The promoter structure differentiation of a MYB transcription factor RLC1 causes red leaf coloration in Empire Red Leaf Cotton under light

Identification and development of functional markers for purple grain genes in durum wheat (Triticum durum Desf.)

KEY MESSAGE

Two allelic variants of Pp-A3 and Pp-B1 were identified in purple durum wheat. Molecular markers at both loci were developed and validated on an independent panel, offering a breakthrough for wheat improvement. Purple wheats are a class of cereals with pigmented kernels of particular interest for their antioxidant and anti-inflammatory properties. Although two complementary loci (Pp-B1 and Pp-A3), responsible for purple pericarp have been pinpointed in bread wheat (Triticum aestivum L.), in durum wheat (Triticum durum Desf.) the causative genes along with functional and non-functional alleles are still unknown. Here, using a quantitative trait loci (QTL) mapping approach on a RIL population derived from purple and non-purple durum wheat genotypes, we identified three major regions on chromosomes 2A, 3A, and 7B explaining the highest phenotypic variation (> 50%). Taking advantage of the Svevo genome, a MYB was reannotated on chromosome 7B and reported as a candidate for Pp-B1. An insertion of ~ 1.6 kb within the first exon led to a non-functional allele (TdPpm1b), whereas the functional allele (TdPpm1a) was characterized and released for the first time in durum wheat. Pp-A3 was instead identified as a duplicated gene, of which only one was functional. The promoter sequencing of the functional allele (TdPpb1a) revealed six 261-bp tandem repeats in purple durum wheat, whereas one unit (TdPpb1b) was found in the yellow once. Functional molecular markers at both loci were developed to precisely discriminate purple and not purple genotypes, representing a valuable resource for selecting superior purple durum lines at early growth stages. Overall, our results expand the understanding of the function of MYB and bHLH activators in durum wheat, paving new ways to explore cis-regulatory elements at the promoter level.

In-silico mining and characterization of MYB family genes in wilt-resistant hybrid guava (Psidium guajava × Psidium molle)

BACKGROUND

The MYB family is one of the most significant groups of transcription factors in plants. However, several MYBs have been linked to secondary metabolism and are important for determining the color of fruit's peel and pulp. Despite being a substantial fruit crop in tropical and subtropical areas of the world, wilt-resistant hybrid guava (Psidium guajava × Psidium molle; PGPM) has not yet been the subject of a thorough examination. This study's goal was to assess the expression of MYB in guava fruit pulp, roots, and seeds to predict its function by in silico analysis of the guava root transcriptome data.

RESULTS

In the current study, we have mined the MYBs family of MYB genes from the transcriptome of the PGPM guava root. We have mined 15 distinct MYB transcription factor genes/transcripts viz MYB3, MYB4, MYB23, MYB86, MYB90, MYB308, MYB5, MYB82, MYB114, MYB6, MYB305, MYB44, MYB51, MYB46, and MYB330. From the analyses, it was found that R2-MYB and R3-MYB domains are conserved in all known guava MYB proteins. The expression of six different MYB TFs was examined using semi-quantitative RT-PCR in "Shweta" pulp (white colour pulp), "Lalit" pulp (red color pulp), "Lalit" root, and "Lalit" seed.

CONCLUSION

There were 15 MYB family members observed in guava. They were unequally distributed across the chromosomes, most likely as a result of gene duplication. Additionally, the expression patterns of the particular MYBs showed that MYB may be involved in the control of wilt, fruit ripening, seed development, and root development. Our results allow for a more thorough functional characterization of the guava MYB family genes and open the door to additional research into one essential MYB transcription factor family of genes and its involvement in the growth and ripening of guava fruit.

Integrated transcriptional and metabolomics signature pattern of pigmented wheat to insight the seed pigmentation and other associated features

Anthocyanin biosynthesis in plants is complex, especially in a polyploid monocot wheat plant. Using whole-genome sequencing, transcriptomics, and LC-MS/MS, we investigated anthocyanin pigmentation patterns in (black, blue, and purple) colored wheat seeds. According to differential gene expression profiling, 2AS-MYC, 7DL-MYB, and WD40 regulatory genes control purple pericarp coloration, 4DL-MYC, 2AS-MYC, 7DL-MYB, WD40 control blue aleurone coloration, and 4DL-MYC, 7DL-MYB, WD40 controls black aleurone color. We hypothesized that at least one MYC and MYB isoform is sufficient to regulate the anthocyanin synthesis in pericarp or aleurone. Transcriptomics and metabolomics revealed that the purple pericarp trait is associated with acylated anthocyanins compared to blue aleurone. Based upon the reduced expressions of the genes belonging to the 4D, SSR molecular marker mapping, variant calling using genome sequencing, and IGV browser gene structure visualization, it was inferred that the advanced black and blue wheat lines were substitution lines (4E{4D}), with very small recombinations. Pericarp anthocyanin pigmentation is controlled by a mutation in chromosome 2AS of purple wheat, and environmental variations influence pigmented pericarp trait. The expression patterns of anthocyanin structural and other genes varied in different colored wheat, corroborating differences in agronomical metrics. Ovate seed shape trait in black and blue wheat dragged with 4E chromosome.

Comparative Metabolome and Transcriptome Analysis of Anthocyanin Biosynthesis in White and Pink Petals of Cotton (Gossypium hirsutum L.)

Upland cotton (Gossypium hirsutum L.) is one of the important fiber crops. Cotton flowers usually appear white (or cream-colored) without colored spots at the petal base, and turn pink on the next day after flowering. In this study, using a mutant showing pink petals with crimson spots at their base, we conducted comparative metabolome and transcriptome analyses to investigate the molecular mechanism of coloration in cotton flowers. Metabolic profiling showed that cyanidin-3-O-glucoside and glycosidic derivatives of pelargonidins and peonidins are the main pigments responsible for the coloration of the pink petals of the mutant. A total of 2443 genes differentially expressed (DEGs) between the white and pink petals were identified by RNA-sequencing. Many DEGs are structural genes and regulatory genes of the anthocyanin biosynthesis pathway. Among them, MYB21, UGT88F3, GSTF12, and VPS32.3 showed significant association with the accumulation of cyanidin-3-O-glucoside in the pink petals. Taken together, our study preliminarily revealed the metabolites responsible for the pink petals and the key genes regulating the biosynthesis and accumulation of anthocyanins in the pink petals. The results provide new insights into the biochemical and molecular mechanism underlying anthocyanin biosynthesis in upland cotton.

Re enhances anthocyanin and proanthocyanidin accumulation to produce red foliated cotton and brown fiber

Red foliated cotton is a typical dominant mutation trait in upland cotton (Gossypium hirsutum). Although mutants have been described, few responsible genes have been identified and characterized. In this study, we performed map-based cloning of the red foliated mutant gene (Re) derived from the cross between G. hirsutum cv. Emian22 and G. barbadense acc. 3-79. Through expression profiling, metabolic pathway analysis, and sequencing of candidate genes, Re was identified as an MYB113 transcription factor. A repeat sequence variation in the promoter region increased the activity of the promoter, which enhanced the expression of Re. Re expression driven by the 35S promoter produced a red foliated phenotype, as expected. When the gene was driven by a fiber elongation-specific promoter, promoter of α-expansin 2 (PGbEXPA2), Re was specifically expressed in 5- to 10-day post-anthesis fibers rather than in other tissues, resulting in brown mature fibers. Re responded to light through phytochrome-interacting factor 4 and formed a dimer with transparent testa 8, which increased its expression as well as that of anthocyanin synthase and UDP-glucose:flavonoid 3-o-glucosyl transferase, and thus activated the entire anthocyanin metabolism pathway. Our research has identified the red foliated mutant gene in cotton, which paves the way for detailed studies of anthocyanin and proanthocyanidin metabolism and pigment accumulation in cotton and provides an alternative strategy for producing brown fiber.

Transcriptome Analysis Reveals Differences in Anthocyanin Accumulation in Cotton (Gossypium hirsutum L.) Induced by Red and Blue Light

Many factors, including illumination, affect anthocyanin biosynthesis and accumulation in plants. light quality is the key factor affecting the process of photoinduced anthocyanin biosynthesis and accumulation. We observed that the red color of the Upland cotton accession Huiyuan with the R1 mutation turned to normal green color under light-emitting diodes (LEDs), which inspired us to investigate the effect of red and blue lights on the biosynthesis and accumulation of anthocyanins. We found that both red and blue lights elevated accumulation of anthocyanins. Comparative transcriptomic analyses, including Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) and GSEA, revealed that genes differentially expressed under different light conditions were enriched with the pathways of circadian rhythm, phenylpropanoid biosynthesis, anthocyanin biosynthesis, and flavone and flavonol biosynthesis. Not surprisingly, all the major structural genes related to biosynthesis of anthocyanins, including the key regulatory MYB transcription factor (GhPAP1D) and anthocyanin transporter (GhGSTF12), were induced by red or blue light treatment. However, LARs and MATEs related to biosynthesis of proanthocyanidins were more significantly up-regulated by red light radiation than by blue light radiation. Vice versa, the accumulation of anthocyanins under red light was not as high as that under blue light. In addition, we demonstrated a potential role of GhHY5, a key regulator in plant circadian rhythms, in regulation of anthocyanin accumulation, which could be achieved via interaction with GhPAP1D. Together, these results indicate different effect of red and blue lights on biosynthesis and accumulation of anthocyanins and a potential module including GhHY5 and GhPAP1D in regulation of anthocyanin accumulation in cotton. These results also suggest that the substrates responsible the synthesis of anthocyanins under blue light is diverted to biosynthesis of proanthocyanidin under red light.

Multilevel regulation of anthocyanin-promoting R2R3-MYB transcription factors in plants

Anthocyanins are common secondary metabolites in plants that confer red, blue, and purple colorations in plants and are highly desired by consumers for their visual appearance and nutritional quality. In the last two decades, the anthocyanin biosynthetic pathway and transcriptional regulation of anthocyanin biosynthetic genes (ABGs) have been well characterized in many plants. From numerous studies on model plants and horticultural crops, many signaling regulators have been found to control anthocyanin accumulation via regulation of anthocyanin-promoting R2R3-MYB transcription factors (so-called R2R3-MYB activators). The regulatory mechanism of R2R3-MYB activators is mediated by multiple environmental factors (e.g., light, temperature) and internal signals (e.g., sugar, ethylene, and JA) in complicated interactions at multiple levels. Here, we summarize the transcriptional control of R2R3-MYB activators as a result of natural variations in the promoter of their encoding genes, upstream transcription factors and epigenetics, and posttranslational modifications of R2R3-MYB that determine color variations of horticultural plants. In addition, we focus on progress in elucidating the integrated regulatory network of anthocyanin biosynthesis mediated by R2R3-MYB activators in response to multiple signals. We also highlight a few gene cascade modules involved in the regulation of anthocyanin-related R2R3-MYB to provide insights into anthocyanin production in horticultural plants.

Incipient diploidization of the medicinal plant Perilla within 10,000 years

Perilla is a young allotetraploid Lamiaceae species widely used in East Asia as herb and oil plant. Here, we report the high-quality, chromosome-scale genomes of the tetraploid (Perilla frutescens) and the AA diploid progenitor (Perilla citriodora). Comparative analyses suggest post Neolithic allotetraploidization within 10,000 years, and nucleotide mutation in tetraploid is 10% more than in diploid, both of which are dominated by G:C → A:T transitions. Incipient diploidization is characterized by balanced swaps of homeologous segments, and subsequent homeologous exchanges are enriched towards telomeres, with excess of replacements of AA genes by fractionated BB homeologs. Population analyses suggest that the crispa lines are close to the nascent tetraploid, and involvement of acyl-CoA: lysophosphatidylcholine acyltransferase gene for high α-linolenic acid content of seed oil is revealed by GWAS. These resources and findings provide insights into incipient diploidization and basis for breeding improvement of this medicinal plant.

The combination of R2R3-MYB gene AmRosea1 and hairy root culture is a useful tool for rapidly induction and production of anthocyanins in Antirrhinum majus L

Anthocyanins are the largest group of water-soluble pigments and beneficial for human health. Although most plants roots have the potential to express natural biosynthesis pathways required to produce specialized metabolites such as anthocyanins, the anthocyanin synthesis is specifically silenced in roots. To explore the molecular mechanism of absence and production ability of anthocyanin in the roots, investigated the effect of a bHLH gene AmDelila, and an R2R3-MYB gene AmRosea1, which are the master regulators of anthocyanin biosynthesis in Antirrhinum majus flowers, by expressing these genes in transformed hairy roots of A. majus. Co-ectopic expression of both AmDelila and AmRosea1 significantly upregulated the expression of the key target structural genes in the anthocyanin biosynthesis pathway. Furthermore, this resulted in strongly enhanced anthocyanin accumulation in transformed hairy roots. Ectopic expression of AmDelila alone did not gives rise to any significant anthocyanin accumulation, however, ectopic expression of AmRosea1 alone clearly upregulated expression of the main structural genes as well as greatly promoted anthocyanin accumulation in transformed hairy roots, where the contents reached 0.773–2.064 mg/g fresh weight. These results suggest that AmRosea1 plays a key role in the regulatory network in controlling the initiation of anthocyanin biosynthesis in roots, and the combination of AmRosea1 and hairy root culture is a powerful tool to study and production of anthocyanins in the roots of A. majus.

Glutathione S-transferases GhGSTF1 and GhGSTF2 involved in the anthocyanin accumulation in Gossypium hirsutum L

The glutathione S-transferases (GSTs) are important enzymes of secondary metabolism in plants. In this study, two putative GSTs, GhGSTF1 and GhGSTF2, were identified as anthocyanin-related GSTs by the transcriptome data of the leaves of Gossypium hirsutum L. TM-1 and T586. The quantitative real-time PCR showed that GhGSTF1 and GhGSTF2 were highly expressed in red leaves and stems of Gossypium hirsutum L. T586. Orthologous genes of GhGSTF2 in two Gossypium barbadense L. 3-79 and Xinhai21 contain bases deletion in N-terminal (GbGSTF2a) and C-terminal (GbGSTF2b) respectively. Among which, GhGSTF1 and GhGSTF2 can restore pigmentation in hypocotyls of Arabidopsis thaliana mutant tt19-7 while GbGSTF2a and GbGSTF2b cannot. Furthermore, in vitro assays showed the recombinant GhGSTF1 and GhGSTF2 had Glutathione S-transferase activities. Fluorescence quenching assays showed that Cya could obviously quench the fluorescence of GhGSTF1, GhGSTF2, GbGSTF2a and GbGSTF2b to lower levels as compared to C3G. Moreover, the transient dual-luciferase assays showed that the promoters of GhGSTF1 and GhGSTF2 could be activated by GhPAP1D at different levels. GUS staining assays showed that their promoters have different activities to light. This study indicated that GhGSTF1 and GhGSTF2 play important roles in anthocyanin accumulation and the regulatory mechanism of anthocyanin accumulation in allotetraploid Gossypium are complicated.

A Stable Agrobacterium rhizogenes-Mediated Transformation of Cotton (Gossypium hirsutum L.) and Plant Regeneration From Transformed Hairy Root via Embryogenesis

Genetic transformation is a powerful tool to study gene function, secondary metabolism pathways, and molecular breeding in crops. Cotton (Gossypium hirsutum L.) is one of the most important economic crops in the world. Current cotton transformation methods take at least seven to culture and are labor-intensive and limited to some cultivars. In this study, we first time achieved plantlet regeneration of cotton via embryogenesis from transformed hairy roots. We inoculated the cotyledon explants of a commercial cultivar Zhongmian-24 with Agrobacterium rhizogenes strain AR1193, harboring a binary vector pBI-35S::GFP that contained the NPT II (neomycin phosphotransferase) gene and the GFP (green fluorescent protein) gene as a fluorescent marker in the T-DNA region. 82.6% explants produced adventitious roots, of which 53% showed GFP expression after transformation. 82% of transformed hairy roots produced embryonic calli, 12% of which regenerated into stable transformed cotton plants after 7 months of culture. The integration of GFP in the transformed cotton genomes were confirmed by PCR (Polymerase chain reaction) and Southern blot analysis as well as the stable expression of GFP were also detected by semi-quantitative RT-PCR analysis. The resultant transformed plantlets were phenotypically, thus avoiding Ri syndrome. Here we report a stable and reproducible method for A. rhizogenes-mediated transformation of cotton using cotyledon as explants, which provides a useful and reliable platform for gene function analysis of cotton.

Flavonoid accumulation in spontaneous cotton mutant results in red coloration and enhanced disease resistance

Cotton, the leading natural fiber, is cultivated worldwide, but its production is seriously threatened by pathogens. Accordingly, the selection of resistant cultivars has become a key priority of cotton breeding programs. In this study, a spontaneous mutant with red coloration (S156) and a control cultivar (S78) were used as experimental materials for a comparative analysis. Metabolomic analysis revealed the enrichment of flavonoids in S156 leaves compared with S78 leaves, and transcriptomic analysis revealed the upregulated expression of flavonoid biosynthesis genes in S156 leaves relative to S78 leaves. In addition, the red mutant showed a significantly increase in resistance to Verticillium dahliae, a fungal pathogen that poses a major threat to cotton production. The pathogen invasion process was suppressed in the red cotton cultivar. This study reveals the mechanism underlying the red coloration of S156 cotton and indicates the great potential of red cotton in pathogen- and insect-resistant breeding of cotton.

Evolutionary comparison of competitive protein-complex formation of MYB, bHLH, and WDR proteins in plants

A protein complex consisting of a MYB, basic Helix-Loop-Helix, and a WDR protein, the MBW complex, regulates five traits, namely the production of anthocyanidin, proanthocyanidin, and seed-coat mucilage, and the development of trichomes and root hairs. For complexes involved in trichome and root hair development it has been shown that the interaction of two MBW proteins can be counteracted by the respective third protein (called competitive complex formation). We examined competitive complex formation for selected MBW proteins from Arabidopsis thaliana, Arabis alpina, Gossypium hirsutum, Petunia hybrida, and Zea mays. Quantitative analyses of the competitive binding of MYBs and WDRs to bHLHs were done by pull-down assays using ProtA- and luciferase-tagged proteins expressed in human HEC cells. We found that some bHLHs show competitive complex formation whilst others do not. Competitive complex formation strongly correlated with a phylogenetic tree constructed with the bHLH proteins under investigation, suggesting a functional relevance. We demonstrate that this different behavior can be explained by changes in one amino acid and that this position is functionally relevant in trichome development but not in anthocyanidin regulation.

Evolutionary Analysis of MBW Function by Phenotypic Rescue in Arabidopsis thaliana

The MBW complex consisting of the three proteins R2R3MYB, bHLH and WDR regulates five traits in Arabidopsis thaliana including trichome and root hair patterning, seed coat color, anthocyanidin production and seed coat mucilage release. The WDR gene TTG1 regulates each trait in specific combinations with different bHLH and R2R3MYB proteins. In this study we analyze to what extent the biochemical properties of the MBW proteins contribute to trait specificity by expressing them in appropriate A. thaliana mutants. We show that the rescue behavior of A. thaliana bHLH and R2R3MYB protein is sufficient to explain the function as derived previously from mutant analysis. When extending this rescue approach using MBW proteins from other species we find that proteins involved in anthocyanidin regulation typically show a rescue of the anthocyanidin phenotype but not of the other traits. Finally, we correlate the rescue abilities of MBW protein from different species with the A. thaliana proteins.

Two transcription factors TaPpm1 and TaPpb1 co-regulate anthocyanin biosynthesis in purple pericarps of wheat

Purple pericarps of bread wheat (Triticum aestivum L.) are a useful source of dietary anthocyanins. Previous mapping results indicated that the purple pericarp trait is controlled by two complementary genes located on chromosomes 7D and 2A. However, the identity of the genes and the mechanisms by which they regulate the trait are unknown. In this study, two transcription factors were characterised as anthocyanin activators in purple pericarps: TaPpm1 (purple pericarp-MYB 1) and TaPpb1 (purple pericarp-bHLH 1). Three non-functional variants were detected in the coding sequence of TaPpm1 from non-purple seed lines, in which the function of TaPpm1 was destroyed either by insertion-induced frame shifts or truncated peptides. There were six 261-bp tandem repeats in the promoter region of TaPpb1 in the purple-grained varieties, while there was only one repeat unit present in the non-purple varieties. Furthermore, using yeast two-hybrid, dual luciferase, yeast one-hybrid, and transient assays, we were able to demonstrate that the interaction of TaPpm1 and TaPpb1 co-regulates the synthesis of anthocyanin. Overall, our results provide a better understanding of the molecular basis of anthocyanin synthesis in the wheat pericarp and indicate the existence of an integrated regulatory mechanism that controls production.

A Dual Repeat Cis-Element Determines Expression of GERANYL DIPHOSPHATE SYNTHASE for Monoterpene Production in Phalaenopsis Orchids

Phalaenopsis bellina is a scented orchid emitting large amount of monoterpenes. GERANYL DIPHOSPHATE SYNTHASE (PbGDPS) is the key enzyme for monoterpene biosynthesis, and shows concomitant expression with the emission of monoterpenes during flower development in P. bellina. Here, we identified a dual repeat cis-element in the GDPS promoter that is critical for monoterpene biosynthesis in Phalaenopsis orchids. A strong correlation between the dual repeat and the monoterpene production was revealed by examination of the GDPS promoter fragments over 12 Phalaenopsis species. Serial-deletion of the 2-kb GDPS promoter fragments demonstrated that the integrity of the dual repeat was crucial for its promoter activities. By screening the Arabidopsis transcription factors (TFs) cDNA library using yeast one-hybrid assay, AtbZIP18, a member of group I of bZIP TFs, was identified to be able to bind the dual repeat. We then identified PbbZIP4 in the transcriptome of P. bellina, showing 83% identity in the DNA binding region with that of AtbZIP18, and the expression level of PbbZIP4 was higher in the scented orchids. In addition, PbbZIP4 transactivated the GDPS promoter fragment containing the dual repeat in dual luciferase assay. Furthermore, transient ectopic expression of PbbZIP4 induced a 10-fold production of monoterpenoids in the scentless orchid. In conclusion, these results indicate that the dual repeat is a real TF-bound cis-element significant for GDPS gene expression, and thus subsequent monoterpene biosynthesis in the scented Phalaenopsis orchids.

TRANSPARENT TESTA GLABRA 1-Dependent Regulation of Flavonoid Biosynthesis

The flavonoid composition of various tissues throughout plant development is of biological relevance and particular interest for breeding. Arabidopsis thaliana TRANSPARENT TESTA GLABRA 1 (AtTTG1) is an essential regulator of late structural genes in flavonoid biosynthesis. Here, we provide a review of the regulation of the pathway's core enzymes through AtTTG1-containing R2R3-MYELOBLASTOSIS-basic HELIX-LOOP-HELIX-WD40 repeat (MBW(AtTTG1)) complexes embedded in an evolutionary context. We present a comprehensive collection of A. thalianattg1 mutants and AtTTG1 orthologs. A plethora of MBW(AtTTG1) mechanisms in regulating the five major TTG1-dependent traits is highlighted.

Overexpression of AmRosea1 Gene Confers Drought and Salt Tolerance in Rice

Ectopic expression of the MYB transcription factor of AmROSEA1 from Antirrhinum majus has been reported to change anthocyanin and other metabolites in several species. In this study, we found that overexpression of AmRosea1 significantly improved the tolerance of transgenic rice to drought and salinity stresses. Transcriptome analysis revealed that a considerable number of stress-related genes were affected by exogenous AmRosea1 during both drought and salinity stress treatments. These affected genes are involved in stress signal transduction, the hormone signal pathway, ion homeostasis and the enzymes that remove peroxides. This work suggests that the AmRosea1 gene is a potential candidate for genetic engineering of crops.

Functional diversification of the potato R2R3 MYB anthocyanin activators AN1, MYBA1, and MYB113 and their interaction with basic helix-loop-helix cofactors

In potato (Solanum tuberosum L.), R2R3 MYBs are involved in the regulation of anthocyanin biosynthesis. We examined sequences of these MYBs in cultivated potatoes, which are more complex than diploid potato due to ploidy and heterozygosity. We found amino acid variants in the C-terminus of the MYB StAN1, termed R0, R1, and R3, due to the presence of a repeated 10-amino acid motif. These variant MYBs showed some expression in both white and pigmented tubers. We found several new alleles or gene family members of R2R3 MYBs,StMYBA1 and StMYB113, which were also expressed in white potato tubers. From functional analysis in tobacco, we showed that the presence of a C-terminal 10-amino acid motif is optimal for activating anthocyanin accumulation. Engineering a motif back into a MYB lacking this sequence enhanced its activating ability. Versions of StMYBA1 and StMYB113 can also activate anthocyanin accumulation in tobacco leaves, with the exception of StMYB113-3, which has a partial R2R3 domain. We isolated five family members of potato StbHLH1, and one StJAF13, to test their ability to interact with MYB variants. The results showed that two alleles of StbHLH1 from white skin and red skin are non-functional, while three other StbHLH1s have different co-regulating abilities, and need to be activated by StJAF13. Combined with expression analysis in potato tuber, results suggest that StbHLH1 and StJAF13a re key co-regulators of anthocyanin biosynthesis, while the transcripts of MYB variants StAN1,StMYBA1, and StMYB113 are well expressed, even in the absence of pigmentation.

Transcriptomic analyses reveal species-specific light-induced anthocyanin biosynthesis in chrysanthemum

BACKGROUND

The flower colour of agricultural products is very important for their commercial value, which is mainly attributed to the accumulation of anthocyanins. Light is one of the key environmental factors that affect the anthocyanin biosynthesis. However, the deep molecular mechanism remains elusive, and many problems regarding the phenotypic change and the corresponding gene regulation are still unclear. In the present study, Chrysanthemum × morifolium 'Purple Reagan', a light-responding pigmentation cultivar, was selected to investigate the mechanism of light-induced anthocyanin biosynthesis using transcriptomic analyses.

RESULTS

Only cyanidin derivatives were identified based on the analyses of the pigmentation in ray florets. Shading experiments revealed that the capitulum was the key organ and that its bud stage was the key phase responding to light. These results were used to design five libraries for transcriptomic analyses, including three capitulum developmental stages and two light conditions. RNA sequences were de novo assembled into 103,517 unigenes, of which 60,712 were annotated against four public protein databases. As many as 2,135 unigenes were differentially expressed between the light and dark libraries with 923 up-regulated and 1,212 down-regulated unigenes in response to shading. Next, interactive pathway analysis showed that the anthocyanin biosynthetic pathway was the only complete metabolic pathway both modulated in response to light and related to capitulum development. Following the shading treatment, nearly all structural genes involved in the anthocyanin biosynthetic pathway were down-regulated. Moreover, three CmMYB genes and one CmbHLH gene were identified as key transcription factors that might participate in the regulation of anthocyanin biosynthesis under light conditions based on clustering analysis and validation by RT-qPCR. Finally, a light-induced anthocyanin biosynthesis pathway in chrysanthemums was inferred.

CONCLUSION

The pigmentation of the ray florets of chrysanthemum cultivar 'Purple Reagan' is dependent on light. During the light-induced pigmentation process, the expression of seven structural genes in the anthocyanin biosynthetic pathway (regulated by at least four transcription factors in response to light) are the main contributors to the pigmentation of chrysanthemums. This information will further our understanding of the molecular mechanisms governing light-induced anthocyanin biosynthesis in ornamental plants.

High AN1 variability and interaction with basic helix-loop-helix co-factors related to anthocyanin biosynthesis in potato leaves

AN1 is a regulatory gene that promotes anthocyanin biosynthesis in potato tubers and encodes a R2R3 MYB transcription factor. However, no clear evidence implicates AN1 in anthocyanin production in leaves, where these pigments might enhance environmental stress tolerance. In our study we found that AN1 displays intraspecific sequence variability in both coding/non-coding regions and in the promoter, and that its expression is associated with high anthocyanin content in leaves of commercial potatoes. Expression analysis provided evidence that leaf pigmentation is associated to AN1 expression and that StJAF13 acts as putative AN1 co-regulator for anthocyanin gene expression in leaves of the red leaf variety 'Magenta Love,' while a concomitant expression of StbHLH1 may contribute to anthocyanin accumulation in leaves of 'Double Fun.' Yeast two-hybrid experiments confirmed that AN1 interacts with StbHLH1 and StJAF13 and the latter interaction was verified and localized in the cell nucleus by bimolecular fluorescence complementation assays. In addition, transgenic tobacco (Nicotiana tabacum) overexpressing a combination of either AN1 with StJAF13 or AN1 with StbHLH1 showed deeper purple pigmentation with respect to AN1 alone. This further confirmed AN1/StJAF13 and AN1/StbHLH1 interactions. Our findings demonstrate that the classical loci identified for potato leaf anthocyanin accumulation correspond to AN1 and may represent an important step to expand our knowledge on the molecular mechanisms underlying anthocyanin biosynthesis in different plant tissues.