Diverse roles for MADS box genes in Arabidopsis development.

Members of the MADS box gene family play important roles in flower development from the early step of determining the identity of floral meristems to specifying the identity of floral organ primordia later in flower development. We describe here the isolation and characterization of six additional members of this family, increasing the number of reported Arabidopsis MADS box genes to 17. All 11 members reported prior to this study are expressed in flowers, and the majority of them are floral specific. RNA expression analyses of the six genes reported here indicate that two genes, AGL11 and AGL13 (AGL for AGAMOUS-like), are preferentially expressed in ovules, but each has a distinct expression pattern. AGL15 is preferentially expressed in embryos, with its onset at or before the octant stage early in embryo development. AGL12, AGL14, and AGL17 are all preferentially expressed in root tissues and therefore represent the only characterized MADS box genes expressed in roots. Phylogenetic analyses showed that the two genes expressed in ovules are closely related to previously isolated MADS box genes, whereas the four genes showing nonfloral expression are more distantly related. Data from this and previous studies indicate that in addition to their proven role in flower development, MADS box genes are likely to play roles in many other aspects of plant development.     

MOSAIC FLORAL ORGANS1, an AGL6-Like MADS Box Gene,Regulates Floral Organ Identity and Meristem Fate in Rice

Floral organ identity and meristem determinacy in plants are controlled by combinations of activities mediated by MADS box genes. AGAMOUS-LIKE6 (AGL6)-like genes are MADS box genes expressed in floral tissues, but their biological functions are mostly unknown. Here, we describe an AGL6-like gene in rice (Oryza sativa), MOSAIC FLORAL ORGANS1 (MFO1/MADS6), that regulates floral organ identity and floral meristem determinacy. In the flower of mfo1 mutants, the identities of palea and lodicule are disturbed, and mosaic organs were observed. Furthermore, the determinacy of the floral meristem was lost, and extra carpels or spikelets developed in mfo1 florets. The expression patterns of floral MADS box genes were disturbed in the mutant florets. Suppression of another rice AGL6-like gene, MADS17, caused no morphological abnormalities in the wild-type background, but it enhanced the phenotype in the mfo1 background, indicating that MADS17 has a minor but redundant function with that of MFO1. Whereas single mutants in either MFO1 or the SEPALLATA-like gene LHS1 showed moderate phenotypes, the mfo1 lhs1 double mutant showed a severe phenotype, including the loss of spikelet meristem determinacy. We propose that rice AGL6-like genes help to control floral organ identity and the establishment and determinacy of the floral meristem redundantly with LHS1.     

MADS box genes expressed in developing inflorescences of rice and sorghum

With the aim of elucidating the complex genetic system controlling flower morphogenesis in cereals, we have characterized two rice and two sorghum MADS box genes isolated from cDNA libraries made from developing inflorescences. The rice clones OsMADS24 and OsMADS45, which share high homology with the Arabidopsis AGL2 and AGL4 MADS box genes, are expressed in the floral meristem, in all the primordia, and in mature floral organs. High expression levels have also been found in developing kernels. The sorghum clone SbMADS1 is also homologous to AGL2 and AGL4: expression analysis and mapping data suggest that it is the ortholog of OsMADS24. The pattern of expression of SbMADS2, the other sorghum MADS box gene, suggests that it may play a role as a meristem identity gene, as does AP1 in Arabidopsis, to which it shows considerable homology. The four genes have been mapped on a rice RFLP genetic map: the results are discussed in terms of synteny among cereals. Received: 25 April 1996 / Accepted: 29 August 1996     

AGL24, SHORT VEGETATIVE PHASE, and APETALA1 redundantly control AGAMOUS during early stages of flower development in Arabidopsis

Loss-of-function alleles of AGAMOUS-LIKE24 (AGL24) and SHORT VEGETATIVE PHASE (SVP) revealed that these two similar MADS box genes have opposite functions in controlling the floral transition in Arabidopsis thaliana, with AGL24 functioning as a promoter and SVP as a repressor. AGL24 promotes inflorescence identity, and its expression is downregulated by APETALA1 (AP1) and LEAFY to establish floral meristem identity. Here, we combine the two mutants to generate the agl24 svp double mutant. Analysis of flowering time revealed that svp is epistatic to agl24. Furthermore, when grown at 30 degrees C, the double mutant was severely affected in flower development. All four floral whorls showed homeotic conversions due to ectopic expression of class B and C organ identity genes. The observed phenotypes remarkably resembled the leunig (lug) and seuss (seu) mutants. Protein interaction studies showed that dimers composed of AP1-AGL24 and AP1-SVP interact with the LUG-SEU corepressor complex. We provide genetic evidence for the role of AP1 in these interactions by showing that the floral phenotype in the ap1 agl24 svp triple mutant is significantly enhanced. Our data suggest that MADS box proteins are involved in the recruitment of the SEU-LUG repressor complex for the regulation of AGAMOUS.     

Multiple interactions amongst floral homeotic MADS box proteins.

Most known floral homeotic genes belong to the MADS box family and their products act in combination to specify floral organ identity by an unknown mechanism. We have used a yeast two-hybrid system to investigate the network of interactions between the Antirrhinum organ identity gene products. Selective heterodimerization is observed between MADS box factors. Exclusive interactions are detected between two factors, DEFICIENS (DEF) and GLOBOSA (GLO), previously known to heterodimerize and control development of petals and stamens. In contrast, a third factor, PLENA (PLE), which is required for reproductive organ development, can interact with the products of MADS box genes expressed at early, intermediate and late stages. We also demonstrate that heterodimerization of DEF and GLO requires the K box, a domain not found in non-plant MADS box factors, indicating that the plant MADS box factors may have different criteria for interaction. The association of PLENA and the temporally intermediate MADS box factors suggests that part of their function in mediating between the meristem and organ identity genes is accomplished through direct interaction. These data reveal an unexpectedly complex network of interactions between the factors controlling flower development and have implications for the determination of organ identity.     

Evolution and function of MADS‐box genes involved in orchid floral development

Orchids are known for their beauty and complexity of flower and ecological strategies. The evolution in orchid floral morphology, structure, and physiological properties has held the fascination of botanists for centuries, from Darwin through to the present. In floral studies, MADS‐box genes contributing to the now famous ABCDE model of floral organ identity control have dominated conceptual thinking. The sophisticated orchid floral organization offers an opportunity to discover new variant genes and different levels of complexity to the ABCDE model. Recently, several remarkable research reports on orchid MADS‐box genes, especially B‐class MADS‐box genes, have revealed the evolutionary track and important functions on orchid floral development. Diversification and fixation of both paleoAP3 gene sequences and expression profiles might be explained by subfunctionalization and even neofunctionalization. Knowledge about MADS‐box genes encoding ABCDE functions in orchids will give insights into the highly evolved floral morphogenetic networks of orchids.     

Plant development going MADS

It has been known for a decade that the plant MADS genesare important regulators of meristem and floral organ identity. The MADS family in Arabidopsis consists of more than 80 members and, until recently, the function of the majority of these genes was unknown. With the enhanced ability to generate loss-of-function mutants and over-expression lines, the function of the MADS gene family members is beginning to be elucidated. Recent progress demonstrates that MADS genes in Arabidopsis are important regulators not only of meristem and floral organ identity but also of flowering timing and cell-type specification in floral organs.     

Ectopic expression of an orchid (Oncidium Gower Ramsey) AGL6-like gene promotes flowering by activating flowering time genes in Arabidopsis thaliana

An AP1/AGL9 group of MADS box gene, OMADS1, with extensive homology to the Arabidopsis AGAMOUS-like 6 gene (AGL6) was characterized from orchid (Oncidium Gower Ramsey). OMADS1 mRNA was detected in apical meristem and in the lip and carpel of flower. Yeast two-hybrid analysis indicated that OMADS1 is able to strongly interact with OMADS3, a TM6-like protein that was involved in flower formation and floral initiation in orchid. Transgenic Arabidopsis and tobacco ectopically expressed OMADS1 showed similar novel phenotypes by significantly reducing plant size, flowering extremely early, and losing inflorescence indeterminacy. In addition, homeotic conversion of sepals into carpel-like structures and petals into staminoid structures were also observed in flowers of 35S::OMADS1 Arabidopsis. This result indicated that OMADS1 was involved in floral formation and initiation in transgenic plants. Further analysis indicated that the expression of flowering time genes FT, SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1) and flower meristem identity genes LEAFY (LFY), APETALA1 (AP1) was significantly up-regulated in 35S::OMADS1 transgenic Arabidopsis plants. Furthermore, ectopic expression of OMADS1 rescued late-flowering phenotype in gi-1, co-3 but not for ft-1 and fwa-1 mutants. These results supported that ectopic expression of OMADS1 influenced flower transition and formation by acting as an activator for FT and SOC1 in Arabidopsis.     

The MADS box gene AOM1 is expressed in reproductive meristems and flowers of the dioecious species Asparagus officinalis

MADS box genes are implicated in different steps of plant development. Some of them are expressed in vegetative organs. Most of them, however, are expressed in flower tissues and are involved in different phases of flower development. Here we describe the isolation and characterization of an Asparagus officinalis MADS box gene, AOM1. The deduced AOM1 protein shows the highest degree of similarity with FBP2 of Petunia hybrida and AGL9 (SEP3), AGL2 (SEP1) and AGL4 (SEP2) of Arabidopsis thaliana. In situ hybridization analyses, however, show that the expression profile of AOM1 is different from that of these genes: AOM1 is expressed not only in flower organs but also in inflorescence and flower meristems. These data indicate a possible function of AOM1 during flower development as well as in earlier stages of the flowering process. Asparagus officinalis is a dioecious species which bears male and female flowers on different individuals. AOM1, which is expressed very early during the process of flowering and has a similar expression profile in male and female flowers, does not seems to be involved in asparagus sex differentiation. Received: 3 July 2000 / Revision accepted: 4 August 2000     

<Emphasis Type="Italic">AOM3</Emphasis> and<Emphasis Type="Italic"> AOM4</Emphasis>: two MADS box genes expressed in reproductive structures of<Emphasis Type="Italic"> Asparagus officinalis</Emphasis>

The MADS box genes participate in different steps of vegetative and reproductive plant development, including the most important phases of the reproductive process. Here we describe the isolation and characterisation of two Asparagus officinalis MADS box genes, AOM3 and AOM4. The deduced AOM3 protein shows the highest degree of similarity with ZAG3 and ZAG5 of maize, OsMADS6 of rice and AGL6 of Arabidopsis thaliana. The deduced AOM4 protein shows the highest degree of similarity with AOM1 of asparagus, the SEP proteins of Arabidopsis and the rice proteins OsMADS8, OsMADS45 and OsMADS7. The high level of identity between AOM1 and AOM4 made impossible the preparation of probes specific for one single gene, so the hybridisation signal previously described for AOM1 is probably due to the expression of both genes. The expression profile of AOM3 and AOM1/AOM4 during flower development is identical, and similar to that of the SEP genes. Asparagus genes, however, are expressed not only in flower organs, but also in the different meristem present on the apical region of the shoot during the flowering season: the apical meristem and the three lateral meristems emerging from the leaf axillary region that will give rise to flowers and lateral inflorescences during flowering season, and to phylloclades and branches during the subsequent vegetative phase. The expression of AOM3 and AOM1/AOM4 in these meristems appears to be correlated with the reproductive function of the apex as the hybridisation signal disappears when the apex switches to vegetative function.     

A petunia MADS box gene involved in the transition from vegetative to reproductive development.

We have identified a novel petunia MADS box gene, PETUNIA FLOWERING GENE (PFG), which is involved in the transition from vegetative to reproductive development. PFG is expressed in the entire plant except stamens, roots and seedlings. Highest expression levels of PFG are found in vegetative and inflorescence meristems. Inhibition of PFG expression in transgenic plants, using a cosuppression strategy, resulted in a unique nonflowering phenotype. Homozygous pfg cosuppression plants are blocked in the formation of inflorescences and maintain vegetative growth. In these mutants, the expression of both PFG and the MADS box gene FLORAL BINDING PROTEIN26 (FBP26), the putative petunia homolog of SQUAMOSA from Antirrhinum, are down-regulated. In hemizygous pfg cosuppression plants initially a few flowers are formed, after which the meristem reverts to the vegetative phase. This reverted phenotype suggests that PFG, besides being required for floral transition, is also required to maintain the reproductive identity after this transition. The position of PFG in the hierarchy of genes controlling floral meristem development was investigated using a double mutant of the floral meristem identity mutant aberrant leaf and flower (alf) and the pfg cosuppression mutant. This analysis revealed that the pfg cosuppression phenotype is epistatic to the alf mutant phenotype, indicating that PFG acts early in the transition to flowering. These results suggest that the petunia MADS box gene, PFG, functions as an inflorescence meristem identity gene required for the transition of the vegetative shoot apex to the reproductive phase and the maintenance of reproductive identity.