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The dahlia is a fabulous cutting flower for the home garden. Cut one bloom, and ten more appear on the plant. Blooming late summer to the first frost of autumn, this native of Mexico provides explosions of color in home gardens. The author's commentary unearths the dahlia from its Aztec origins and imparts practical, hands-on knowledge for growing and overwintering these tropical plants in wintry climes.

Garden dahlias (Dahlia variabilis) are autoallooctoploids with redundant genes producing wide color variations in flowers. There are no pure white dahlia cultivars, despite its long breeding history. However, the white areas of bicolor flower petals appear to be pure white. The objective of this experiment was to elucidate the mechanism by which the pure white color is expressed in the petals of some bicolor cultivars. A pigment analysis showed that no flavonoid derivatives were detected in the white areas of petals in a star-type cultivar 'Yuino' and the two seedling cultivars 'OriW1' and 'OriW2' borne from a red-white bicolor cultivar, 'Orihime', indicating that their white areas are pure white. Semi-quantitative RT-PCR showed that in the pure white areas, transcripts of two chalcone synthases (CHS), DvCHS1 and DvCHS2 which share 69% nucleotide similarity with each other, were barely detected. Premature mRNA of DvCHS1 and DvCHS2 were detected, indicating that these two CHS genes are silenced post-transcriptionally. RNA gel blot analysis revealed that small interfering RNAs (siRNAs) derived from CHSs were produced in these pure white areas. By high-throughput sequence analysis of small RNAs in the pure white areas with no mismatch acceptance, small RNAs were mapped to two alleles of DvCHS1 and two alleles of DvCHS2 expressed in 'Yuino' petals. Therefore, we concluded that simultaneous siRNA-mediated post-transcriptional gene silencing of redundant CHS genes results in the appearance of pure white color in dahlias.

Dahlias (Dahlia variabilis) exhibit a wide range of flower colours because of accumulation of anthocyanin and other flavonoids in their ray florets. Two lateral mutants were used that spontaneously occurred in 'Michael J' (MJW) which has yellow ray florets with orange variegation. MJOr, a bud mutant producing completely orange ray florets, accumulates anthocyanins, flavones, and butein, and MJY, another mutant producing completely yellow ray florets, accumulates flavones and butein. Reverse transcription–PCR analysis showed that expression of chalcone synthase 1 (DvCHS1), flavanone 3-hydroxylase (DvF3H), dihydroflavonol 4-reductase (DvDFR), anthocyanidin synthase (DvANS), and DvIVS encoding a basic helix–loop–helix transcription factor were suppressed, whereas that of chalcone isomerase (DvCHI) and DvCHS2, another CHS with 69% nucleotide identity with DvCHS1, was not suppressed in the yellow ray florets of MJY. A 5.4 kb CACTA superfamily transposable element, transposable element of Dahlia variabilis 1 (Tdv1), was found in the fourth intron of the DvIVS gene of MJW and MJY, and footprints of Tdv1 were detected in the variegated flowers of MJW. It is shown that only one type of DvIVS gene was expressed in MJOr, whereas these plants are likely to have three types of the DvIVS gene. On the basis of these results, the mechanism regulating the formation of orange and yellow ray florets in dahlia is discussed.

Black color in flowers is a highly attractive trait in the floricultural industry, but its underlying mechanisms are largely unknown. This study was performed to identify the bases of the high accumulation of anthocyanidins in black cultivars and to determine whether the high accumulation of total anthocyanidins alone leads to the black appearance. Our approach was to compare black dahlia (Dahlia variabilis) cultivars with purple cultivars and a purple flowering mutant of a black cultivar, using pigment and molecular analyses. Black cultivars characteristically exhibited low lightness, high petal accumulation of cyanidin and total anthocyanidins without flavones, and marked suppression of flavone synthase (DvFNS) expression. A comparative study using black and purple cultivars revealed that neither the absence of flavones nor high accumulation of total anthocyanidins is solely sufficient for black appearance, but that cyanidin content in petals is also an important factor in the phenotype. A study comparing the black cultivar 'Kokucho' and its purple mutant showed that suppression of DvFNS abolishes the competition between anthocyanidin and flavone synthesis and leads to accumulation of cyanidin and total anthocyanidins that produce a black appearance. Surprisingly, in black cultivars the suppression of DvFNS occurred in a post-transcriptional manner, as determined by small RNA mapping.

Main conclusion Tobacco streak virus suppressed post-transcriptional gene silencing and caused a flower color change in black dahlias, which supported the role of cyanidin-based anthocyanins for black flower appearance. Black flower color of dahlia (Dahlia variabilis) has been attributed, in part, to the high accumulation of cyanidinbased anthocyanins that occurs when flavone synthesis is reduced because of post-transcriptional gene silencing (PTGS) of flavone synthase II (DvFNS). There are also purple-flowering plants that have emerged from a black cultivar 'Kokucho'. We report that the purple color is not caused by a mutation, as previously thought, but by infection with tobacco streak virus (TSVdahlia), which suppresses the PTGS of DvFNS. When TSVdahlia was eliminated from the purple-flowering 'Kokucho' by leaf primordia-free shoot apical meristem culture, the resulting flowers were black. TSVdahlia-infected purple flowers had lower numbers of siRNAs to DvFNS than black flowers, suggesting that TSVdahlia has a silencing suppressor. The graft inoculation of other black cultivars with TS Vdahlia altered their flower color drastically except for 'Fidalgo Blacky', a very deep black cultivar with the highest amount of cyanidin-based anthocyanins. The flowers of all six TSVdahlia-infected cultivars accumulated increased amounts of flavones and reduced amounts of cyanidin-based anthocyanins. 'Fidalgo Blacky' remained black despite the change in pigment accumulation, and the amounts of cyanidin-based anthocyanins in its TSVdahlia-infected plants were still higher than those of other cultivars. We propose that black flower color in dahlia is controlled by two different mechanisms that increase the amount of cyanidin-based anthocyanins: DvFNS PTGS-dependent and -independent mechanisms. If both mechanisms occur simultaneously, the flower color will be blacker than if only a single mechanism is active.

Petal color lability is a prominent feature of bicolor dahlia cultivars, and causes plants to produce not only original bicolor petals with colored bases and pure white tips, but also frequently single-colored petals without white tips. In this study, we analysed the molecular mechanisms that are associated with petal color lability using the red-white bicolor cultivar ‘Yuino’. Red single-colored petals lose their white tips as a result of recover of flavonoid biosynthesis. Among flavonoid biosynthetic genes including four chalcone synthase (CHS)-like genes (DvCHS1, DvCHS2, DvCHS3, and DvCHS4), DvCHS1 and DvCHS2 had significantly lower expression levels in the white part of bicolor petals than in red petals, while DvCHS3, DvCHS4, and other flavonoid biosynthetic genes had almost the same expression levels. Small RNAs from the white part of a bicolor petal were mapped onto DvCHS1 and DvCHS2, while small RNAs from a red single-colored petal were not mapped onto any of the four CHS genes. A relationship between petal color and leaf flavonoid accumulation has previously been demonstrated, whereby red petal-producing plants accumulate flavonoids in their leaves, while bicolor petal-producing plants tend not to. The expression level of DvCHS2 was down-regulated in flavonoid-poor leaves and small RNAs from flavonoid-poor leaves were mapped onto DvCHS2, suggesting that the down-regulation of DvCHS2 in flavonoid-poor leaves occurs post-transcriptionally. Genomic analysis also suggested that DvCHS2 is the key gene involved in bicolor formation. Together, these results suggest that post-transcriptional gene silencing of DvCHS2 plays a key role in phenotypic lability in this bicolor dahlia.

Main conclusionTwo glycosyltransferase genes belonging to UGT88 family were identified to have 6ʹ-deoxychalcone 4ʹ-glucosyltransferase activity in dahlia.6ʹ-Deoxychalcones (isoliquiritigenin and butein) are important pigments for yellow and orange to red flower color. 6ʹ-Deoxychalcones are glucosylated at the 4ʹ-position in vivo, but the genes encoding 6ʹ-deoxychalcone 4ʹ-glucosyltransferase have not yet been identified. In our previous study, it was indicated that snapdragon (Antirrhinum majus) chalcone 4ʹ-O-glucosyltransferase (Am4ʹCGT) has isoliquiritigenin 4ʹ-glucosylation activity. Therefore, to identify genes encoding 6ʹ-deoxychalcone 4ʹ-glucosyltransferase in dahlia (Dahlia variabilis), genes expressed in ray florets that shared high homology with Am4ʹCGT were explored. As a result, c34671_g1_i1 and c35662_g1_i1 were selected as candidate genes for 6ʹ-deoxychalcone 4ʹ-glucosyltransferases in dahlia. We conducted transient co-overexpression of three genes (c34671_g1_i1 or c35662_g1_i1, dahlia aldo-keto reductase1 (DvAKR1) or soybean (Glycine max) chalcone reductase5 (GmCHR5), and chili pepper (Capsicum annuum) MYB transcription factor (CaMYBA)) in Nicotiana benthamiana by agroinfiltration. Transient overexpression of c34671_g1_i1, DvAKR1, and CaMYBA resulted in increase in the accumulation of isoliquiritigenin 4ʹ-glucosides, isoliquiritigenin 4ʹ-O-glucoside, and isoliquiritigenin 4ʹ-O-[6-O-(malonyl)-glucoside]. However, transient overexpression of c35662_g1_i1, DvAKR1, and CaMYBA did not increase accumulation of isoliquiritigenin 4ʹ-glucosides. Using GmCHR5 instead of DvAKR1 showed similar results suggesting that c34671_g1_i1 has isoliquiritigenin 4ʹ-glucosyltransferase activity. In addition, we conducted co-overexpression of four genes (c34671_g1_i1, c35662_g1_i1 or Am4ʹCGT, DvAKR1 or GmCHR5, CaMYBA, and chalcone 3-hydroxylase from dahlia). Accumulation of butein 4ʹ-O-glucoside and butein 4ʹ-O-[6-O-(malonyl)-glucoside] was detected for c35662_g1_i1, suggesting that c35662_g1_i1 has butein 4ʹ-glucosyltransferase activity. Recombinant enzyme analysis also supported butein 4ʹ-glucosyltransferases activity of c35662_g1_i1. Therefore, our results suggested that both c34671_g1_i1 and c35662_g1_i1 are 6ʹ-deoxychalcone 4ʹ-glucosyltransferases but with different substrate preference.

The study was aimed to identify the factors that regulate the intensity of flower color in cyanic dahlia (Dahlia variabilis), using fifteen cultivars with different color intensities in their petals. The cultivars were classified into three groups based on their flavonoid composition: ivory white cultivars with flavones; purple and pink cultivars with flavones and anthocyanins; and red cultivars with flavones, anthocyanins, and chalcones. Among the purple, pink, and ivory white cultivars, an inverse relationship was detected between lightness, which was used as an indicator for color intensity and anthocyanin content. A positive correlation was detected between anthocyanin contents and the expression of some structural genes in the anthocyanin synthesis pathway that are regulated by DvIVS, a basic helix-loop-helix transcription factor. A positive correlation between anthocyanin content and expression of DvIVS was also found. The promoter region of DvIVS was classified into three types, with cultivars carrying Type. 1 promoter exhibited deep coloring, those carrying Type 2 and/or Type 3 exhibited pale coloring, and those carrying Type 1 and Type 2 and/or Type 3 exhibited medium coloring. The transcripts of the genes from these promoters encoded fulllength predicted proteins. These results suggested that the genotype of the promoter region in DvIVS is one of the key factors determining the flower color intensity.

Selenium nanoparticles (SeNPs) synthesized using Dahlia pinnata L tuber extract exhibit significant antimicrobial, antidiabetic, and anti-inflammatory properties. This study describes the green synthesis of SeNPs using D. pinnata L. tuber extract, a sustainable approach that leverages plant-based compounds. Green synthesis was confirmed via UV-Vis spectroscopy (280 nm peak), XRD trigonal crystal structure with planes (100), (101), (110), (111), and (201), and TEM (spherical particles, 17.37 nm average size). FTIR revealed functional groups (C–H, C = O, Se–Se), while HPLC identified gallic acid (1070.58 µg/g) and chlorogenic acid (903.87 µg/g). In antimicrobial studies, SeNPs demonstrated strong efficacy against 70 Escherichia coli isolates from laboratory samples of diabetic patients. For 15 multidrug-resistant (MDR, 21.4%) isolates, SeNPs had MICs of 25–50 µg/ml (mean: 35 ± 12 µg/ml) and MBCs of 50–100 µg/ml (mean: 76.6 ± 26 µg/ml). Non-MDR isolates (78.6%) were more sensitive, with MICs of 10–25 µg/ml (mean: 15 ± 4.5 µg/ml) and MBCs of 25–50 µg/ml (mean: 35 ± 12 µg/ml), outperforming selenium precursors and vancomycin. SeNPs also showed antidiabetic potential through α-amylase inhibition (IC 50  = 50.32 µg/ml) and α-glucosidase inhibition (IC 50  = 31.55 µg/ml), though less effective than acarbose (IC 50  = 5.85 µg/ml and 3.93 µg/ml, respectively). In anti-inflammatory assays, SeNPs achieved dose-dependent hemolysis inhibition, with 96.0% inhibition at 1000 µg/ml and an IC 50 of 11.53 µg/ml, compared to indomethacin’s IC 50 of 4.51 µg/ml. These findings demonstrate the promising in vitro bioactivities of SeNPs synthesized from D. pinnata L. tuber extract and support their potential for further preclinical investigation.

In this brief note, we review the taxonomic history of dahlia mosaic virus (DMV) and related viruses. DMV is the only officially recognized caulimovirus known to infect dahlia (Dahlia variabilis) plants, although this virus appears to be relatively rare as a pathogen compared to a more recently described but unclassified caulimovirus called dahlia common mosaic virus (DCMV). We have undertaken a new set of analyses to test the hypothesis that DCMV represents a new caulimovirus species whose members infect dahlia, but we ultimately reject this hypothesis. A probable sequencing error was identified in the reference genome sequence of DMV, and consequently, we recommend that an alternative virus isolate be nominated as the exemplar for this species. In accordance with the new binomial nomenclatural system, it is proposed that the virus species be called “Caulimovirus dahliae”.