CaMADS1, an AGAMOUS homologue from hazelnut, produces floral homeotic conversion when expressed in Arabidopsis

CaMADS1 is a floral-specific MADS box gene of hazelnut (Corylus avellana) which, according to its sequence and expression pattern, belongs to the AGAMOUS gene sub-family. To investigate whether CaMADS1 plays a role in specifying stamen and carpel identity, this gene was ectopically expressed in Arabidopsis. The constitutive expression of CaMADS1 in transgenic plants produced the homeotic conversion of first and second whorl organs: the first whorl exhibited carpelloid sepals and the second whorl showed staminoid features. This was expected on the basis of the ABC model, according to which ectopic expression of a functional AGAMOUS (a gene of class C) orthologue would suppress the A class homeotic function in the first and second whorls, leading to transformation of these whorls into carpels and stamen, respectively. These results indicate a functional equivalency between AGAMOUS and CaMADS1, for which CaMADS1 might behave like a class C homeotic gene, controlling the determination of stamen and carpel identity in hazelnut Received: 31 July 2000 / Revision accepted: 28 September 2000     

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 Microscopic observations revealed that the innermost-whorl carpels had been replaced by stamen-like organs, which resembled the flowers of the previously described Arabidopsis thaliana (L.) Heynh.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.     

Morphological alterations by ectopic expression of the rice OsMADS4 gene in tobacco plants

OsMADS4, a rice MADS-box gene, is a member of the GLO/PI family that specifies the identity of petals and stamens in combination with other MADS-box genes. We report here the ectopic expression of OsMADS4 fused to the CaMV 35S promoter in tobacco plants. Transgenic plants carrying the CaMV 35S promoter::OsMADS4 construct generated mutant flowers with a mosaic carpel, in which the tissue around the nectary was elongated and the styles reduced. The fruits were distorted, but viable seeds did develop. These phenotypes mimicked those of transgenic tobacco plants that ectopically express Antirrhinum GLO. However, unlike GLO, OsMADS4 did not cause any homeotic change in the first whorl of the transgenic flowers. These results suggest that the functional role of OsMADS4 in the outer whorls has diverged from that of its dicot counterparts.     

Separable roles of UFO during floral development revealed by conditional restoration of gene function

The UNUSUAL FLORAL ORGANS (UFO) gene is required for several aspects of floral development in Arabidopsis including specification of organ identity in the second and third whorls and the proper pattern of primordium initiation in the inner three whorls. UFO is expressed in a dynamic pattern during the early phases of flower development. Here we dissect the role of UFO by ubiquitously expressing it in ufo loss-of-function flowers at different developmental stages and for various durations using an ethanol-inducible expression system. The previously known functions of UFO could be separated and related to its expression at specific stages of development. We show that a 24- to 48-hour period of UFO expression from floral stage 2, before any floral organs are visible, is sufficient to restore normal petal and stamen development. The earliest requirement for UFO is during stage 2, when the endogenous UFO gene is transiently expressed in the centre of the wild-type flower and is required to specify the initiation patterns of petal, stamen and carpel primordia. Petal and stamen identity is determined during stages 2 or 3, when UFO is normally expressed in the presumptive second and third whorl. Although endogenous UFO expression is absent from the stamen whorl from stage 4 onwards, stamen identity can be restored by UFO activation up to stage 6. We also observed floral phenotypes not observed in loss-of-function or constitutive gain-of-function backgrounds, revealing additional roles of UFO in outgrowth of petal primordia.     

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.     

Male and female flowers of the dioecious plant sorrel show different patterns of MADS box gene expression.

Male and female flowers of the dioecious plant sorrel (Rumex acetosa) each produce three whorls of developed floral organs: two similar whorls of three perianth segments and either six stamens (in the male) or a gynoecium consisting of a fertile carpel and two sterile carpels (in the female). In the developing male flower, there is no significant proliferation of cells in the center of the flower, in the position normally occupied by the carpels of a hermaphrodite plant. In the female flower, small stamen primordia are formed. To determine whether the organ differences are associated with differences in the expression of organ identity genes, cDNA clones representing the putative homologs of B and C function MADS box genes were isolated and used in an in situ hybridization analysis. The expression of RAD1 and RAD2 (two different DEFICIENS homologs) in males and females was confined to the stamen whorl; the lack of expression in the second, inner perianth whorl correlated with the sepaloid nature of the inner whorl of perianth segments. Expression of RAP1 (a PLENA homolog) occurred in the carpel and stamen whorls in very young flower primordia from both males and females. However, as soon as the inappropriate set of organs ceased to develop, RAP1 expression became undetectable in those organs. The absence of expression of RAP1 may be the cause of the arrest in organ development or may be a consequence.     

重瓣紫蓝大岩桐组培苗的花同源异型现象（简报）

Six types of floral homeotic variants of in vitro seedlings were observed in doubleflower sinningia. Type I, red and green mosaic petals exist in the outermost whorl of petal-whorls, 2.38%. Type II, the outermost whorl of petal-whorls exhibit green petals with thin yellow edge, 25.0%. Type III, green petals exist in the innermost side of normal red petal whorls, 1.78%. Type IV, multiple whorls of green petals exist in the inner side of normal sepals, no stamen and carpel, 1.67%. Type V, it exhibits duplicated whorls of sepals in the outermost, 7.14%. Type VI, it exists multiple whorls of green sepals, no petal, stamen and carpel, 0.12%. The total percentage of all types of floral homeotic variants is up to 38.1%. The distribution of nodal site of homeotic flowers were analyzed, and the results showed that the homeotic flower occurred mainly at the fourth and fifth nodes.     

GRCD1, an AGL2-like MADS box gene, participates in the C function during stamen development in Gerbera hybrida

Despite the differences in flower form, the underlying mechanism in determining the identity of floral organs is largely conserved among different angiosperms, but the details of how the functions of A, B, and C are specified varies greatly among plant species. Here, we report functional analysis of a Gerbera MADS box gene, GRCD1, which is orthologous to AGL2-like MADS box genes. Members of this group of genes are being reported in various species in growing numbers, but their functions remained largely unsettled. GRCD1 expression is detected in all four whorls, but the strongest signal is seen in the developing stamen and carpel. Downregulating GRCD1 expression by antisense transformation revealed that lack of GRCD1 caused homeotic changes in one whorl only: sterile staminodes, which normally develop in whorl 3 of marginal female florets, were changed into petals. This indicates that the GRCD1 gene product is active in determining stamen identity. Transgenic downregulation of GRCD1 causes a homeotic change similar to that in the downregulation of the Gerbera C function genes GAGA1 and GAGA2, but one that is limited to whorl 3. Downregulation of GRCD1 expression does not reduce expression of GAGA1 or GAGA2, or vice versa; and in yeast two-hybrid analysis, GRCD1 is able to interact with GAGA1 and GAGA2. We propose that a heterodimer between the GRCD1 and GAGA1/2 gene products is needed to fulfill the C function in whorl 3 in Gerbera.