Ectopic Expression of a Fagopyrum esculentum APETALA1 Ortholog only Rescues Sepal Development in Arabidopsis ap1 Mutant

APETALA2-like Floral Homeotic Protein Up-Regulating FaesAP1_2 Gene Involved in Floral Development in Long-Homostyle Common Buckwheat

In the rosid species Arabidopsis thaliana, the AP2-type AP2 transcription factor (TF) is required for specifying the sepals and petals identities and confers a major A-function to antagonize the C-function in the outer floral whorls. In the asterid species Petunia, the AP2-type ROB TFs are required for perianth and pistil development, as well as repressing the B-function together with TOE-type TF BEN. In Long-homostyle (LH) Fagopyrum esculentum, VIGS-silencing showed that FaesAP2 is mainly involved in controlling filament and style length, but FaesTOE is mainly involved in regulating filament length and pollen grain development. Both FaesAP2 (AP2-type) and FaesTOE (TOE-type) are redundantly involved in style and/or filament length determination instead of perianth development. However, neither FaesAP2 nor FaesTOE could directly repress the B and/or C class genes in common buckwheat. Moreover, the FaesAP1_2 silenced flower showed tepal numbers, and filament length decreased obviously. Interestingly, yeast one-hybrid (Y1H) and dual-luciferase reporter (DR) further suggested that FaesTOE directly up-regulates FaesAP1_2 to be involved in filament length determination in LH common buckwheat. Moreover, the knockdown of FaesTOE expression could result in expression down-regulation of the directly target FaesAP1_2 in the FaesTOE-silenced LH plants. Our findings uncover a stamen development pathway in common buckwheat and offer deeper insight into the functional evolution of AP2 orthologs in the early-diverging core eudicots.

AtSAMS regulates floral organ development by DNA methylation and ethylene signaling pathway

S-adenosylmethionine synthase is the key enzyme involved in the biosynthesis of S-adenosylmethionine, which serves as the universal methyl group donor and a common precursor for the biosynthesis of ethylene and polyamines. However, little is known about how SAMS controls plant development. Here, we report that the abnormal floral organ development in the AtSAMS-overexpressing plants is caused by DNA demethylation and ethylene signaling. The whole-genome DNA methylation level decreased, and ethylene content increased in SAMOE. Wild-type plants treated with DNA methylation inhibitor mimicked the phenotypes and the ethylene levels in SAMOE, suggesting that DNA demethylation enhanced ethylene biosynthesis, which led to abnormal floral organ development. DNA demethylation and elevated ethylene resulted in changes in the expression of ABCE genes, which is essential for floral organ development. Furthermore, the transcript levels of ACE genes were highly correlated to their methylation levels, except for the down-regulation of the B gene, which might have resulted from demethylation-independent ethylene signaling. SAMS-mediated methylation and ethylene signaling might create crosstalk in the process of floral organ development. Together, we provide evidence that AtSAMS regulates floral organ development by DNA methylation and ethylene signaling pathway.

Genome-Wide Identification of MADS-Box Family Genes in Safflower (Carthamus tinctorius L.) and Functional Analysis of CtMADS24 during Flowering

Safflower is an important economic crop with a plethora of industrial and medicinal applications around the world. The bioactive components of safflower petals are known to have pharmacological activity that promotes blood circulation and reduces blood stasis. However, fine-tuning the genetic mechanism of flower development in safflower is still required. In this study, we report the genome-wide identification of MADS-box transcription factors in safflower and the functional characterization of a putative CtMADS24 during vegetative and reproductive growth. In total, 77 members of MADS-box-encoding genes were identified from the safflower genome. The phylogenetic analysis divided CtMADS genes into two types and 15 subfamilies. Similarly, bioinformatic analysis, such as of conserved protein motifs, gene structures, and cis-regulatory elements, also revealed structural conservation of MADS-box genes in safflower. Furthermore, the differential expression pattern of CtMADS genes by RNA-seq data indicated that type II genes might play important regulatory roles in floral development. Similarly, the qRT-PCR analysis also revealed the transcript abundance of 12 CtMADS genes exhibiting tissue-specific expression in different flower organs. The nucleus-localized CtMADS24 of the AP1 subfamily was validated by transient transformation in tobacco using GFP translational fusion. Moreover, CtMADS24-overexpressed transgenic Arabidopsis exhibited early flowering and an abnormal phenotype, suggesting that CtMADS24 mediated the expression of genes involved in floral organ development. Taken together, these findings provide valuable information on the regulatory role of CtMADS24 during flower development in safflower and for the selection of important genes for future molecular breeding programs.

FaesAP3_1 Regulates the FaesELF3 Gene Involved in Filament-Length Determination of Long-Homostyle Fagopyrum esculentum

The identification downstream genes of floral organ identity regulators are critical to revealing the molecular mechanisms underlying floral morphogenesis. However, a general regulatory pathway between floral organ identity genes and their downstream targets is still unclear because of the lack of studies in nonmodel species. Here, we screened a direct downstream target gene, FaesELF3, of a stamen identity transcription factor, FaesAP3_1, in long-homostyle (LH) Fagopyrum esculentum moench by using yeast one-hybrid (Y1H) and dual-luciferase reporter (DR) assays. Furthermore, FaesAP3_1-silenced LH plants that produced flowers with part stamens or anthers homeotically converted into a tepaloid structure, and FaesELF3-silenced plants that had flowers with part stamens consisting of a short filament and empty anther (male sterile anther). All these suggested that transcription factor (TF) FaesAP3_1 directly activates FaesELF3 in order to regulate filament elongation and pollen grain development in LH buckwheat. Our data also suggested that other stamen development pathways independent of FaesAP3_1 remain in F. esculentum.

Characterization of PISTILLATA-like Genes and Their Promoters from the Distyly Fagopyrum esculentum

Arabidopsis PISTILLATA (PI) encodes B-class MADS-box transcription factor (TF), and works together with APETALA3 (AP3) to specify petal and stamen identity. However, a small-scale gene duplication event of PI ortholog was observed in common buckwheat and resulted in FaesPI_1 and FaesPI_2. FaesPI_1 and FaesPI_2 were expressed only in the stamen of dimorphic flower (thrum and pin) of Fagopyrum esculentum. Moreover, intense beta-glucuronidase (GUS) staining was found in the entire stamen (filament and anther) in pFaesPI_1::GUS transgenic Arabidopsis, while GUS was expressed only in the filament of pFaesPI_2::GUS transgenic Arabidopsis. In addition, phenotype complementation analysis suggested that pFaesPI_1::FaesPI_1/pFaesPI_2::FaesPI_2 transgenic pi-1 Arabidopsis showed similar a flower structure with stamen-like organs or filament-like organs in the third whorl. This suggested that FaesPI_2 only specified filament development, but FaesPI_1 specified stamen development. Meanwhile, FaesPI_1 and FaesPI_2 were shown to function redundantly in regulating filament development, and both genes work together to require a proper stamen identity. The data also provide a clue to understanding the roles of PI-like genes involved in floral organ development during the early evolution of core eudicots and also suggested that FaesPI_1 and FaesPI_2 hold the potential application in bioengineering to develop a common buckwheat male sterile line.

Isolation and Characterization of APETALA3 Orthologs and Promoters from the Distylous Fagopyrum esculentum

Common buckwheat (Fagopyrum esculentum) produces distylous flowers with undifferentiated petaloid tepals, which makes it obviously different from flowers of model species. In model species Arabidopsis, APETALA3 (AP3) is expressed in petal and stamen and specifies petal and stamen identities during flower development. Combining with our previous studies, we found that small-scale gene duplication (GD) event and alternative splicing (AS) of common buckwheat AP3 orthologs resulted in FaesAP3_1, FaesAP3_2 and FaesAP3_2a. FaesAP3_2 and FaesAP3_2a were mainly expressed in the stamen of thrum and pin flower. Promoters functional analysis suggested that intense GUS staining was observed in the whole stamen in pFaesAP3_2::GUS transgenic Arabidopsis, while intense GUS staining was observed only in the filament of stamen in pFaesAP3_1::GUS transgenic Arabidopsis. These suggested that FaesAP3_1 and FaesAP3_2 had overlapping functions in specifying stamen filament identity and work together to determine normal stamen development. Additionally, FaesAP3_2 and FaesAP3_2a owned the similar ability to rescue stamen development of Arabidopsis ap3-3 mutant, although AS resulted in a frameshift mutation and consequent omission of the complete PI-derived motif and euAP3 motif of FaesAP3_2a. These suggested that the MIK region of AP3-like proteins was crucial for determining stamen identity, while the function of AP3-like proteins in specifying petal identity was gradually obtained after AP3 Orthologs acquiring a novel C-terminal euAP3 motif during the evolution of core eudicots. Our results also provide a clue to understanding the early evolution of the functional specificity of euAP3-type proteins involving in floral organ development in core eudicots, and also suggested that FaesAP3_2 holds the potential application for biotechnical engineering to develop a sterile male line of F. esculentum.

High-Resolution Transcriptome Atlas and Improved Genome Assembly of Common Buckwheat, Fagopyrum esculentum

Common buckwheat (Fagopyrum esculentum) is an important non-cereal grain crop and a prospective component of functional food. Despite this, the genomic resources for this species and for the whole family Polygonaceae, to which it belongs, are scarce. Here, we report the assembly of the buckwheat genome using long-read technology and a high-resolution expression atlas including 46 organs and developmental stages. We found that the buckwheat genome has an extremely high content of transposable elements, including several classes of recently (0.5-1 Mya) multiplied TEs ("transposon burst") and gradually accumulated TEs. The difference in TE content is a major factor contributing to the three-fold increase in the genome size of F. esculentum compared with its sister species F. tataricum. Moreover, we detected the differences in TE content between the wild ancestral subspecies F. esculentum ssp. ancestrale and buckwheat cultivars, suggesting that TE activity accompanied buckwheat domestication. Expression profiling allowed us to test a hypothesis about the genetic control of petaloidy of tepals in buckwheat. We showed that it is not mediated by B-class gene activity, in contrast to the prediction from the ABC model. Based on a survey of expression profiles and phylogenetic analysis, we identified the MYB family transcription factor gene tr_18111 as a potential candidate for the determination of conical cells in buckwheat petaloid tepals. The information on expression patterns has been integrated into the publicly available database TraVA: http://travadb.org/browse/Species=Fesc/. The improved genome assembly and transcriptomic resources will enable research on buckwheat, including practical applications.

Ectopic expression of an AGAMOUS homologue gene in Jatropha curcas causes early flowering and heterostylous phenotypes

Jatropha curcasseeds are abundant in biodiesel, and low seed yields are linked to poor quality female flowers, which creates a bottleneck for Jatropha seed utilization. Therefore, identifying the genes associated with flowering is crucial for the genetic enrichment of seed yields. Here, we identified an AGAMOUS homologue gene (JcAG) from J. curcas. We found that reproductive organs had higher JcAG expression than vegetative organs, particularly the carpel. Rosette leaves were small and misshapen in 35S:JcAG transgenic lines in comparison with those in wild-type plants. JcAG overexpression caused an extremely early flowering, delayed perianth and stamen filament development, small flowers, and significantly shorter Arabidopsis plants with little fruit. In the JcAG-overexpressing line, the homeotic transformation of sepals into pistillate organs was observed, and floral meristem and organ identity genes were regulated. This study provides insights into the JcAG's function and benefits to our knowledge of the underlying the genetic mechanisms related to floral sex differentiation in Jatropha.