Ectopic expression of a hyacinth AGL6 homolog caused earlier flowering and homeotic conversion in Arabidopsis

A transcriptional atlas identifies key regulators and networks for the development of spike tissues in barley

Grain number and size determine grain yield in crops and are closely associated with spikelet fertility and grain filling in barley (Hordeum vulgare). Abortion of spikelet primordia within individual barley spikes causes a 30%-50% loss in the potential number of grains during development from the awn primordium stage to the tipping stage, after that grain filling is the primary factor regulating grain size. To identify transcriptional signatures associated with spike development, we use a six-rowed barley cultivar (Morex) to develop a spatiotemporal transcriptome atlas containing 255 samples covering 17 stages and 5 positions along the spike. We identify several fundamental regulatory networks, in addition to key regulators of spike development and morphology. Specifically, we show HvGELP96, encoding a GDSL domain-containing protein, as a regulator of spikelet fertility and grain number. Our transcriptional atlas offers a powerful resource to answer fundamental questions in spikelet development and degeneration in barley.

Genome-wide identification and analysis of MIKC-type MADS-box genes expression in Chimonanthus salicifolius

BACKGROUND

MIKC type MADS-box transcription factors are one of the largest gene families and play a pivotal role in flowering time and flower development. Chimonanthus salicifolius belongs to the family Calycanthaceae and has a unique flowering time and flowering morphology compared to other Chimonanthus species, but the research on MIKC type MADS-box gene family of C. salicifolius has not been reported.

OBJECTIVE

Identification, comprehensive bioinformatic analysis, the expression pattern of MIKC-type MADS-box gene family from different tissues of C. salicifolius.

METHODS

Genome-wide investigation and expression pattern under different tissues of the MIKC-type MADS-box gene family in C. salicifolius, and their phylogenetic relationships, evolutionary characteristics, gene structure, motif distribution, promoter cis-acting element were performed.

RESULTS

A total of 29 MIKC-type MADS-box genes were identified from the whole genome sequencing. Interspecies synteny analysis revealed more significant collinearity between C. salicifolius and the magnoliids species compared to eudicots and monocots. MIKC-type MADS-box genes from the same subfamily share similar distribution patterns, gene structure, and expression patterns. Compared with Arabidopsis thaliana, Nymphaea colorata, and Chimonanthus praecox, the FLC genes were absent in C. salicifolius, while the AGL6 subfamily was expanded in C. salicifolius. The selectively expanded promoter (AGL6) and lack of repressor (FLC) genes may explain the earlier flowering in C. salicifolius. The loss of the AP3 homologous gene in C. salicifolius is probably the primary cause of the morphological distinction between C. salicifolius and C. praecox. The csAGL6a gene is specifically expressed in the flowering process and indicates the potential function of promoting flowering.

CONCLUSION

This study offers a genome-wide identification and expression profiling of the MIKC-types MADS-box genes in the C. salicifolius, and establishes the foundation for screening flowering development genes and understanding the potential function of the MIKC-types MADS-box genes in the C. salicifolius.

Functional Divergence Analysis of AGL6 Genes in Prunus mume

The AGAMOUS-LIKE6 (AGL6) lineage is an important clade of MADS-box transcription factors that play essential roles in floral organ development. The genome of Prunus mume contains two homoeologous AGL6 genes that are replicated as gene fragments. In this study, two AGL6 homologs, PmAGL6-1 and PmAGL6-2, were cloned from P. mume and then functionally characterized. Sequence alignment and phylogenetic analyses grouped both genes into the AGL6 lineage. The expression patterns and protein-protein interaction patterns showed significant differences between the two genes. However, the ectopic expression of the two genes in Arabidopsis thaliana resulted in similar phenotypes, including the promotion of flowering, alteration of floral organ structure, participation in the formation of the floral meristem and promotion of pod bending. Therefore, gene duplication has led to some functional divergence of PmAGL6-1 and PmAGL6-2 but their functions are similar. We thus speculated that AGL6 genes play a crucial role in flower development in P. mume.

CRISPR/Cas9-Mediated Editing of AGAMOUS-like Genes Results in a Late-Bolting Phenotype in Chinese Cabbage (Brassica rapa ssp. pekinensis)

Due to the sudden change in temperature in spring, Chinese cabbage, a leafy vegetable cultivated for consumption, loses its commercial value due to the onset of bolting—the phenomenon of switching from vegetative to reproductive growth. In this study, we applied clustered regularly interspaced short palindromic repeats/(CRISPR)-associated system 9 (CRISPR/Cas9) technology to analyze AGAMOUS-like genes. We performed functional analysis of AGL19 and AGL24 genes related to bolting and flowering using CRISPR/Cas9-mediated Chinese cabbage transformation. Single-guide RNA (sgRNA) sequences were created with a low off-targeting probability to construct gene-editing vectors. Agrobacterium-mediated transformation was conducted, and tentative E0 AGL-edited lines were analyzed using molecular biotechnological methods. Two AGL19-edited lines with nucleotide sequence mutations in the target sequence of the AGL19 genes and four AGL24-edited lines with nucleotide sequence mutations in the target sequence of the AGL24 genes showed particularly late bolting compared to the inbred line ‘CT001.’ Generational progression using bud pollination obtained T-DNA-free E1 AGL-edited lines, which also showed late bolting. The loss of function of the AGL protein was caused by the occurrence of an indel mutation in the AGL19 and AGL24 genes, which results in an early stop codon. Furthermore, frameshift mutations led to structural changes and the introduction of an early stop codon in the AGL19 and AGL24 proteins. Our results indicate that CRISPR/Cas9-mediated editing of AGAMOUS-like genes results in a late-bolting phenotype and that CRISPR/Cas9 is a useful technology for analyzing gene function in Chinese cabbage (Brassica rapa ssp. pekinensis).

Transcriptome analysis reveals dual action of salicylic acid application in the induction of flowering in Malus domestica

In the apple tree, insufficient flower bud production is an intractable challenge, and very little information is available in this field due to the fact that research done in this sector is very rare owing to its extended life cycles and low rate of genetic transformation. Here we display novel changes and events in spur buds of Malus × domestica trees after they were exposed to salicylic acid (SA) treatment during the flower induction period. We found a significant increase in morphological indexes, followed by a wider and well-defined shoot apical meristem in SA-treated spur buds. Additionally, we observed increased oxidative stress markers and enzymatic antioxidants in control-treated buds during the flower induction period, while non-enzymatic antioxidants were recorded higher in SA-treated buds. Maximum flowering was observed in SA-treated trees in the next year. Furthermore, ultra-high-performance liquid chromatography (u-HPLC) analysis displays that SA treatment enhances SA and indole acetic acid (IAA), while having an antagonistic effect on gibberellin (GA). At different time points, transcriptome analysis was conducted to analyze the transcriptional response of CK and SA treated buds. Pathway enrichment was detected in differentially expressed genes (DEGs). Agamous (AGL) and SQUAMOSA-promoter binding protein-like (SPL) family related flowering genes display a positive signal for the increased flowering in SA-treated trees, which confirms our findings. As far as we know, there is no report available on the response of spur buds to SA treatment during the flower induction period. This data provides a new theoretical reference for the management of apple tree flowering and also provides an essential basis for future analysis of the regulation and control of flowering in M. domestica.

DiZF-C3H1, a zinc finger transcription factor from the dove tree (Davidia involucrata Baill.), plays a negative role in seed development and plant growth in Arabidopsis and tobacco

Low seed fertility seriously limits the survival and adaption of rare plant species. Here, we identified a seed-specific gene, DiZF-C3H1, from the dove tree and verified its function. Overexpression of DiZF-C3H1 caused retarded root development, delayed anthesis, abnormal floral organs, and deformed siliques in transgenic Arabidopsis lines. No offspring were obtained in transgenic Arabidopsis lines due to serious seed abortion. Therefore, we performed further verification in tobacco. Similarly, overexpression of DiZF-C3H1 retarded root development and reduced berry size and seed yield in transgenic tobacco lines. Moreover, although transgenic tobacco offspring were obtained, the viability of transgenic seeds was reduced and their germination was delayed. In addition, faded flowers were observed in transgenic tobacco lines. Taken together, DiZF-C3H1 was verified to play a negative role in root growth, floral organ development, and especially seed development in Arabidopsis and tobacco. This appears to be a deleterious gene for these model plants with high seed fertility. However, this function might be of special significance for Davidia, whose seed dormancy period is extremely long; DiZF-C3H1 might play a critical role in the distinctive reproduction strategy adopted by this rare and endangered species.

Gene duplication led to divergence of expression patterns, protein-protein interaction patterns and floral development functions of AGL6-like genes in the basal angiosperm Magnolia wufengensis (Magnoliaceae)

The MADS-box family genes play critical roles in the regulation of growth and development of flowering plants. AGAMOUS-LIKE 6 (AGL6)-like genes are one of the most enigmatic subfamilies of the MADS-box family because of highly variable expression patterns and ambiguous functions, which have long puzzled researchers. A lot of AGL6 homologs have been identified from gymnosperms and angiosperms. However, only a few have been characterized, especially for basal angiosperm taxa. Magnolia wufengensis is a woody basal angiosperm from the family Magnoliaceae. In the current study, the phylogenesis, expression and protein-protein interaction (PPI) patterns, and functions of two AGL6 homologs from M. wufengensis, MawuAGL6-1 and MawuAGL6-2, were analyzed. Phylogenetic analysis indicated that the two AGL6 duplicates may have arisen by gene duplication before the divergence of Magnoliaceae and Lauraceae, with the diversification of their expression and PPI patterns after gene duplication. Functional analysis revealed that, in addition to common functions in accelerating flowering, MawuAGL6-1 might be responsible for flower meristem determinacy, while MawuAGL6-2 is preferentially recruited to regulate tepal morphogenesis. These findings further advance our understanding of the evolution of phylogenesis, expression, interaction and functions of AGL6 lineage genes from basal angiosperms, as well as the entire AGL6 lineage genes, and the significance of AGL6 lineage genes in the evolution and biological diversity.

Two C3H Type Zinc Finger Protein Genes, CpCZF1 and CpCZF2, from Chimonanthus praecox Affect Stamen Development in Arabidopsis

Wintersweet (Chimonanthus praecox) is a popular garden plant because of its flowering time, sweet fragrance, and ornamental value. However, research into the molecular mechanism that regulates flower development in wintersweet is still limited. In this study, we sought to investigate the molecular characteristics, expression patterns, and potential functions of two C3H-type zinc finger (CZF) protein genes, CpCZF1 and CpCZF2, which were isolated from the wintersweet flowers based on the flower developmental transcriptome database. CpCZF1 and CpCZF2 were more highly expressed in flower organs than in vegetative tissues, and during the flower development, their expression profiles were associated with flower primordial differentiation, especially that of petal and stamen primordial differentiation. Overexpression of either CpCZF1 or CpCZF2 caused alterations on stamens in transgenic Arabidopsis. The expression levels of the stamen identity-related genes, such as AGAMOUS (AG), PISTILLATA (PI), SEPALLATA1 (SEP1), SEPALLATA2 (SEP2), SEPALLATA3 (SEP3), APETALA1 (AP1), APETALA2 (AP2), and boundary gene RABBIT EAR (RBE) were significantly up-regulated in CpCZF1 overexpression lines. Additionally, the transcripts of AG, PI, APETALA3SEP1-3, AP1, and RBE were markedly increased in CpCZF2 overexpressed plant inflorescences. Moreover, CpCZF1 and CpCZF2 could interact with each other by using yeast two-hybrid and bimolecular fluorescence complementation assays. Our results suggest that CpCZF1 and CpCZF2 may be involved in the regulation of stamen development and cause the formation of abnormal flowers in transgenic Arabidopsis plants.

The role of ABC genes in shaping perianth phenotype in the basal angiosperm Magnolia

It is generally accepted that the genus Magnolia is characterised by an undifferentiated perianth, typically organised into three whorls of nearly identical tepals. In some species, however, we encountered interesting and significant perianth modifications. In Magnolia acuminata, M. liliiflora and M. stellata the perianth elements of the first whorl are visually different from the others. In M. stellata the additional, spirally arranged perianth elements are present above the first three whorls, which suggests that they have been formed within the domain of stamen primordia. In these three species, we analysed expression patterns of the key flower genes (AP1, AGL6, AP3, PI, AG) responsible for the identity of flower elements and correlated them with results of morphological and anatomical investigations. In all studied species the elements of the first whorl lacked the identity of petals (lack of AP3 and PI expression) but also that of leaves (presence of AGL6 expression), and this seems to prove their sepal character. The analysis of additional perianth elements of M. stellata, spirally arranged on the elongated floral axis, revealed overlapping and reduced activity of genes involved in specification of the identity of the perianth (AGL6) but also of generative parts (AG), even though no clear gradient of morphological changes could be observed. In conclusion, Magnolia genus is capable of forming, in some species, a perianth differentiated into a calyx (sepals) and corolla (petals). Spirally arranged, additional perianth elements of M. stellata, despite activity of AG falling basipetally, resemble petals.

Analysis of the floral MADS-box genes from monocotyledonous Trilliaceae species indicates the involvement of SEPALLATA3-like genes in sepal-petal differentiation

The evolution of greenish sepals from petaloid outer tepals has occurred repeatedly in various lineages of non-grass monocots. Studies in distinct monocot species showed that the evolution of sepals could be explained by the ABC model; for example, the defect of B-class function in the outermost whorl was linked to the evolution of sepals. Here, floral MADS-box genes from three sepal-bearing monocotyledonous Trilliaceae species, Trillium camschatcense, Paris verticillata, and Kinugasa japonica were examined. Unexpectedly, expression of not only A- but also B-class genes was detected in the sepals of all three species. Although the E-class gene is generally expressed across all floral whorls, no expression was detected in sepals in the three species examined here. Overexpression of the E-class SEPALLATA3-like gene from T. camschatcense (TcamSEP) in Arabidopsis thaliana produced phenotypes identical to those reported for orthologs in other monocots. Additionally, yeast hybrid experiments indicated that TcamSEP could form a higher-order complex with an endogenous heterodimer of B-class APETALA3/DEFICIENS-like (TcamDEF) and PISTILLATA/GLOBOSA-like (TcamGLO) proteins. These results suggest a conserved role for Trilliaceae SEPALLATA3-like genes in functionalization of the B-class genes, and that a lack of SEPALLATA3-like gene expression in the outermost whorl may be related to the formation of greenish sepals.

Characterization of Osmads6-5, a null allele, reveals that OsMADS6 is a critical regulator for early flower development in rice (Oryza sativa L.)

AGL6-clade genes are a subfamily of MADS-box genes and preferentially expressed in floral organs. OsMADS6 and OsMADS17 are two AGL6-like genes in rice. OsMADS17 has been shown to play a minor role in floral development and appears to result from a duplication of OsMADS6. OsMADS6 was initially named as MFO1 for mosaic floral organs based on its moderate mutant phenotypes. So far, four moderate or weak mutant alleles of OsMADS6 have been described, providing valuable insights into its role in flower development. Here, we report a null allele of OsMADS6 (Osmads6-5), which exhibited a strong mutant phenotype in spikelet without affecting vegetative traits, causing all floral organs except lemma homeotically transformed into lemma-like organs (LLOs) as well as an indeterminate floral meristem, thus resulting in a mutant floret consisting of reiterating whorls of lemma and LLOs. In consistently, over-expression of OsMADS6 led to additional lodicule-, stamen- and carpel-like organs. Expression analysis showed that OsMADS6 controls the formation of the incipient primordia of lodicule, stamen and carpel via regulating the expression of class B, C and SEP-like MADS-box genes. Taken together, our results revealed that OsMADS6 acts as a critical regulator for early flower development in rice and provide novel insights into the molecular mechanism of OsMADS6.

The expression of floral organ identity genes in contrasting water lily cultivars

The floral organs of typical eudicots such as Arabidopsis thaliana are arranged in four characteristic whorls, namely the sepal, petal, stamen and carpel, and the "ABC" floral organ identity model has been based on this arrangement. However, the floral organs in most basal angiosperms are spirally arranged with a gradual transition from the inside to outside, and an alternative model referred to as "fading borders" was developed to take account of this. The flower morphology of the water lily was tested against the "fading borders" model by determining the expression profile of the six primary floral organ identity genes AP2, AGL6, AP3, PI, AG and SEP1 in two cultivars showing contrasting floral morphology. In addition, to get accurate floatation of the genes expression level from outer to inner, we divided the floral organs into eight whorls according to morphological features. All these genes were expressed throughout all whorls of the flower, but their expression level changed gradually from the outside of the flower to its inside. This pattern was consistent with the "fading borders" model.

Genetic interaction of OsMADS3, DROOPING LEAF, and OsMADS13 in specifying rice floral organ identities and meristem determinacy

Grass plants develop unique floral patterns that determine grain production. However, the molecular mechanism underlying the specification of floral organ identities and meristem determinacy, including the interaction among floral homeotic genes, remains largely unknown in grasses. Here, we report the interactions of rice (Oryza sativa) floral homeotic genes, OsMADS3 (a C-class gene), OsMADS13 (a D-class gene), and DROOPING LEAF (DL), in specifying floral organ identities and floral meristem determinacy. The interaction among these genes was revealed through the analysis of double mutants. osmads13-3 osmads3-4 displayed a loss of floral meristem determinacy and generated abundant carpelloid structures containing severe defective ovules in the flower center, which were not detectable in the single mutant. In addition, in situ hybridization and yeast two-hybrid analyses revealed that OsMADS13 and OsMADS3 did not regulate each other's transcription or interact at the protein level. This indicates that OsMADS3 plays a synergistic role with OsMADS13 in both ovule development and floral meristem termination. Strikingly, osmads3-4 dl-sup6 displayed a severe loss of floral meristem determinacy and produced supernumerary whorls of lodicule-like organs at the forth whorl, suggesting that OsMADS3 and DL synergistically terminate the floral meristem. Furthermore, the defects of osmads13-3 dl-sup6 flowers appeared identical to those of dl-sup6, and the OsMADS13 expression was undetectable in dl-sup6 flowers. These observations suggest that DL and OsMADS13 may function in the same pathway specifying the identity of carpel/ovule and floral meristem. Collectively, we propose a model to illustrate the role of OsMADS3, DL, and OsMADS13 in the specification of flower organ identity and meristem determinacy in rice.

OsMADS6 plays an essential role in endosperm nutrient accumulation and is subject to epigenetic regulation in rice (Oryza sativa)

MADS-box transcription factors are known for their roles in plant growth and development. The regulatory mechanisms of spatial and temporal specific expression of MADS-box genes and the function of MADS-box genes in other biological processes are still to be explored. Here, we report that OsMADS6 is highly expressed in flower and endosperm in Oryza sativa (rice). In addition to displaying a homeotic organ identity phenotype in all the four whorls of the flowers, the endosperm development is severely affected in its mutant. At least 32% of the seeds lacked starch filling and aborted. For seeds that have starch filling and develop to maturity, the starch content is reduced by at least 13%. In addition, the seed shape changes from elliptical to roundish, and the protein content increases from 12.1 to 15.0% (P < 0.05). Further investigation shows that ADP-glucose pyrophosphorylase genes, encoding the rate-limiting step enzyme in the starch synthesis pathway, are subject to the regulation of OsMADS6. Chromatin immunoprecipitation (ChIP)-PCR analyses on the chromatin of the OsMADS6 gene find that H3K27 is trimethylated in tissues where OsMADS6 is silenced, and that H3K36 is trimethylated in tissues where OsMADS6 is highly activated. Point mutation analysis reveals that leucine at position 83 is critical to OsMADS6 function.

Evolutionary trends in the floral transcriptome: insights from one of the basalmost angiosperms, the water lily Nuphar advena (Nymphaeaceae)

Current understanding of floral developmental genetics comes primarily from the core eudicot model Arabidopsis thaliana. Here, we explore the floral transcriptome of the basal angiosperm, Nuphar advena (water lily), for insights into the ancestral developmental program of flowers. We identify several thousand Nuphar genes with significantly upregulated floral expression, including homologs of the well-known ABCE floral regulators, deployed in broadly overlapping transcriptional programs across floral organ categories. Strong similarities in the expression profiles of different organ categories in Nuphar flowers are shared with the magnoliid Persea americana (avocado), in contrast to the largely organ-specific transcriptional cascades evident in Arabidopsis, supporting the inference that this is the ancestral condition in angiosperms. In contrast to most eudicots, floral organs are weakly differentiated in Nuphar and Persea, with staminodial intermediates between stamens and perianth in Nuphar, and between stamens and carpels in Persea. Consequently, the predominantly organ-specific transcriptional programs that characterize Arabidopsis flowers (and perhaps other eudicots) are derived, and correlate with a shift towards morphologically distinct floral organs, including differentiated sepals and petals, and a perianth distinct from stamens and carpels. Our findings suggest that the genetic regulation of more spatially discrete transcriptional programs underlies the evolution of floral morphology.

Expression divergence of the AGL6 MADS domain transcription factor lineage after a core eudicot duplication suggests functional diversification

BACKGROUND

Because of their known role as transcriptional regulators of key plant developmental processes, the diversification of MADS-box gene function is thought to be a major driving force in the developmental evolution of plants. Yet the function of some MADS-box gene subfamilies has remained elusive thus far. One such lineage, AGL6, has now been functionally characterized in three angiosperm species, but a phylogenetic framework for comparison of AGL6 gene function is currently missing.

RESULTS

Based on phylogenetic analyses of newly isolated and EST-based sequences, we describe the duplication history of the AGL6 subfamily in angiosperms. Our analyses provide support for four ancient duplications in the evolution of the AGL6 lineage: one at the base of core eudicots resulting in euAGL6 and AGL6-like gene clades, one during basal angiosperm diversification and two in monocot evolution. To investigate whether the spatial domains in which AGL6 genes function have diverged after duplication, we use quantitative Real Time PCR. We show that the core eudicot AGL6-like clade acquired expression in vegetative tissues, while its paralog euAGL6 remains predominantly confined to reproductive tissues.

CONCLUSIONS

These and previous data lead us to propose that the AGL6 lineage in core eudicots, in addition to functions related to the expression in reproductive structures, may have acquired a function in developmental transitions of vegetative shoots.

Control of lateral organ development and flowering time by the Arabidopsis thaliana MADS-box Gene AGAMOUS-LIKE6

MADS-domain transcription factors play pivotal roles in various developmental processes. The lack of simple loss-of-function phenotypes provides impediments to understand the biological function of some of the MADS-box transcription factors. Here we have characterized the potential role of the Arabidopsis thaliana AGAMOUS-LIKE6 (AGL6) gene by fusing full-length coding sequence with transcriptional activator and repressor domains and suggest a role for AGL6 in lateral organ development and flowering. Upon photoperiodic induction of flowering, AGL6 becomes expressed in abaxial and proximal regions of cauline leaf primordia, as well as the cryptic bracts subtending flowers. In developing flowers, AGL6 is detected in the proximal regions of all floral organs and in developing ovules. Converting AGL6 into a strong activator through fusion to the VP16 domain triggers bract outgrowth, implicating AGL6 in the development of bractless flowers in Arabidopsis. In addition, ectopic reproductive structures form on both bracts and flowers in gAGL6::VP16 transgenic plants, which is dependent on B and C class homeotic genes, but independent of LEAFY. Overexpression of both AGL6 and its transcriptional repressor form, AGL6::EAR, causes precocious flowering and terminal flower formation, suggesting that AGL6 suppresses the function of a floral repressor.

The AGL6-like gene OsMADS6 regulates floral organ and meristem identities in rice

Although AGAMOUS-LIKE6 (AGL6) MADS-box genes are ancient with wide distributions in gymnosperms and angiosperms, their functions remain poorly understood. Here, we show the biological role of the AGL6-like gene, OsMADS6, in specifying floral organ and meristem identities in rice (Oryza sativa L.). OsMADS6 was strongly expressed in the floral meristem at early stages. Subsequently, OsMADS6 transcripts were mainly detectable in paleas, lodicules, carpels and the integument of ovule, as well as in the receptacle. Compared to wild type plants, osmads6 mutants displayed altered palea identity, extra glume-like or mosaic organs, abnormal carpel development and loss of floral meristem determinacy. Strikingly, mutation of a SEPALLATA (SEP)-like gene, OsMADS1 (LHS1), enhanced the defect of osmads6 flowers, and no inner floral organs or glume-like structures were observed in whorls 2 and 3 of osmads1-z osmads6-1 flowers. Furthermore, the osmads1-z osmads6-1 double mutants developed severely indeterminate floral meristems. Our finding, therefore, suggests that the ancient OsMADS6 gene is able to specify "floral state" by determining floral organ and meristem identities in monocot crop rice together with OsMADS1.

The petunia AGL6 gene has a SEPALLATA-like function in floral patterning

SEPALLATA (SEP) MADS-box genes are required for the regulation of floral meristem determinacy and the specification of sepals, petals, stamens, carpels and ovules, specifically in angiosperms. The SEP subfamily is closely related to the AGAMOUS LIKE6 (AGL6) and SQUAMOSA (SQUA) subfamilies. So far, of these three groups only AGL6-like genes have been found in extant gymnosperms. AGL6 genes are more similar to SEP than to SQUA genes, both in sequence and in expression pattern. Despite the ancestry and wide distribution of AGL6-like MADS-box genes, not a single loss-of-function mutant exhibiting a clear phenotype has yet been reported; consequently the function of AGL6-like genes has remained elusive. Here, we characterize the Petunia hybrida AGL6 (PhAGL6, formerly called PETUNIA MADS BOX GENE4/pMADS4) gene, and show that it functions redundantly with the SEP genes FLORAL BINDING PROTEIN2 (FBP2) and FBP5 in petal and anther development. Moreover, expression analysis suggests a function for PhAGL6 in ovary and ovule development. The PhAGL6 and FBP2 proteins interact in in vitro experiments overall with the same partners, indicating that the two proteins are biochemically quite similar. It will be interesting to determine the functions of AGL6-like genes of other species, especially those of gymnosperms.

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.

Evolution of AGL6-like MADS box genes in grasses (Poaceae): ovule expression is ancient and palea expression is new

AGAMOUS-like6 (AGL6) genes encode MIKC-type MADS box transcription factors and are closely related to SEPALLATA and AP1/FUL-like genes. Here, we focus on the molecular evolution and expression of the AGL6-like genes in grasses. We have found that AGL6-like genes are expressed in ovules, lodicules (second whorl floral organs), paleas (putative first whorl floral organs), and floral meristems. Each of these expression domains was acquired at a different time in evolution, indicating that each represents a distinct function of the gene product and that the AGL6-like genes are pleiotropic. Expression in the inner integument of the ovule appears to be an ancient expression pattern corresponding to the expression of the gene in the megasporangium and integument in gymnosperms. Expression in floral meristems appears to have been acquired in the angiosperms and expression in second whorl organs in monocots. Early in grass evolution, AGL6-like orthologs acquired a new expression domain in the palea. Stamen expression is variable. Most grasses have a single AGL6-like gene (orthologous to the rice [Oryza sativa] gene MADS6). However, rice and other species of Oryza have a second copy (orthologous to rice MADS17) that appears to be the result of an ancient duplication.

bearded-ear encodes a MADS box transcription factor critical for maize floral development

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.