aminoácidos , Arabidopsis/genética , Clonagem Molecular , Regulação da Expressão Gênica de Plantas , Genes de Plantas , Hordeum/genética , Modelos Genéticos , Dados de Sequência Molecular , Oryza/genética , Filogenia , Folhas de Planta/genética , Proteínas de Plantas/metabolismo , Alinhamento de Sequência , Zea mays/crescimento & desenvolvimentoRESUMO
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.