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 Microscopic observations revealed that the innermost-whorl carpels had been replaced by stamen-like organs, which resembled the flowers of the previously described Arabidopsis thaliana (L.) Heynh.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