SUPERWOMAN1 and DROOPING LEAF genes control floral organ identity in rice

We analyzed recessive mutants of two homeotic genes in rice, SUPERWOMAN1 (SPW1) and DROOPING LEAF (DL). The homeotic mutation spw1 transforms stamens and lodicules into carpels and palea-like organs, respectively. Two spw1 alleles, spw1-1 and spw1-2, show the same floral phenotype and did not affect vegetative development. We show that SPW1 is a rice APETALA3 homolog, OsMADS16. In contrast, two strong alleles of the dl locus, drooping leaf-superman1 (dl-sup1) and drooping leaf-superman2 (dl-sup2), cause the complete transformation of the gynoecium into stamens. In these strong mutants, many ectopic stamens are formed in the region where the gynoecium is produced in the wild-type flower and they are arranged in a non-whorled, alternate pattern. The intermediate allele dl-1 (T65), results in an increase in the number of stamens and stigmas, and carpels occasionally show staminoid characteristics. In the weakest mutant, dl-2, most of the flowers are normal. All four dl alleles cause midrib-less drooping leaves. The flower of the double mutant, spw1 dl-sup, produces incompletely differentiated organs indefinitely after palea-like organs are produced in the position where lodicules are formed in the wild-type flower. These incompletely differentiated organs are neither stamens nor carpels, but have partial floral identity. Based on genetic and molecular results, we postulate a model of stamen and carpel specification in rice, with DL as a novel gene controlling carpel identity and acting mutually and antagonistically to the class B gene, SPW1.     

Ectopic expression of OsMADS3, a rice ortholog of AGAMOUS, caused a homeotic transformation of lodicules to stamens in transgenic rice plants

In order to clarify the evolutionary relationship of floral organs between grasses and dicots, we expressed OsMADS3, a rice (Oryza sativa L.) AGAMOUS(AG) ortholog, in rice plants under the control of an Actin1 promoter. As a consequence of the ectopic expression of the OsMADS3, lodicules were homeotically transformed into stamens. In total, the transformation of lodicules to staminoid organs was observed in 18 out of 26 independent transgenic lines. In contrast to the almost complete transformation occurring in lodicules, none of the transgenic plants exhibited any morphological alterations in the palea or the lemma. Our results confirmed the prediction that the lodicule is an equivalent of a dicot petal and that the ABC model can be applied to rice at least for organ specification in lodicules and stamens.     

Unequal genetic redundancy of rice PISTILLATA orthologs, OsMADS2 and OsMADS4, in lodicule and stamen development

Two homologs of PISTILLATA have been identified in rice: OsMADS2 and OsMADS4. However, their roles in floral organ development are controversial. Here, we demonstrate that the genes show unequal redundancy of class B function. Although OsMADS2 plays an important role in lodicule development, OsMADS4 also supports the specification of lodicule identity. In contrast, the genes are roughly equally important in stamen development. Consistent with their redundant functions, both OsMADS2 and OsMADS4 interact with the unique rice AP3 ortholog SPW1.     

水稻花器官突变体apl (abnormal palea and lodicules) 的表型分析与基因初定位

水稻(Oryza sativa)是重要的粮食作物, 其花器官的正常起始及形态建成直接影响水稻的产量。为了深入分析水稻小花发育的调控机理, 从已构建的水稻EMS诱变突变体库中筛选获得了一个花器官异常发育的突变体apl (abnormal palea and lodicules)。与野生型相比, apl突变体小花的内稃膨大, 浆片伸长或转换成稃状结构, 雄蕊数目减少, 表明APL基因可能参与调控水稻内稃、浆片和雄蕊等多轮花器官属性的建成。遗传学分析表明, 该突变体性状受1个隐性单基因控制。通过图位克隆, 将APL基因初步定位于1号染色体上。该工作为深入研究APL基因在水稻花器官形态建成中的作用机制奠定了基础。     

一个水稻畸形颖壳突变体ah的鉴定及其突变性状的遗传分析

水稻畸形颖壳突变体ah是双胚苗品系W2555中自然突变产生的。该突变体的内外稃畸形,退化;雄蕊雌蕊化,雌蕊败育;浆片同源转化为类内外稃的结构,推测该突变体可能影响B功能基因的正常发育。与野生型相比,突变体的小穗分支稀疏,每级枝梗上颖花数目减少,一般为4～6朵;小穗顶端的颖花经常不能成熟,表现为颖花始终泛白,不能转绿,因此该突变也影响花序分生组织的发育。进一步的研究证明,该突变体的发育受外界环境的影响。突变性状的遗传分析表明,该突变体由单隐性基因控制。     

Alteration of floral organ identity in rice through ectopic expression of<Emphasis Type="Italic"> OsMADS16</Emphasis>

We used a transgenic approach and yeast two-hybrid experiments to study the role of the rice ( Oryza sativa L.) B-function MADS-box gene, OsMADS16. Transgenic rice plants were generated that ectopically expressed OsMADS16 under the control of the maize ( Zea mays L.) ubiquitin1 promoter. Microscopic observations revealed that the innermost-whorl carpels had been replaced by stamen-like organs, which resembled the flowers of the previously described Arabidopsis thaliana (L.) Heynh. mutation superman as well as those ectopically expressing the AP3 gene. These results indicate that expression of OsMADS16 in the innermost whorl induces stamen development. Occasionally, carpels had completely disappeared. In addition, ectopic expression of OsMADS16 enhanced expression of OsMADS4, another B-function gene, causing superman phenotypes. In the yeast two-hybrid system, OsMADS16 did not form a homodimer but, rather, the protein interacted with OsMADS4. OsMADS16 also interacted with OsMADS6 and OSMADS8, both of which are homologous to SEPALLATA proteins required for the proper function of class-B and class-C genes in Arabidopsis. Based on the gene expression pattern and our yeast two-hybrid data, we discuss a quartet model of MADS-domain protein interactions in the lodicule and stamen whorls of rice florets.     

Identification of class B and class C floral organ identity genes from rice plants

The functions of two rice MADS-box genes were studied by the loss-of-function approach. The first gene, OsMADS4, shows a significant homology to members in the PISTILLATA (PI) family, which is required to specify petal and stamen identity. The second gene, OsMADS3, is highly homologous to the members in the AGAMOUS (AG) family that is essential for the normal development of the internal two whorls, the stamen and carpel, of the flower. These two rice MADS box cDNA clones were connected to the maize ubiquitin promoter in an antisense orientation and the fusion molecules were introduced to rice plants by the Agrobacterium-mediated transformation method. Transgenic plants expressing antisense OsMADS4 displayed alterations of the second and third whorls. The second-whorl lodicules, which are equivalent to the petals of dicot plants in grasses, were altered into palea/lemma-like organs, and the third whorl stamens were changed to carpel-like organs. Loss-of-function analysis of OsMADS3 showed alterations in the third and fourth whorls. In the third whorl, the filaments of the transgenic plants were changed into thick and fleshy bodies, similar to lodicules. Rather than making a carpel, the fourth whorl produced several abnormal flowers. These phenotypes are similar to those of the agamous and plena mutants in Arabidopsis and Antirrhinum, respectively. These results suggest that OsMADS4 belongs to the class B gene family and OsMADS3 belongs to the class C gene family of floral organ identity determination.     

Identification and Gene Mapping of a Novel Mutant supernumerary lodicules(snl) in Rice

In order to gain a better understanding of rice flower development, a rice flower mutant supernumerary lodicules (snl), which was identified from ethyl methane sulfonate (EMS)-treated Jinhui10 (Oryza sativa L. ssp. indica) was used in the present study. In the snl mutant, the palea obtained lemma identity, additional glume-like organs formed, lodicules increased and elongated, stamens decreased, and a few aberrant carpels formed. These phenotypes suggest that SNL is involved in the entire rice flower develo...     

Functional diversification of the two C-class MADS box genes OSMADS3 and OSMADS58 in Oryza sativa

The C-class MADS box gene AGAMOUS (AG) plays crucial roles in Arabidopsis thaliana development by regulating the organ identity of stamens and carpels, the repression of A-class genes, and floral meristem determinacy. To examine the conservation and diversification of C-class gene function in monocots, we analyzed two C-class genes in rice (Oryza sativa), OSMADS3 and OSMADS58, which may have arisen by gene duplication before divergence of rice and maize (Zea mays). A knockout line of OSMADS3, in which the gene is disrupted by T-DNA insertion, shows homeotic transformation of stamens into lodicules and ectopic development of lodicules in the second whorl near the palea where lodicules do not form in the wild type but carpels develop almost normally. By contrast, RNA-silenced lines of OSMADS58 develop astonishing flowers that reiterate a set of floral organs, including lodicules, stamens, and carpel-like organs, suggesting that determinacy of the floral meristem is severely affected. These results suggest that the two C-class genes have been partially subfunctionalized during rice evolution (i.e., the functions regulated by AG have been partially partitioned into two paralogous genes, OSMADS3 and OSMADS58, which were produced by a recent gene duplication event in plant evolution).     

Identification of a novel mutant spp1 that specifies the identity of inflorescence meristem in rice

Abstract

Homeotic mutations can affect the genetic architecture of flower structure. Some genes have been identified that specify shoot and floral meristem development. ABCDE and floral quartet are two widely accepted models that explain how genes interact to form all the whorls of a flower. In the current study, we identified an spp1 (SUPER PISTIL 1) mutant controlled by a single recessive nuclear gene mapping to chromosome 6 near RM50. Compared to wild type, spp1 mutants showed similar agronomic traits, except for panicle length and 1000-grain weight, which were significantly lower in the spp1 mutant plants. The mutation in the SPP1 gene induced complete homeotic transformations of paleae, lodicules, and stamens into carpelloid structures. Although lemmata were only marginally affected in the spp1 mutants, they developed a stigma-like structure on the top instead of an awn. Interestingly, stigma-like structures were also observed at the base of panicle branches. From the results, we propose that the SPP1 gene plays an important role in specifying the identity of lemmata/paleae, lodicules, stamens, and inflorescence meristem in rice. Identification of this mutant not only provides further evidence for validity of the ABCDE model in monocots, but also contributes to the understanding of meristem development.