RHA2b-mediated MYB30 degradation facilitates MYB75-regulated, sucrose-induced anthocyanin biosynthesis in Arabidopsis seedlings

Anthocyanins play diverse roles in plant physiology and stress adaptation. In Arabidopsis, the MYB-bHLH-WD40 (MBW) complex has a crucial role in the regulation of anthocyanin synthesis. Here, we report that the R2R3-MYB transcription factor MYB30 and the ubiquitin E3 ligase RHA2b participate in anthocyanin biosynthesis through regulation of the MBW complex. MYB30 was found to negatively regulate sucrose-induced anthocyanin biosynthesis in Arabidopsis seedlings. Expression of multiple genes involved in flavonoid or anthocyanin biosynthesis was affected in the myb30 mutant, and MYB30 directly repressed the expression of MYB75, which encodes a core component of the MBW complex, by binding to its promoter. Moreover, MYB30 physically interacted with MYB75 to inhibit its activity by repressing MBW complex assembly. In addition, sucrose treatment significantly promoted MYB30 degradation via the action of RHA2b. The ubiquitination and degradation of MYB30 were significantly attenuated in the rha2b mutant under high-sucrose treatment, and further analysis showed that MYB75 directly promoted RHA2b expression in response to high sucrose. Our work thus reveals an anthocyanin biosynthetic regulatory module, RHA2b-MYB30, that controls the function of the MBW complex via MYB75. The repression of MYB75 by MYB30 is released by MYB75-induced RHA2b expression, thus ensuring the self-activation of MYB75 when anthocyanin synthesis is needed.

Anthocyanin biosynthesis in goji berry is inactivated by deletion in a bHLH transcription factor LrLAN1b promoter

Black goji berry (Lycium ruthenicum Murray) contains a rich source of health-promoting anthocyanins which are used in herbal medicine and nutraceutical foods in China. A natural variant producing white berries allowed us to identify two key genes involved in the regulation of anthocyanin biosynthesis in goji berries: one encoding a MYB transcription factor (LrAN2-like) and one encoding a basic helix-loop-helix (bHLH) transcription factor (LrAN1b). We previously found that LrAN1b expression was lost in the white berry variant, but the molecular basis for this phenotype was unknown. Here, we identified the molecular mechanism for loss of anthocyanins in white goji berries. In white goji, the LrAN1b promoter region has a 229-bp deletion that removes 3 MYB-binding elements and 1 bHLH-binding element, which are key to its expression. Complementation of the white goji berry LrAN1b allele with the LrAN1b promoter restored pigmentation. Virus-induced gene silencing of LrAN1b in black goji berry reduced fruit anthocyanin biosynthesis. Molecular analyses showed that LrAN2-like and another bHLH transcription factor LrJAF13 can activate LrAN1b by binding directly to the MYB-recognizing element (MRE) and bHLH-recognizing element (BRE) of its promoter-deletion region. LrAN1b expression is enhanced by the interaction of LrAN2-like with LrJAF13 and the WD40 protein LrAN11. LrAN2-like and LrAN11 interact with either LrJAF13 or LrAN1b to form two MYB-bHLH-WD40 (MBW) complexes, which hierarchically regulate anthocyanin biosynthesis in black goji berry. This study on a natural variant builds a comprehensive anthocyanin regulatory network that may be manipulated to tailor goji berry traits.

Multiple mechanisms explain loss of anthocyanins from betalain-pigmented Caryophyllales, including repeated wholesale loss of a key anthocyanidin synthesis enzyme

In this study, we investigate the genetic mechanisms responsible for the loss of anthocyanins in betalain-pigmented Caryophyllales, considering our hypothesis of multiple transitions to betalain pigmentation. Utilizing transcriptomic and genomic datasets across 357 species and 31 families, we scrutinize 18 flavonoid pathway genes and six regulatory genes spanning four transitions to betalain pigmentation. We examined evidence for hypotheses of wholesale gene loss, modified gene function, altered gene expression, and degeneration of the MBW (MYB-bHLH-WD40) trasnscription factor complex, within betalain-pigmented lineages. Our analyses reveal that most flavonoid synthesis genes remain conserved in betalain-pigmented lineages, with the notable exception of TT19 orthologs, essential for the final step in anthocyanidin synthesis, which appear to have been repeatedly and entirely lost. Additional late-stage flavonoid pathway genes upstream of TT19 also manifest strikingly reduced expression in betalain-pigmented species. Additionally, we find repeated loss and alteration in the MBW transcription complex essential for canonical anthocyanin synthesis. Consequently, the loss and exclusion of anthocyanins in betalain-pigmented species appear to be orchestrated through several mechanisms: loss of a key enzyme, downregulation of synthesis genes, and degeneration of regulatory complexes. These changes have occurred iteratively in Caryophyllales, often coinciding with evolutionary transitions to betalain pigmentation.

Role of epigenetic and post-translational modifications in anthocyanin biosynthesis: A review

Anthocyanins are a class of flavonoids having antioxidant and anti-inflammatory properties. They defend plants against various biotic and abiotic stresses and are synthesized by a specific branch of the flavonoid biosynthetic pathway. Different regulatory mechanisms have been found to regulate anthocyanin biosynthesis in plants. These include the MYB-bHLH-WDR (MBW) MBW trimeric complex consisting of bHLH, R2R3 MYB, and WD40 transcription factors. Epigenetic and Post-translational modification (PTMs) of MBW complex and various other transcription factors play important role in both plant developmental processes and modulating plant response to different environmental conditions. Recent studies have broadened our understanding of the role of various epigenetic (methylation and histone modification) and PTMs (phosphorylation, acetylation, ubiquitylation, sumoylation, etc.) mechanisms in regulating anthocyanin biosynthesis in plants. In this review, we are updating various epigenetic and PTMs modifications of various transcription factors which regulate anthocyanin biosynthesis in various plants. In addition to this, we have also briefly discussed in which direction future research on epigenetic and PTMs can be taken so that we can engineer medicinal plants for enhanced secondary metabolite biosynthesis.

SlMYB7, an AtMYB4-Like R2R3-MYB Transcription Factor, Inhibits Anthocyanin Accumulation in Solanum lycopersicum Fruits

Tomato is a horticultural crop with an incomplete flavonoid metabolic pathway that does not typically accumulate anthocyanins in the fruit. In recent years, intensive studies of the loci Anthocyanin fruit (Aft) and atroviolacium (atv) have clarified the functions of positive regulators (R2R3-MYBs) and a negative regulator (CPC-MYB) in anthocyanin biosynthesis in the fruits. However, little is known about the R2R3-MYB repressors. Here, we used transient overexpression analysis to show that SlMYB7, a subgroup 4 AtMYB4-like R2R3-MYB, inhibited anthocyanin accumulation and reduced expression of anthocyanin synthase genes in the 'black pearl' tomato fruits, which usually accumulate high concentrations of anthocyanins. These findings revealed that SlMYB7 served as a repressor of anthocyanin production. Furthermore, SlMYB7 actively repressed SlANS expression by binding its promoter and passively inhibited anthocyanin synthesis by interacting with the basic helix-loop-helix (bHLH) proteins SlJAF13 and SlAN1, which are involved in the formation of MBW complexes. Thus, SlMYB7 and the MBW complex may coregulate the anthocyanin content of 'black pearl' tomato fruits via a negative feedback loop. These findings provide a theoretical basis for the future enhancement of tomato anthocyanin contents through genetic manipulation of the biosynthetic regulatory network.

Environmental Stimuli and Phytohormones in Anthocyanin Biosynthesis: A Comprehensive Review

Anthocyanin accumulation in plants plays important roles in plant growth and development, as well as the response to environmental stresses. Anthocyanins have antioxidant properties and play an important role in maintaining the reactive oxygen species (ROS) homeostasis in plant cells. Furthermore, anthocyanins also act as a "sunscreen", reducing the damage caused by ultraviolet radiation under high-light conditions. The biosynthesis of anthocyanin in plants is mainly regulated by an MYB-bHLH-WD40 (MBW) complex. In recent years, many new regulators in different signals involved in anthocyanin biosynthesis were identified. This review focuses on the regulation network mediated by different environmental factors (such as light, salinity, drought, and cold stresses) and phytohormones (such as jasmonate, abscisic acid, salicylic acid, ethylene, brassinosteroid, strigolactone, cytokinin, and auxin). We also discuss the potential application value of anthocyanin in agriculture, horticulture, and the food industry.

Genome-Wide Identification of PAP1 Direct Targets in Regulating Seed Anthocyanin Biosynthesis in Arabidopsis

Anthocyanins are widespread water-soluble pigments in the plant kingdom. Anthocyanin accumulation is activated by the MYB-bHLH-WD40 (MBW) protein complex. In Arabidopsis, the R2R3-MYB transcription factor PAP1 activates anthocyanin biosynthesis. While prior research primarily focused on seedlings, seeds received limited attention. This study explores PAP1's genome-wide target genes in anthocyanin biosynthesis in seeds. Our findings confirm that PAP1 is a positive regulator of anthocyanin biosynthesis in Arabidopsis seeds. PAP1 significantly increased anthocyanin content in developing and mature seeds in Arabidopsis. Transcriptome analysis at 12 days after pollination reveals the upregulation of numerous genes involved in anthocyanin accumulation in 35S:PAP1 developing seeds. Chromatin immunoprecipitation and dual luciferase reporter assays demonstrate PAP1's direct promotion of ten key genes and indirect upregulation of TT8, TTG1, and eight key genes during seed maturation, thus enhancing seed anthocyanin accumulation. These findings enhance our understanding of PAP1's novel role in regulating anthocyanin accumulation in Arabidopsis seeds.

Molecular understanding of anthocyanin biosynthesis activated by PAP1 and regulated by 2, 4-dichlorophenoxyacetic acid in engineered red Artemisia annua cells

MAIN CONCLUSION

Eight promoters were cloned, from which AC and G-box cis-elements were identified. PAP1 enhanced the promoter activity. 2,4-D reduced the anthocyanin biosynthesis via downregulating the expression of the PAP1 transgene. Artemisia annua is an effective antimalarial medicinal crop. We have established anthocyanin-producing red cell cultures from this plant with the overexpression of Production of Anthocyanin Pigment 1 (PAP1) encoding a R2R3MYB transcription factor. To understand the molecular mechanism by which PAP1 activated the entire anthocyanin pathway, we mined the genomic sequences of A. annua and obtained eight promoters of the anthocyanin pathway genes. Sequence analysis identified four types of AC cis-elements from six promoters, the MYB response elements (MRE) bound by PAP1. In addition, six promoters were determined to have at least one G-box cis-element. Eight promoters were cloned for activity analysis. Dual luciferase assays showed that PAP1 significantly enhanced the promoting activity of seven promoters, indicating that PAP1 turned on the biosynthesis of anthocyanins via the activation of these pathway gene expression. To understand how 2,4-dichlorophenoxyacetic acid (2,4-D), an auxin, regulates the PAP1-activated anthocyanin biosynthesis, five different concentrations (0, 0.05, 0.5, 2.5, and 5 µM) were tested to characterize anthocyanin production and profiles. The resulting data showed that the concentrations tested decreased the fresh weight of callus growth, anthocyanin levels, and the production of anthocyanins per Petri dish. HPLC-qTOF-MS/MS-based profiling showed that these concentrations did not alter anthocyanin profiles. Real-time RT-PCR was completed to characterize the expression PAP1 and four representative pathway genes. The results showed that the five concentrations reduced the expression levels of the constitutive PAP1 transgene and three pathway genes significantly and eliminated the expression of the chalcone synthase gene either significantly or slightly. These data indicate that the constitutive PAP1 expression depends on gradients added in the medium. Based on these findings, the regulation of 2,4-D is discussed for anthocyanin engineering in red cells of A. annua.

SnRK1 inhibits anthocyanin biosynthesis through both transcriptional regulation and direct phosphorylation and dissociation of the MYB/bHLH/TTG1 MBW complex

Plants have evolved an extensive specialized secondary metabolism. The colorful flavonoid anthocyanins, for example, not only stimulate flower pollination and seed dispersal, but also protect different tissues against high light, UV and oxidative stress. Their biosynthesis is highly regulated by environmental and developmental cues and induced by high sucrose levels. Expression of the biosynthetic enzymes involved is controlled by a transcriptional MBW complex, comprising (R2R3) MYB- and bHLH-type transcription factors and the WD40 repeat protein TTG1. Anthocyanin biosynthesis is not only useful, but also carbon- and energy-intensive and non-vital. Consistently, the SnRK1 protein kinase, a metabolic sensor activated in carbon- and energy-depleting stress conditions, represses anthocyanin biosynthesis. Here we show that Arabidopsis SnRK1 represses MBW complex activity both at the transcriptional and post-translational level. In addition to repressing expression of the key transcription factor MYB75/PAP1, SnRK1 activity triggers MBW complex dissociation, associated with loss of target promoter binding, MYB75 protein degradation and nuclear export of TTG1. We also provide evidence for direct interaction with and phosphorylation of multiple MBW complex proteins. These results indicate that repression of expensive anthocyanin biosynthesis is an important strategy to save energy and redirect carbon flow to more essential processes for survival in metabolic stress conditions.

CsMYBL2 homologs modulate the light and temperature stress-regulated anthocyanin and catechins biosynthesis in tea plants (Camellia sinensis)

Anthocyanin and catechin production in tea (Camellia sinensis) leaves can positively affect tea quality; however, their regulatory mechanisms are not fully understood. Here we report that, while the CsMYB75- or CsMYB86-directed MYB-bHLH-WD40 (MBW) complexes differentially activate anthocyanin or catechin biosynthesis in tea leaves, respectively, CsMYBL2a and CsMYBL2b homologs negatively modified the light- and temperature-induced anthocyanin and catechin production in both Arabidopsis and tea plants. The MBW complexes activated both anthocyanin synthesis genes and the downstream repressor genes CsMYBL2a and CsMYBL2b. Overexpression of CsMYBL2b, but not CsMYBL2a, repressed Arabidopsis leaf anthocyanin accumulation and seed coat proanthocyanin production. CsMYBL2b strongly and CsMYBL2a weakly repressed the activating effects of CsMYB75/CsMYB86 on CsDFR and CsANS, due to their different EAR and TLLLFR domains and interactions with CsTT8/CsGL3, interfering with the functions of activating MBW complexes. CsMYBL2b and CsMYBL2a in tea leaves play different roles in fine-tuning CsMYB75/CsMYB86-MBW activation of biosynthesis of anthocyanins and catechins, respectively. The CsbZIP1-CsmiR858a-CsMYBL2 module mediated the UV-B- or cold-activated CsMYB75/CsMYB86 regulation of anthocyanin/catechin biosynthesis by repressing CsMYBL2a and CsMYBL2b. Similarly, the CsCOP1-CsbZIP1-CsPIF3 module, and BR signaling as well, mediated the high temperature repression of anthocyanin and catechin biosynthesis through differentially upregulating CsMYBL2b and CsMYBL2a, respectively. The present study provides new insights into the complex regulatory networks in environmental stress-modified flavonoid production in tea plant leaves.

A novel R2R3-MYB transcription factor FaMYB5 positively regulates anthocyanin and proanthocyanidin biosynthesis in cultivated strawberries (Fragaria × ananassa)

Flavonoids have a major contribution to the fruit quality in cultivated strawberries and are regulated by MYB, bHLH and WD40 transcriptional factors. We reported here the identification of the FaMYB5, an R2R3-MYB transcription factor, which positively regulated the accumulation of anthocyanins and proanthocyanidins through the trans-activation of the F3'H and LAR. The strawberry FaEGL3 and FaLWD1/FaLWD1-like interact with the R2R3-FaMYB5 to form an MYB-bHLH-WD40 complex (MBW), enhancing the regulatory efficiency. The R2R3-FaMYB5 was constitutively expressed in various tissues and in fruits of different developmental stages, which was strikingly contrasting to the fruit-specific expression patterns of FaMYB10. Meanwhile, R2R3-FaMYB5 failed to promote a stable accumulation of anthocyanin glycosides in the mature fruits of the myb10 mutant, mainly due to the suppressed expression of TT19. The R2R3-FaMYB5 was regulated by an antisense long noncoding RNA lncRNA-myb5. Additionally, the R2R3-FaMYB5 protein could interact with FaBT2 and was degraded through the ubiquitin/26 S proteasome pathway. Transcriptome and metabolome data showed that R2R3-FaMYB5 enhanced the gene expression and the metabolite accumulation involved in the flavonoid, phenylpropanoid and lignin biosynthesis pathways. Collectively, we conclude that the FaMYB5 is an R2R3-MYB activator involved in the composition of MBW, which positively regulates the biosynthesis of anthocyanin and proanthocyanidin. These findings provided new insights into the molecular mechanisms that regulate flavonoids in strawberry fruits.

Impact of High Light Intensity and Low Temperature on the Growth and Phenylpropanoid Profile of Azolla filiculoides

Exposure to high light intensity (HL) and cold treatment (CT) induces reddish pigmentation in Azolla filiculoides, an aquatic fern. Nevertheless, how these conditions, alone or in combination, influence Azolla growth and pigment synthesis remains to be fully elucidated. Likewise, the regulatory network underpinning the accumulation of flavonoids in ferns is still unclear. Here, we grew A. filiculoides under HL and/or CT conditions for 20 days and evaluated the biomass doubling time, relative growth rate, photosynthetic and non-photosynthetic pigment contents, and photosynthetic efficiency by chlorophyll fluorescence measurements. Furthermore, from the A. filiculoides genome, we mined the homologs of MYB, bHLH, and WDR genes, which form the MBW flavonoid regulatory complex in higher plants, to investigate their expression by qRT-PCR. We report that A. filiculoides optimizes photosynthesis at lower light intensities, regardless of the temperature. In addition, we show that CT does not severely hamper Azolla growth, although it causes the onset of photoinhibition. Coupling CT with HL stimulates the accumulation of flavonoids, which likely prevents irreversible photoinhibition-induced damage. Although our data do not support the formation of MBW complexes, we identified candidate MYB and bHLH regulators of flavonoids. Overall, the present findings are of fundamental and pragmatic relevance to Azolla's biology.

UV-C Promotes the Accumulation of Flavane-3-ols in Juvenile Fruit of Grape through Positive Regulating VvMYBPA1

Flavane-3-ol monomers are the precursors of proanthocyanidins (PAs), which play a crucial role in grape resistance. Previous studies showed that UV-C positively regulated leucoanthocyanidin reductase (LAR) enzyme activity to promote the accumulation of total flavane-3-ols in juvenile grape fruit, but its molecular mechanism was still unclear. In this paper, we found that the contents of flavane-3-ol monomers increased dramatically at the early development stage grape fruit after UV-C treatment, and the expression of its related transcription factor VvMYBPA1 was also enhanced significantly. The contents of (-)-epicatechin and (+)-catechin, the expression level of VvLAR1 and VvANR, and the activities of LAR and anthocyanidin reductase (ANR) were improved significantly in the VvMYBPA1 overexpressed grape leaves compared to the empty vector. Both VvMYBPA1 and VvMYC2 could interact with VvWDR1 using bimolecular fluorescence complementation (BiFC) and yeast two hybrid (Y2H). Finally, VvMYBPA1 was proven to bind with the promoters of VvLAR1 and VvANR by yeast one hybrid (Y1H). To sum up, we found that the expression of VvMYBPA1 increased in the young stage of grape fruit after UV-C treatment. VvMYBPA1 formed a trimer complex with VvMYC2 and VvWDR1 to regulate the expression of VvLAR1 and VvANR, thus positively promoting the activities of LAR and ANR enzyme, and eventually improved the accumulation of flavane-3-ols in grape fruit.

Genome-wide identification of WD40 transcription factors and their regulation of the MYB-bHLH-WD40 (MBW) complex related to anthocyanin synthesis in Qingke (Hordeum vulgare L. var. nudum Hook. f.)

BACKGROUND

WD40 transcription factors, a large gene family in eukaryotes, are involved in a variety of growth regulation and development pathways. WD40 plays an important role in the formation of MYB-bHLH-WD (MBW) complexes associated with anthocyanin synthesis, but studies of Qingke barley are lacking.

RESULTS

In this study, 164 barley HvWD40 genes were identified in the barley genome and were analyzed to determine their relevant bioinformatics. The 164 HvWD40 were classified into 11 clusters and 14 subfamilies based on their structural and phylogenetic protein profiles. Co-lineage analysis revealed that there were 43 pairs between barley and rice, and 56 pairs between barley and maize. Gene ontology (GO) enrichment analysis revealed that the molecular function, biological process, and cell composition were enriched. The Kyoto Encyclopedia of Genes and Genomes (KEGG) results showed that the RNA transport pathway was mainly enriched. Based on the identification and analysis of the barley WD40 family and the transcriptome sequencing (RNA-seq) results, we found that HvWD40-140 (WD40 family; Gene ID: r1G058730), HvANT1 (MYB family; Gene ID: HORVU7Hr1G034630), and HvANT2 (bHLH family; Gene ID: HORVU2Hr1G096810) were important components of the MBW complex related to anthocyanin biosynthesis in Qingke, which was verified via quantitative real-time fluorescence polymerase chain reaction (qRT-PCR), subcellular location, yeast two-hybrid (Y2H), and bimolecular fluorescent complimentary (BiFC) and dual-luciferase assay analyses.

CONCLUSIONS

In this study, we identified 164 HvWD40 genes in barley and found that HvnANT1, HvnANT2, and HvWD40-140 can form an MBW complex and regulate the transcriptional activation of the anthocyanin synthesis related structural gene HvDFR. The results of this study provide a theoretical basis for further study of the mechanism of HvWD40-140 in the MBW complex related to anthocyanin synthesis in Qingke.

Regulation of flavonoids in strawberry fruits by FaMYB5/FaMYB10 dominated MYB-bHLH-WD40 ternary complexes

Anthocyanins endowing strawberry fruit red color are regulated by the MYB-bHLH-WD40 complex. By analyzing the MYBs involved in the flavonoid biosynthesis in strawberry, we found that R2R3-FaMYB5 promoted the content of anthocyanin and proanthocyanidins in strawberry fruits. Yeast two-hybrid and BiFC assays confirmed that MBW complexes connected with flavonoid metabolism were FaMYB5/FaMYB10-FaEGL3 (bHLH)-FaLWD1/FaLWD1-like (WD40). Transient overexpression and qRT-PCR analysis revealed that disparate MBW models hold different patterns in the regulation of flavonoid biosynthesis in strawberry fruits. Compared with FaMYB10, FaMYB5 and its dominant complexes showed a more specific regulatory range on strawberry flavonoid biosynthetic pathway, while FaMYB10 was more extensive. In addition, the complexes involved in FaMYB5 facilitated PAs accumulation primarily through the LAR tributary while FaMYB10 mainly by the ANR branch. FaMYB9 and FaMYB11 tremendously elicited the accumulation of proanthocyanidins by up-regulating the expression levels of both LAR and ANR, and also affected anthocyanin metabolism by changing the ratio of Cy3G and Pg3G which were constituted as two major anthocyanin monomers in strawberries. Our study also illustrated that FaMYB5-FaEGL3-FaLWD1-like directly targeted the promoters of F3'H, LAR, and AHA10 thus committing to flavonoid accumulation. These results allow the specific members involved in the MBW complex to be deciphered and provide new insights into the regulatory mechanisms of anthocyanins and proanthocyanidins regulated by the MBW complex.

Molecular mechanism of phosphorous signaling inducing anthocyanin accumulation in Arabidopsis

Anthocyanins, flavonoid compounds derived from secondary metabolic pathways, play important roles in various biological processes. Phosphorus (P) is an essential macroelement for plant growth and development, and P-starvation usually results in anthocyanin accumulation. However, the molecular mechanism of P deficiency promotes anthocyanin biosynthesis has not been well characterized. Here, we provided evidence that the P signaling core protein PHOSPHATE STARVATION RESPONSE1 (PHR1) is physically associate with transcription factors (TFs) involved in anthocyanidin biosynthesis, including PRODUCTION OF ANTHOCYANIN PIGMENTS1 (PAP1/MYB75), MYB DOMAIN PROTEIN 113 (MYB113) and TRANSPARENT TESTA 8 (TT8). PHR1 and its homologies positively regulated anthocyanin accumulation in Arabidopsis seedlings under P-deficient conditions. Disruption of PHR1 simultaneously rendered seedlings hyposensitive to limiting P, whereas the overexpression of PHR1 enhanced P- deficiency-induced anthocyanin accumulation. Genetic analysis demonstrated that 35S:PHR1-2HA-5 seedlings partially recovers the P deficiency insensitive phenotype of myb-RNAi and tt8 mutants. In summary, our study indicated that protein complexes formed by PHR1 and MBW complex directly mediate the process of P-deficiency-induced anthocyanin accumulation, providing a new mechanistic understanding of how P-deficient signaling depends on the endogenous anthocyanin synthesis pathway to promote anthocyanin accumulation in Arabidopsis.

RNAseq-Based Working Model for Transcriptional Regulation of Crosstalk between Simultaneous Abiotic UV-B and Biotic Stresses in Plants

Plants adjust their secondary metabolism by altering the expression of corresponding genes to cope with both abiotic and biotic stresses. In the case of UV-B radiation, plants produce protective flavonoids; however, this reaction is impeded during pattern-triggered immunity (PTI) induced by pathogens. Pathogen attack can be mimicked by the application of microbial associated molecular patterns (e.g., flg22) to study crosstalk between PTI and UV-B-induced signaling pathways. Switching from Arabidopsis cell cultures to in planta studies, we analyzed whole transcriptome changes to gain a deeper insight into crosstalk regulation. We performed a comparative transcriptomic analysis by RNAseq with four distinct mRNA libraries and identified 10778, 13620, and 11294 genes, which were differentially expressed after flg22, UV-B, and stress co-treatment, respectively. Focusing on genes being either co-regulated with the UV-B inducible marker gene chalcone synthase CHS or the flg22 inducible marker gene FRK1 identified a large set of transcription factors from diverse families, such as MYB, WRKY, or NAC. These data provide a global view of transcriptomic reprogramming during this crosstalk and constitute a valuable dataset for further deciphering the underlying regulatory mechanism(s), which appear to be much more complex than previously anticipated. The possible involvement of MBW complexes in this context is discussed.

Investigation and Expression Analysis of R2R3-MYBs and Anthocyanin Biosynthesis-Related Genes during Seed Color Development of Common Bean (Phaseolus vulgaris)

Anthocyanins are responsible for the coloration of common bean seeds, and their accumulation is positively correlated with the expression level of anthocyanin biosynthetic genes. The MBW (MYB-bHLH-WD40) complex is thought to regulate the expression of these genes, and MYB proteins, which are a key factor in activating anthocyanin pathway genes, have been identified in several plants. This study demonstrated gene structures, chromosomal placements, gene duplications of R2R3-MYBs, miRNAs associated with R2R3-MYBs, and the interaction of these genes with other flavonoid regulatory genes. qRT-PCR was used to investigate the role of specific R2R3-MYBs and flavonoid genes in common bean seed color development. As a result of a comprehensive analysis with the help of in silico tools, we identified 160 R2R3-MYB genes in the common bean genome. We divided these genes into 16 classes on the basis of their intron-exon and motif structures. Except for three, the rest of the common bean R2R3-MYB members were distributed to all chromosomes with different densities, primarily located on chromosomes 3 and 8. We identified a total of 44 duplicated gene pairs dispersed across 11 chromosomes and evolved under purifying selection (Ka/Ks  <  1), 19 of which were derived from a whole-genome duplication. Our research uncovered 25 putative repressor PvMYB proteins that contain the EAR motif. Additionally, fifty different cis-regulatory elements regulated by light, stress, and hormone were identified. Within the genome of the common bean, we discovered a total of 36 microRNAs that target a total of 72 R2R3-MYB transcripts. The effect of 16 R2R3-MYB genes and 16 phenylpropanoid pathway genes, selected on the basis of their interaction in the protein-protein interaction map, playing role in the regulation of seed coat color development was evaluated using qRT-PCR in 5 different tissues at different developmental stages. The results revealed that these specific genes have different expression levels during different developmental periods, with higher levels in the pod filling and early pod stages than in the rest of the developmental periods. Furthermore, it was shown that PvTT8 (bHLH), PvTT2 (PvMYB42), PvMYB113, PvTTG1, and PvWD68 genes have effects on the regulation of seed coat color. The findings of this study, which is the first to use whole-genome analysis to identify and characterize the R2R3-MYB genes in common bean, may serve as a reference for future functional research in the legume.

An R3-MYB repressor, BnCPC forms a feedback regulation with MBW complex to modulate anthocyanin biosynthesis in Brassica napus

BACKGROUND

Anthocyanins are metabolites of phenylpropanoid pathway, and involves in diverse processes of plant development and adaptation, which are regulated by the MYB-bHLH-WD40 (MBW) protein complexes. Many R2R3-MYB activators have been well characterized, but the MYB repressors in anthocyanin biosynthesis were recognized recently, which are also important in modulating phenylpropanoid metabolism in plants. The regulatory mechanism of anthocyanin biosynthesis in oil crop Brassica napus remains to be revealed.

RESULTS

In this study, we identified an anthocyanin repressor BnCPC in B. napus. BnCPC encoded a typical R3-MYB protein containing a conserved [D/E]Lx2[R/K]x3Lx6Lx3R motif for interaction with bHLH proteins. Overexpression of BnCPC in B. napus inhibited anthocyanin accumulation, especially under anthocyanin inducible conditions. Protein-protein interaction and dual-luciferase assays confirmed that BnCPC could compete with BnPAP1 to interact with bHLHs (BnTT8 and BnEGL3), and repress the expression of anthocyanin biosynthetic genes (e.g., BnDFR) that activated by MBW complexes. Moreover, we found BnCPC inhibited the MBW complex-induced BnCPC activity.

CONCLUSIONS

Overall, this research demonstrated that BnCPC repressed anthocyanin biosynthesis by affecting the formation of MBW complex, and formed a feedback loop to regulate anthocyanin accumulation in B. napus.

RsTTG1, a WD40 Protein, Interacts with the bHLH Transcription Factor RsTT8 to Regulate Anthocyanin and Proanthocyanidin Biosynthesis in Raphanus sativus

MBW complexes, consisting of MYB, basic helix–loop–helix (bHLH), and WD40 proteins, regulate multiple traits in plants, including anthocyanin and proanthocyanidin (PA) biosynthesis and the determination of epidermal cell fate. Here, a WD40 gene from Raphanus sativus, designated TRANSPARENT TESTA GLABRA 1 (RsTTG1), was cloned and functionally characterized. Heterologous expression of RsTTG1 in the Arabidopsis thaliana mutant ttg1-22 background restored accumulation of anthocyanin and PA in the mutant and rescued trichome development. In radish, RsTTG1 was abundantly expressed in all root and leaf tissues, independently of anthocyanin accumulation, while its MBW partners RsMYB1 and TRANSPARENT TESTA 8 (RsTT8) were expressed at higher levels in pigment-accumulating tissues. In yeast two-hybrid analysis, the full-length RsTTG1 protein interacted with RsTT8. Moreover, transient protoplast co-expression assays demonstrated that RsTTG1, which localized to both the cytoplasm and nucleus, moves from the cytoplasm to the nucleus in the presence of RsTT8. When co-expressed with RsMYB1 and RsTT8, RsTTG1 stably activated the promoters of the anthocyanin biosynthesis genes CHALCONE SYNTHASE (RsCHS) and DIHYDROFLAVONOL 4-REDUCTASE (RsDFR). Transient expression of RsTTG1 in tobacco leaves exhibited an increase in anthocyanin accumulation due to activation of the expression of anthocyanin biosynthesis genes when simultaneously expressed with RsMYB1 and RsTT8. These results indicate that RsTTG1 is a vital regulator of pigmentation and trichome development as a functional homolog of AtTTG1.

A dual-function transcription factor, SlJAF13, promotes anthocyanin biosynthesis in tomato

Unlike modern tomato (Solanum lycopersicum) cultivars, cv. LA1996 harbors the dominant Aft allele, which is associated with anthocyanin synthesis in tomato fruit peel. However, the control of Aft anthocyanin biosynthesis remains unclear. Here, we used ethyl methanesulfonate-induced and CRISPR/Cas9-mediated mutation of LA1996 to show, respectively, that two class IIIf basic helix-loop-helix (bHLH) transcription factors, SlJAF13 and SlAN1, are involved in the control of anthocyanin synthesis. These transcription factors are key components of the MYB-bHLH-WD40 (MBW) complex, which positively regulates anthocyanin synthesis. Molecular and genetic analyses showed that SlJAF13 functions as an upstream activation factor of SlAN1 by binding directly to the G-Box motif of its promoter region. On the other hand, SlJAZ2, a JA signaling repressor, interferes with formation of the MBW complex to suppress anthocyanin synthesis by directly binding these two bHLH components. Unexpectedly, the transcript level of SlJAZ2 was in turn repressed in a SlJAF13-dependent manner. Mechanistically, SlJAF13 interacts with SlMYC2, inhibiting SlMYC2 activation of SlJAZ2 transcription, thus constituting a negative feedback loop governing anthocyanin accumulation. Taken together, our findings support a sophisticated regulatory network, in which SlJAF13 acts as an upstream dual-function regulator that fine tunes anthocyanin biosynthesis in tomato.

The R2R3-MYB gene family in Cicer arietinum: genome-wide identification and expression analysis leads to functional characterization of proanthocyanidin biosynthesis regulators in the seed coat

We identified 119 typical CaMYB encoding genes and reveal the major components of the proanthocyanidin regulatory network. CaPARs emerged as promising targets for genetic engineering toward improved agronomic traits in C. arietinum. Chickpea (Cicer arietinum) is among the eight oldest crops and has two main types, i.e., desi and kabuli, whose most obvious difference is the color of their seeds. We show that this color difference is due to differences in proanthocyanidin content of seed coats. Using a targeted approach, we performed in silico analysis, metabolite profiling, molecular, genetic, and biochemical studies to decipher the transcriptional regulatory network involved in proanthocyanidin biosynthesis in the seed coat of C. arietinum. Based on the annotated C. arietinum reference genome sequence, we identified 119 typical CaMYB encoding genes, grouped in 32 distinct clades. Two CaR2R3-MYB transcription factors, named CaPAR1 and CaPAR2, clustering with known proanthocyanidin regulators (PARs) were identified and further analyzed. The expression of CaPAR genes correlated well with the expression of the key structural proanthocyanidin biosynthesis genes CaANR and CaLAR and with proanthocyanidin levels. Protein-protein interaction studies suggest the in vivo interaction of CaPAR1 and CaPAR2 with the bHLH-type transcription factor CaTT8. Co-transfection analyses using Arabidopsis thaliana protoplasts showed that the CaPAR proteins form a MBW complex with CaTT8 and CaTTG1, able to activate the promoters of CaANR and CaLAR in planta. Finally, transgenic expression of CaPARs in the proanthocyanidin-deficient A. thaliana mutant tt2-1 leads to complementation of the transparent testa phenotype. Taken together, our results reveal main components of the proanthocyanidin regulatory network in C. arietinum and suggest that CaPARs are relevant targets of genetic engineering toward improved agronomic traits.

RrTTG1 promotes fruit prickle development through an MBW complex in Rosa roxburghii

Fruit prickles are widely distributed on the pericarp and exhibit polymorphic traits at different developmental stages. Although they are multicellular appendages that are well-known for helping plants defend against biotic and abiotic stresses, their origination and molecular mechanism are still less known. Here, we studied the origination and molecular mechanism of fruit prickles in Rosa roxburghii. Using morphological and histological observations, we found that the fruit prickle primordium of R. roxburghii originated from the ground meristem that underwent cell division to form flagelliform prickles, continued to enlarge, and finally lignified to form mature fruit prickles. We amplified a homolog of candidate gene TRANSPARENT TESTA GLABRA1 (TTG1) from R. roxburghii, named RrTTG1. RrTTG1 harbored four conserved WD-repeat domains and was exclusively nuclear-localized. Using qRT-PCR and in situ hybridization, we found that RrTTG1 was constitutively expressed and highly expressed during the initiation and cell expansion phases of fruit prickles. Ectopic expression analysis in Arabidopsis proved that RrTTG1 substantially enhanced the number of trichome and pigmentation production and inhibited root hair formation. Besides, RrTTG1 complemented the phenotypes of the ttg1 mutant in Arabidopsis, thus indicating that RrTTG1 played pleiotropic roles akin to AtTTG1. We demonstrated that the RrTTG1 only interacted with RrEGL3, a homolog of ENHANCER OF GLABRA3 (EGL3), via yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays. Briefly, RrTTG1 might positively regulate the initiation of fruit prickle primordium and cell enlargement by forming the RrTTG1-RrEGL3-RrGL1 complex in R. roxburghii. Therefore, our results help characterize the RrTTG1 in R. roxburghii and also elucidate the establishment of the prickles regulatory system in the Rosaceae plants.

Genetic effects of Red Lettuce Leaf genes on red coloration in leaf lettuce under artificial lighting conditions

Some cultivars of lettuce accumulate anthocyanins, which act as functional food ingredients. Leaf lettuce has been known to be erratic in exhibiting red color when grown under artificial light, and there is a need for cultivars that more stably exhibit red color in artificial light cultivation. In this study, we aimed to dissect the genetic architecture for red coloring in various leaf lettuce cultivars grown under artificial light. We investigated the genotype of Red Lettuce Leaf (RLL) genes in 133 leaf lettuce strains, some of which were obtained from publicly available resequencing data. By studying the allelic combination of RLL genes, we further analyzed the contribution of these genes to producing red coloring in leaf lettuce. From the quantification of phenolic compounds and corresponding transcriptome data, we revealed that gene expression level-dependent regulation of RLL1 (bHLH) and RLL2 (MYB) is the underlying mechanism conferring high anthocyanin accumulation in red leaf lettuce under artificial light cultivation. Our data suggest that different combinations of RLL genotypes cause quantitative differences in anthocyanin accumulation among cultivars, and some genotype combinations are more effective at producing red coloration even under artificial lighting.

Uncovering Hierarchical Regulation among MYB-bHLH-WD40 Proteins and Manipulating Anthocyanin Pigmentation in Rice

Anthocyanins accumulate in various organs of rice, and the regulatory genes involved in pigmentation of specific organs, such as pericarp, hull, leaf, apiculus, and stigma have been elucidated. However, the corresponding gene for rice culm pigmentation has not been clarified. The well-known MYB-bHLH-WD40 (MBW) complex plays vital role in regulating the anthocyanin biosynthesis pathway in plants. However, the core members of MBW and the hierarchical regulation between these members are not fully elucidated in rice. Here, by map-based cloning, we identified the culm-specific pigmentation gene S1 whose alleles are also known for hull/pericarp pigmentation. We also clarified that one WD40 protein encoding gene, WA1, is indispensable for anthocyanin biosynthesis in rice. In the cascading regulation among MBW members, S1 (bHLH) acts as the master gene by activating the expression of C1 (MYB), and then C1 activates the expression of WA1 (WD40), which is unique in plant species. This enables MBW members to be coordinated in a common way to efficiently regulate anthocyanin biosynthesis genes. Based on these studies, we explored the minimal gene set required for anthocyanin biosynthesis in rice. These findings will help us design new rice varieties with anthocyanin accumulation in specific organs as needed.

Genome-Wide Analysis of Anthocyanin Biosynthesis Regulatory WD40 Gene FcTTG1 and Related Family in Ficus carica L

WD40 proteins serve as crucial regulators in a broad spectrum of plant developmental and physiological processes, including anthocyanin biosynthesis. However, in fig (Ficus carica L.), neither the WD40 family nor any member involved in anthocyanin biosynthesis has been elucidated. In the present study, 204 WD40 genes were identified from the fig genome and phylogenetically classified into 5 clusters and 12 subfamilies. Bioinformatics analysis prediction localized 109, 69, and 26 FcWD40 proteins to the cytoplasm, nucleus and other cellular compartments, respectively. RNA-seq data mining revealed 127 FcWD40s expressed at FPKM > 10 in fig fruit. Most of these genes demonstrated higher expression in the early stages of fruit development. FcWD40-97 was recruited according to three criteria: high expression in fig fruit, predicted nuclear localization, and closest clustering with TTG1s identified in other plants. FcWD40-97, encoding 339 amino acids including 5 WD-repeat motifs, showed 88.01 and 87.94% amino acid sequence similarity to apple and peach TTG1, respectively. The gene is located on fig chromosome 4, and is composed of 1 intron and 2 exons. Promoter analysis revealed multiple light-responsive elements, one salicylic acid-responsive element, three methyl jasmonate-responsive elements, and one MYB-binding site involved in flavonoid biosynthesis gene regulation. FcWD40-97 was in the FPKM > 100 expression level group in fig fruit, and higher expression was consistently found in the peel compared to the flesh at the same development stages. Expression level did not change significantly under light deprivation, whereas in leaves and roots, its expression was relatively low. Transient expression verified FcWD40-97's localization to the nucleus. Yeast two-hybrid (Y2H) and biomolecular fluorescence complementation (BiFC) assays revealed that FcWD40-97 interacts with FcMYB114, FcMYB123, and FcbHLH42 proteins in vitro and in vivo, showing that FcWD40-97 functions as a member of the MYB-bHLH-WD40 (MBW) complex in anthocyanin-biosynthesis regulation in fig. We therefore renamed FcWD40-97 as FcTTG1. Our results provide the first systematic analysis of the FcWD40 family and identification of FcTTG1 in fig pigmentation.

New insight into the pigment composition and molecular mechanism of flower coloration in tulip (Tulipa gesneriana L.) cultivars with various petal colors

Pigmentation of various components leads to different colors in tulip flowers. To understand the molecular basis of the petal coloration in tulip, integrative analyses of the pigment components and transcriptome profiles were conducted on four tulip cultivars with different petal colors. A total of four major anthocyanins and 46 carotenoids were identified. The anthocyanin cyanidin 3-O-galactoside showed markedly higher abundances in the B cultivar than in the other varieties, and among the 46 kinds of carotenoids, (E/Z)-phytoene, violaxanthin myristate and violaxanthin palmitate were the major components. The RNA-seq and qRT-PCR results indicated that the pigment accumulation was linked to the expression of genes involved in the anthocyanin and carotenoid biosynthesis pathways. Yeast two-hybrid (Y2H) assays showed the interaction between different regulator factors in tulip MYB-bHLH-WD40 (MBW) complexes. Co-expression analyses of genes were performed, which include anthocyanin and carotenoid biosynthesis genes and transcription factors involved in MYB, bHLH, WRKY, AUX-IAA and MADS-box. The co-expression network and related analysis provide a basis for the discovery of color regulatory factors. Taken together, our study sheds light on the anthocyanin and carotenoid synthesis pathways and candidate regulatory transcription factors underlying flower coloration and shows the potential of flower breeding or pigments engineering in tulips.

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.

Loss of the R2R3 MYB Transcription Factor RsMYB1 Shapes Anthocyanin Biosynthesis and Accumulation in Raphanus sativus

The red or purple color of radish (Raphanus sativus L.) taproots is due to anthocyanins, which have nutritional and aesthetic value, as well as antioxidant properties. Moreover, the varied patterns and levels of anthocyanin accumulation in radish roots make them an interesting system for studying the transcriptional regulation of anthocyanin biosynthesis. The R2R3 MYB transcription factor RsMYB1 is a key positive regulator of anthocyanin biosynthesis in radish. Here, we isolated an allele of RsMYB1, named RsMYB1Short, in radish cultivars with white taproots. The RsMYB1Short allele carried a 4 bp insertion in the first exon causing a frame-shift mutation of RsMYB1, generating a truncated protein with only a partial R2 domain at the N-terminus. Unlike RsMYB1Full, RsMYB1Short was localized to the nucleus and the cytoplasm and failed to interact with their cognate partner RsTT8. Transient expression of genomic or cDNA sequences for RsMYB1Short in radish cotyledons failed to induce anthocyanin accumulation, but that for RsMYB1Full activated it. Additionally, RsMYB1Short showed the lost ability to induce pigment accumulation and to enhance the transcript level of anthocyanin biosynthetic genes, while RsMYB1Full promoted both processes when co-expressed with RsTT8 in tobacco leaves. As the result of the transient assay, co-expressing RsTT8 and RsMYB1Full, but not RsMYB1Short, also enhanced the promoter activity of RsCHS and RsDFR. We designed a molecular marker for RsMYB1 genotyping, and revealed that the RsMYB1Short allele is common in white radish cultivars, underscoring the importance of variation at the RsMYB1 locus in anthocyanin biosynthesis in the radish taproot. Together, these results indicate that the nonsense mutation of RsMYB1 generated the truncated protein, RsMYB1Short, that had the loss of ability to regulate anthocyanin biosynthesis. Our findings highlight that the frame shift mutation of RsMYB1 plays a key role in anthocyanin biosynthesis in the radish taproot.

An ancient RAB5 governs the formation of additional vacuoles and cell shape in petunia petals

Homologous ("canonical") RAB5 proteins regulate endosomal trafficking to lysosomes in animals and to the central vacuole in plants. Epidermal petal cells contain small vacuoles (vacuolinos) that serve as intermediate stations for proteins on their way to the central vacuole. Here, we show that transcription factors required for vacuolino formation in petunia induce expression of RAB5a. RAB5a defines a previously unrecognized clade of canonical RAB5s that is evolutionarily and functionally distinct from ARA7-type RAB5s, which act in trafficking to the vacuole. Loss of RAB5a reduces cell height and abolishes vacuolino formation, which cannot be rescued by the ARA7 homologs, whereas constitutive RAB5a (over)expression alters the conical cell shape and promotes homotypic vacuolino fusion, resulting in oversized vacuolinos. These findings provide a rare example of how gene duplication and neofunctionalization increased the complexity of membrane trafficking during evolution and suggest a mechanism by which cells may form multiple vacuoles with distinct content and function.

OsMYB3 is a R2R3-MYB gene responsible for anthocyanin biosynthesis in black rice

Black rice is a rare type of rice germplasm with various health benefits that are largely attributed to anthocyanin pigment accumulation in the pericarps. The anthocyanin biosynthesis in plant tissues is activated mainly by the MBW complexes, consisting of three types of transcription factors R2R3-MYB, bHLH, and WDR. In black rice, the bHLH and WDR components regulating anthocyanin biosynthesis in pericarps have been characterized, while the R2R3-MYB factor remains unknown. By examining the expression correlation between all putative rice MYB genes and anthocyanin biosynthesis-related genes based on transcriptome data of pericarps in combination with further molecular and genetic analysis, we proved that OsMYB3 (LOC_Os03g29614) was the determinant R2R3-MYB gene for anthocyanin biosynthesis in rice pericarps. The expression level of OsMYB3 in pericarps of black rice was significantly higher than that of white rice. The knockout of OsMYB3 in a black rice variety caused significant downregulation of 19 anthocyanin metabolites and many other flavonoids in grains. Our research deepens the understanding of regulatory system for anthocyanin biosynthesis in rice pericarps and provides implications for breeding black rice varieties with high anthocyanin level.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-021-01244-x.

Discrete bHLH transcription factors play functionally overlapping roles in pigmentation patterning in flowers of Antirrhinum majus

Floral pigmentation patterning is important for pollinator attraction as well as aesthetic appeal. Patterning of anthocyanin accumulation is frequently associated with variation in activity of the Myb, bHLH and WDR transcription factor complex (MBW) that regulates anthocyanin biosynthesis. Investigation of two classic mutants in Antirrhinum majus, mutabilis and incolorata I, showed they affect a gene encoding a bHLH protein belonging to subclade bHLH-2. The previously characterised gene, Delila, which encodes a bHLH-1 protein, has a bicoloured mutant phenotype, with residual lobe-specific pigmentation conferred by Incolorata I. Both Incolorata I and Delila induce expression of the anthocyanin biosynthetic gene DFR. Rosea 1 (Myb) and WDR1 proteins compete for interaction with Delila, but interact positively to promote Incolorata I activity. Delila positively regulates Incolorata I and WDR1 expression. Hierarchical regulation can explain the bicoloured patterning of delila mutants, through effects on both regulatory gene expression and the activity of promoters of biosynthetic genes like DFR that mediate MBW regulation. bHLH-1 and bHLH-2 proteins contribute to establishing patterns of pigment distribution in A. majus flowers in two ways: through functional redundancy in regulating anthocyanin biosynthetic gene expression, and through differences between the proteins in their ability to regulate genes encoding transcription factors.

Conserved and non-conserved functions of the rice homologs of the Arabidopsis trichome initiation-regulating MBW complex proteins

BACKGROUND

Trichome initiation in Arabidopsis is regulated by a MYB-bHLH-WD40 (MBW) transcriptional activator complex formed by the R2R3 MYB transcription factor GLABRA1 (GL1), MYB23 or MYB82, the bHLH transcription factor GLABRA3 (GL3), ENHANCER OF GLABRA3 (EGL3) or TRANSPARENT TESTA8 (TT8), and the WD40-repeat protein TRANSPARENT TESTA GLABRA1 (TTG1). However, the functions of the rice homologs of the MBW complex proteins remained uncharacterized.

RESULTS

Based on amino acid sequence identity and similarity, and protein interaction prediction, we identified OsGL1s, OsGL3s and OsTTG1s as rice homologs of the MBW complex proteins. By using protoplast transfection, we show that OsGL1D, OsGL1E, OsGL3B and OsTTG1A were predominantly localized in the nucleus, OsGL3B functions as a transcriptional activator and is able to interact with GL1 and TTG1. By using yeast two-hybrid and protoplast transfection assays, we show that OsGL3B is able to interact with OsGL1E and OsTTG1A, and OsGL1E and OsTTG1A are also able to interact with GL3. On the other hand, we found that OsGL1D functions as a transcription activator, and it can interact with GL3 but not OsGL3B. Furthermore, our results show that expression of OsTTG1A in the ttg1 mutant restored the phenotypes including alternations in trichome and root hair formation, seed color, mucilage production and anthocyanin biosynthesis, indicating that OsTTG1A and TTG1 may have similar functions.

CONCLUSION

These results suggest that the rice homologs of the Arabidopsis MBW complex proteins are able to form MBW complexes, but may have conserved and non-conserved functions.

A single amino acid substitution in the R2R3 conserved domain of the BrPAP1a transcription factor impairs anthocyanin production in turnip (Brassica rapa subsp. rapa)

The purple pigmentation in the epidermis of swollen roots of 'Tsuda' turnip (Brassica rapa subsp. rapa) is induced by light, providing a good system to investigate the genetic mechanism of light-dependent anthocyanin biosynthesis in B. rapa. Here, we identified the R2R3 MYB transcription factor gene PRODUCTION OF ANTHOCYANIN PIGMENT1 (BrPAP1a) as the critical gene in the anthocyanin-defective mutant w68. A nucleotide mutation in the turn region of the R3 domain was screened, which caused an amino acid substitution from glycine to serine (G94S). Functional analysis showed that the interaction of BrPAP1a with two bHLH factors ENHANCER OF GLABRA 3 (BrEGL3) and TRANSPARENT TESTA 8 (BrTT8) were impaired by the mutation. Expression of BrTT8 was activated by BrPAP1a and enhanced by MYB-bHLH-WDR (MBW) complexes, but blocked by the mutation. Furthermore, BrPAP1a directly bound the MYB-recognizing element (MRE) in the BrTT8 promoter, while the G94S substitution caused a loss of DNA-binding activity. Our findings indicate that G94 is required for protein interaction with BrTT8 and BrEGL3 and DNA-binding of BrPAP1a to activate BrTT8 expression, which leads to anthocyanin biosynthesis. Collectively, our data indicate the importance of the highly conserved amino acids within R2R3 MYB proteins in regulating anthocyanin biosynthesis and could aid programs to increase anthocyanins in turnip roots.

Recent Insights into Anthocyanin Pigmentation, Synthesis, Trafficking, and Regulatory Mechanisms in Rice (Oryza sativa L.) Caryopsis

Anthocyanins are antioxidants used as natural colorants and are beneficial to human health. Anthocyanins contribute to reactive oxygen species detoxification and sustain plant growth and development under different environmental stresses. They are phenolic compounds that are broadly distributed in nature and are responsible for a wide range of attractive coloration in many plant organs. Anthocyanins are found in various parts of plants such as flowers, leaves, stems, shoots, and grains. Considering their nutritional and health attributes, anthocyanin-enriched rice or pigmented rice cultivars are a possible alternative to reduce malnutrition around the globe. Anthocyanin biosynthesis and storage in rice are complex processes in which several structural and regulatory genes are involved. In recent years, significant progress has been achieved in the molecular and genetic mechanism of anthocyanins, and their synthesis is of great interest to researchers and the scientific community. However, limited studies have reported anthocyanin synthesis, transportation, and environmental conditions that can hinder anthocyanin production in rice. Rice is a staple food around the globe, and further research on anthocyanin in rice warrants more attention. In this review, metabolic and pre-biotic activities, the underlying transportation, and storage mechanisms of anthocyanins in rice are discussed in detail. This review provides potential information for the food industry and clues for rice breeding and genetic engineering of rice.

Looking into 'hair tonics' for cotton fiber initiation

Cotton fiber is the most important source of cellulose for the global textile industry. These hair-like single-celled trichomes develop from ovule epidermis. They are classified into long spinnable lint and short fuzz. A key objective in the cotton industry is to breed elite cultivars with fuzzless seeds carrying high lint yield. Molecular basis underlying lint and fuzz initiation remains obscure. Recent studies indicate fiber initiation is under the control of MYB-bHLH-WDR (MBW) transcription factor complex. Based on molecular genetic studies and gene expression patterns linking fiber phenotypes, we propose that specific but different sets of MBW genes are required to precisely regulate the initiation of the lint and fuzz fibers. Emerging evidence further points to sugar signaling as a 'hair-tonic' to boost fiber initiation through interaction with MBW complex and auxin signaling. An integrative model is provided as a conceptual framework for future studies to dissect the molecular network responsible for cotton fiber initiation.

Maize SRO1e represses anthocyanin synthesis through regulating the MBW complex in response to abiotic stress

Plants experiencing abiotic stress react by generating reactive oxygen species (ROS), compounds that, if allowed to accumulate to excess, repress plant growth and development. Anthocyanins induced by abiotic stress are strong antioxidants that neutralize ROS, whereas their over-accumulation retards plant growth. Although the mechanism of anthocyanin synthesis has been revealed, how plants balance anthocyanin synthesis under abiotic stress to maintain ROS homeostasis is unknown. Here, ROS-related proteins, SIMILAR TO RCD-ONEs (SROs), were analysed in Zea mays (maize), and all six SRO1 genes were inducible by a variety of abiotic stress agents. The constitutive expression of one of these genes, ZmSRO1e, in maize as well as in Arabidopsis thaliana increased the sensitivity of the plant to abiotic stress, but repressed anthocyanin biosynthesis and ROS scavenging activity. Loss-of-function mutation of ZmSRO1e enhanced ROS tolerance and anthocyanin accumulation. We showed that ZmSRO1e competed with ZmR1 (a core basic helix-loop-helix subunit of the MYB-bHLH-WD40 transcriptional activation complex) for binding with ZmPL1 (a core MYB subunit of the complex). Thus, during the constitutive expression of ZmSRO1e, the formation of the complex was compromised, leading to the repression of genes, such as ZmA4 (encoding dihydroflavonol reductase), associated with anthocyanin synthesis. Overall, the results have revealed a mechanism that allows the products of maize SRO1e to participate in the abiotic stress response.

Alternative Splicing of the Basic Helix-Loop-Helix Transcription Factor Gene CmbHLH2 Affects Anthocyanin Biosynthesis in Ray Florets of Chrysanthemum (Chrysanthemum morifolium)

Chrysanthemum is an important ornamental crop worldwide. Some white-flowered chrysanthemum cultivars produce red ray florets under natural cultivation conditions, but little is known about how this occurs. We compared the expression of anthocyanin biosynthetic and transcription factor genes between white ray florets and those that turned red based on cultivation conditions to comprehend the underlying mechanism. Significant differences in the expression of CmbHLH2 were detected between the florets of different colors. CmbHLH2 generated two alternatively spliced transcripts, designated CmbHLH2^Full and CmbHLH2^Short . Compared with CmbHLH2^Full , CmbHLH2^Short encoded a truncated protein with only a partial MYB-interaction region and no other domains normally present in the full-length protein. Unlike the full-length form, the splicing variant protein CmbHLH2^Short localized to the cytoplasm and the nucleus and could not interact with CmMYB6. Additionally, CmbHLH2^Short failed to activate anthocyanin biosynthetic genes and induce pigment accumulation in transiently transfected tobacco leaves, whereas CmbHLH2^Full promoted both processes when simultaneously expressed with CmMYB6. Co-expressing CmbHLH2^Full and CmMYB6 also enhanced the promoter activities of CmCHS and CmDFR. Notably, the Arabidopsis tt8-1 mutant, which lacks red pigmentation in the leaves and seeds, could be complemented by the heterologous expression of CmbHLH2^Full, which restored red pigmentation and resulted in red pigmentation in high anthocyanin and proanthocyanidin contents in the leaves and seeds, respectively, whereas expression of CmbHLH2^Short did not. Together, these results indicate that CmbHLH2 and CmMYB6 interaction plays a key role in the anthocyanin pigmentation changes of ray florets in chrysanthemum. Our findings highlight alternative splicing as a potential approach to modulate anthocyanin biosynthesis in specific tissues.

Identification of MBW Complex Components Implicated in the Biosynthesis of Flavonoids in Woodland Strawberry

Flavonoids belong to the family of polyphenolic secondary metabolites and contribute to fruit quality traits. It has been shown that MBW complexes (MYB-bHLH-WD40) regulate the flavonoids biosynthesis in different plants, but only a limited number of MBW complexes have been identified in strawberry species in general. In this study, we identified 112 R2R3-MYB proteins in woodland strawberry; 12 of them were found to have potential functions in regulating flavonoids biosynthesis by phylogenetic analysis. qRT-PCR assays showed that FvMYB3, FvMYB9, FvMYB11, FvMYB22, FvMYB64, and FvMYB105 mostly expressed at green stage of fruit development, aligned with proanthocyanidins accumulation; FvMYB10 and FvMYB41 showed higher expression levels at turning and ripe stages, aligned with anthocyanins accumulation. These results suggest that different MYBs might be involved in flavonoids biosynthesis at specific stages. Furthermore, FvMYB proteins were demonstrated to interact with FvbHLH proteins and induce expression from the promoters of CHS2 and DFR2 genes, which encode key enzymes in flavonoids biosynthesis. The co-expression of FvMYB and FvbHLH proteins in strawberry fruits also promoted the accumulation of proanthocyanidins. These findings confirmed and provided insights into the biofunction of MBW components in the regulation of flavonoid biosynthesis in woodland strawberry.

A bHLH gene NnTT8 of Nelumbo nucifera regulates anthocyanin biosynthesis

Lotus is an important aquatic ornamental plant, whose flower color is one of the key horticultural traits that determines its ornamental value. Previous studies revealed that anthocyanins largely determined the red color of lotus flower, which are also the main component that has beneficial effects on human health. However, the regulation mechanism of flower pigmentation in lotus flower remains unclear. In the present study, in order to further understand the regulatory mechanism underlying the anthocyanin biosynthesis, a bHLH gene NnTT8 was characterized to be phylogenetically close to AtTT8 and the bHLH proteins from other plant species that have been indicated to be involved in the positive regulation of anthocyanin biosynthesis. Complementation analysis in Arabidopsis tt8 mutant showed that NnTT8 could function similarly to AtTT8 in regulating anthocyanin and proanthocyanin biosynthesis. An MYB transcription factor capable of interacting with NnTT8 was also characterized from lotus. The identification of a bHLH transcription factor playing regulatory roles in anthocyanin biosynthesis is crucial, as it might help to obtain more in-depth insight into the coloration of lotus and help in breeding high anthocyanin content lotus variety that can be explored for lotus flower beverages.

TRANSPARENT TESTA GLABRA1, a Key Regulator in Plants with Multiple Roles and Multiple Function Mechanisms

TRANSPARENT TESTA GLABRA1 (TTG1) is a WD40 repeat protein. The phenotypes caused by loss-of-function of TTG1 were observed about half a century ago, but the TTG1 gene was identified only about twenty years ago. Since then, TTG1 has been found to be a plant-specific regulator with multiple roles and multiple functional mechanisms. TTG1 is involved in the regulation of cell fate determination, secondary metabolisms, accumulation of seed storage reserves, plant responses to biotic and abiotic stresses, and flowering time in plants. In some processes, TTG1 may directly or indirectly regulate the expression of downstream target genes via forming transcription activator complexes with R2R3 MYB and bHLH transcription factors. Whereas in other processes, TTG1 may function alone or interact with other proteins to regulate downstream target genes. On the other hand, the studies on the regulation of TTG1 are very limited. So far, only the B3-domain family transcription factor FUSCA3 (FUS3) has been found to regulate the expression of TTG1, phosphorylation of TTG1 affects its interaction with bHLH transcription factor TT2, and TTG1 proteins can be targeted for degradation by the 26S proteasome. Here, we provide an overview of TTG1, including the identification of TTG1, the functions of TTG1, the possible function mechanisms of TTG1, and the regulation of TTG1. We also proposed potential research directions that may shed new light on the regulation and functional mechanisms of TTG1 in plants.

The Conserved and Particular Roles of the R2R3-MYB Regulator FhPAP1 from Freesia hybrida in Flower Anthocyanin Biosynthesis

Anthocyanin biosynthesis is mainly controlled by MYB-bHLH-WD40 (MBW) complexes that modulate the expression of anthocyanin biosynthetic genes (ABGs). The MYB regulators involved in anthocyanin biosynthesis arose early during plant evolution and thus might function divergently in different evolutionary lineages. Although the anthocyanin-promoting R2R3-MYB regulators in eudicots have been comprehensively explored, little consensus has been reached about functional discrepancies versus conservation among MYB regulators from different plant lineages. Here, we integrated transcriptome analysis, gene expression profiles, gain-of-function experiments and transient protoplast transfection assays to functionally characterize the monocot Freesia hybrida anthocyanin MYB regulator gene FhPAP1, which showed correlations with late ABGs. FhPAP1 could activate ABGs as well as TT8-clade genes FhTT8L, AtTT8 and NtAN1 when overexpressed in Freesia, Arabidopsis and tobacco, respectively. Consistently, FhPAP1 could interact with FhTT8L and FhTTG1 to form the conserved MBW complex and shared similar target genes with its orthologs from Arabidopsis. Most prominently, FhPAP1 displayed higher transactivation capacity than its homologs in Arabidopsis and tobacco, which was instantiated in its powerful regulation on ABGs. Moreover, we found that FhPAP1 might be the selected gene during the domestication and rapid evolution of the wild Freesia species to generate intensive flower pigmentation. These results showed that while the MBW complex was highly evolutionarily conserved between tested monocot and core eudicot plants, participating MYB regulators showed functional differences in transactivation capacity according to their activation domain and played important roles in the flower coloration domestication and evolution of angiosperms.

Multiple MYB Activators and Repressors Collaboratively Regulate the Juvenile Red Fading in Leaves of Sweetpotato

Juvenile red fading describes the phenomenon in plants whereby red young leaves gradually turn green as they mature. While this phenomenon is commonly observed, the underlying molecular mechanism is still obscure as the classic model plants do not exhibit this process. Here, the molecular mechanism for the loss of anthocyanins during juvenile red fading were explored in the sweetpotato (Ipomoea batatas L.) cultivar "Chuanshan Zi". The MYB-bHLH-WDR (MBW) regulatory complexes for anthocyanins were examined with five stages of leaf development from C1 to C5. Alternating accumulation of anthocyanins and chlorophylls caused the leaf color change. Five anthocyanin components were identified by ultra performance liquid chromatography/tandem mass spectrometry (UPLC-MS/MS), and their contents were highest at stage C2. Transcriptomic analysis showed massive gene expression alteration during leaf development. The anthocyanin structural genes expressed in sweetpotato leaves were screened and found to be highly comparable with those identified in morning glories. The screened anthocyanin regulatory genes included one bHLH (IbbHLH2), one WDR (IbWDR1), three MYB activators (IbMYB1, IbMYB2, and IbMYB3), and five MYB repressors (IbMYB27, IbMYBx, IbMYB4a, IbMYB4b, and IbMYB4c). The expression trends of MYBs were key to the red fading process: the activators were highly expressed in early red leaves and were all accompanied by simultaneously expressed MYB repressors, which may act to prevent excessive accumulation of anthocyanins. The only antagonistic repressor, IbMYB4b, was highly expressed in green leaves, and may be critical for declined anthocyanin content at later stages. Further functional verification of the above transcription factors were conducted by promoter activation tests. These tests showed that the MBW complexes of IbMYB1/IbMYB2/IbMYB3-IbbHLH2-IbWDR1 not only activated promoters of anthocyanin structural genes IbCHS-D and IbDFR-B, but also promoters for IbbHLH2 and IbMYB27, indicating both hierarchical and feedback regulations. This study outlines the elaborate regulatory network of MBW complexes involving multiple MYBs which allow for the timely accumulation of anthocyanins in sweetpotato leaves. These results may also provide clues for similar studies of juvenile red fading in other plant species.

Alternative Splicing in the Anthocyanin Fruit Gene Encoding an R2R3 MYB Transcription Factor Affects Anthocyanin Biosynthesis in Tomato Fruits

Tomato (Solanum lycopersicum) fruits are typically red at ripening, with high levels of carotenoids and a low content in flavonoids. Considerable work has been done to enrich the spectrum of their health-beneficial phytochemicals, and interspecific crosses with wild species have successfully led to purple anthocyanin-colored fruits. The Aft (Anthocyanin fruit) tomato accession inherited from Solanum chilense the ability to accumulate anthocyanins in fruit peel through the introgression of loci controlling anthocyanin pigmentation, including four R2R3 MYB transcription factor-encoding genes. Here, we carried out a comparative functional analysis of these transcription factors in wild-type and Aft plants, and tested their ability to take part in the transcriptional complexes that regulate the biosynthetic pathway and their efficiency in inducing anthocyanin pigmentation. Significant differences emerged for SlAN2like, both in the expression level and protein functionality, with splicing mutations determining a complete loss of function of the wild-type protein. This transcription factor thus appears to play a key role in the anthocyanin fruit pigmentation. Our data provide new clues to the long-awaited genetic basis of the Aft phenotype and contribute to understand why domesticated tomato fruits display a homogeneous red coloration without the typical purple streaks observed in wild tomato species.

A Transcriptional Network Promotes Anthocyanin Biosynthesis in Tomato Flesh

Dietary anthocyanins are important health-promoting antioxidants that make a major contribution to the quality of fruits. It is intriguing that most tomato cultivars do not produce anthocyanins in fruit. However, the purple tomato variety Indigo Rose, which has the dominant Aft locus combined with the recessive atv locus from wild tomato species, exhibits light-dependent anthocyanin accumulation in the fruit skin. Here, we report that Aft encodes a functional anthocyanin activator named SlAN2-like, while atv encodes a nonfunctional version of the anthocyanin repressor SlMYBATV. The expression of SlAN2-like is responsive to light, and the functional SlAN2-like can activate the expression of both anthocyanin biosynthetic genes and their regulatory genes, suggesting that SlAN2-like acts as a master regulator in the activation of anthocyanin biosynthesis. We further showed that cultivated tomatoes contain nonfunctional alleles of SlAN2-like and therefore fail to produce anthocyanins. Consistently, expression of a functional SlAN2-like gene driven by the fruit-specific promoter in a tomato cultivar led to the activation of the entire anthocyanin biosynthesis pathway and high-level accumulation of anthocyanins in both the peel and flesh. Taken together, our study exemplifies that efficient engineering of complex metabolic pathways could be achieved through tissue-specific expression of master transcriptional regulators.

Genome-Wide Identification of Direct Targets of the TTG1-bHLH-MYB Complex in Regulating Trichome Formation and Flavonoid Accumulation in Arabidopsis Thaliana

Extensive studies have shown that the MBW complex consisting of three kinds of regulatory proteins, MYB and basic helix–loop–helix (bHLH) transcription factors and a WD40 repeat protein, TRANSPARENT TESTA GLABRA1 (TTG1), acts in concert to promote trichome formation and flavonoid accumulation in Arabidopsis thaliana. TTG1 functions as an essential activator in these two biological processes. However, direct downstream targets of the TTG1-dependent MBW complex have not yet been obtained in the two biological processes at the genome-wide level in A. thaliana. In the present study, we found, through RNA sequencing and quantitative real-time PCR analysis, that a great number of regulatory and structural genes involved in both trichome formation and flavonoid accumulation are significantly downregulated in the young shoots and expanding true leaves of ttg1-13 plants. Post-translational activation of a TTG1-glucocorticoid receptor fusion protein and chromatin immunoprecipitation assays demonstrated that these downregulated genes are directly or indirectly targeted by the TTG1-dependent MBW complex in vivo during trichome formation and flavonoid accumulation. These findings further extend our understanding of the role of TTG1-dependent MBW complex in the regulation of trichome formation and flavonoid accumulation in A. thaliana.

The MYB5-driven MBW complex recruits a WRKY factor to enhance the expression of targets involved in vacuolar hyper-acidification and trafficking in grapevine

The accumulation of secondary metabolites and the regulation of tissue acidity contribute to the important traits of grape berry and influence plant performance in response to abiotic and biotic factors. In several plant species a highly conserved MYB-bHLH-WD (MBW) transcriptional regulatory complex controls flavonoid pigment synthesis and transport, and vacuolar acidification in epidermal cells. An additional component, represented by a WRKY-type transcription factor, physically interacts with the complex increasing the expression of some target genes and adding specificity for other targets. Here we investigated the function of MBW(W) complexes involving two MYBs (VvMYB5a and VvMYB5b) and the WRKY factor VvWRKY26 in grapevine (Vitis vinifera L.). Using transgenic grapevine plants we showed that these complexes affected different aspects of morphology, plant development, pH regulation, and pigment accumulation. Transcriptomic analysis identified a core set of putative target genes controlled by VvMYB5a, VvMYB5b, and VvWRKY26 in different tissues. Our data indicated that VvWRKY26 enhances the expression of selected target genes induced by VvMYB5a/b. Among these targets are genes involved in vacuolar hyper-acidification, such as the P-type ATPases VvPH5 and VvPH1, and trafficking, and genes involved in the biosynthesis of flavonoids. In addition, VvWRKY26 is recruited specifically by VvMYB5a, reflecting the functional diversification of VvMYB5a and VvMYB5b. The expression of MBWW complexes in vegetative organs, such as leaves, indicates a possible function of vacuolar hyper-acidification in the repulsion of herbivores and/or in developmental processes, as shown by defects in transgenic grape plants where the complex is inactivated.

A functional homologue of Arabidopsis TTG1 from Freesia interacts with bHLH proteins to regulate anthocyanin and proanthocyanidin biosynthesis in both Freesia hybrida and Arabidopsis thaliana

The MBW complex, consisting of MYB, basic helix-loop-helix (bHLH) and WD40 proteins, regulates multiple traits in plants, such as anthocyanin and proanthocyanidin biosynthesis and cell fate determination. The complex has been widely identified in dicot plants, whereas few studies are concentrated on monocot plants which are of crucial importance to decipher its functional diversities among angiosperms during evolution. In present study, a WD40 gene from Freesia hybrida, designated as FhTTG1, was cloned and functionally characterized. Real-time PCR analysis indicated that it was expressed synchronously with the accumulation of both proanthocyanidins and anthocyanins in Freesia flowers. Transient protoplast transfection and biomolecular fluorescence complementation (BiFC) assays demonstrated that FhTTG1 could interact with FhbHLH proteins (FhTT8L and FhGL3L) to constitute the MBW complex. Moreover, the transportation of FhTTG1 to nucleus was found to rely on FhbHLH factors. Outstandingly, FhTTG1 could highly activate the anthocyanin or proanthocyanidin biosynthesis related gene promoters when co-transfected with MYB and bHLH partners, implying that FhTTG1 functioned as a member of MBW complex to control the anthocyanin or proanthocyanidin biosynthesis in Freesia hybrida. Further ectopic expression assays in Arabidopsis ttg1-1 showed the defective phenotypes of ttg1-1 were partially restored. Molecular biological assays validated FhTTG1 might interact with the endogenous bHLH factors to up-regulate genes responsible for anthocyanin and proanthocyanidin biosynthesis and trichome formation, indicating that FhTTG1 might perform exchangeable roles with AtTTG1. These results will not only contribute to the characterization of FhTTG1 in Freesia but also shed light on the establishment of flavonoid regulatory system in monocot plants, especially in Freesia hybrida.

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.

Repression of anthocyanin biosynthesis by R3-MYB transcription factors in lily (Lilium spp.)

Lily R3-MYB transcription factors are involved in negative regulation to limit anthocyanin accumulation in lily flowers and leaves and create notable color patterns on ectopically expressed petunia flowers. In eudicots, both positive and negative regulators act to precisely regulate the level of anthocyanin accumulation. The R3-MYB transcription factor is among the main factors repressing anthocyanin biosynthesis. Although, in monocots, the positive regulators have been well characterized, the negative regulators have not been examined. Two R3-MYBs, LhR3MYB1 and LhR3MYB2, which were identified in lily transcriptomes, were characterized in this study to understand the regulatory mechanisms of anthocyanin biosynthesis. LhR3MYB1 and LhR3MYB2 had a C2 suppressor motif downstream of a single MYB repeat; the similar amino acid motif appears only in AtMYBL2 among the eudicot R3-MYB proteins. Stable and transient overexpression of LhR3MYB1 and LhR3MYB2 in tobacco plants showed suppression of anthocyanin biosynthesis by both; however, suppression by LhR3MYB2 was stronger than that by LhR3MYB1. In the lily plant, the LhR3MYB2 transcript was detected in leaves with light stimulus-induced anthocyanin accumulation and in pink tepals. Although LhR3MYB1 was expressed in some, but not all tepals, its expression was not linked to anthocyanin accumulation. In addition, LhR3MYB1 expression levels in the leaves remained unchanged by the light stimulus, and LhR3MYB1 transcripts predominantly accumulated in the ovaries, which did not accumulate anthocyanins. Thus, although LhR3MYB1 and LhR3MYB2 have an ability to repress anthocyanin accumulation, LhR3MYB2 is more strongly involved in the negative regulation to limit the accumulation than that by LhR3MYB1. In addition, the overexpression of LhR3MYB2 generated notable color patterns in petunia flowers; thus, the usefulness of the LhR3MYB genes for creating unique color patterns by genetic engineering is discussed.