The TRANSPARENT TESTA GLABRA1 locus, which regulates trichome differentiation and anthocyanin biosynthesis in Arabidopsis, encodes a WD40 repeat protein

Ant13 Encodes Regulatory Factor WD40 Controlling Anthocyanin and Proanthocyanidin Synthesis in Barley (Hordeum vulgare L.)

Flavonoid compounds like anthocyanins and proanthocyanidins are important plant secondary metabolites having wide biological activities for humans. In this study, the molecular function of the Ant13 locus, which is one of the key loci governing flavonoid synthesis in barley, was determined. It was found that Ant13 encodes a WD40-type regulatory protein, which is required for transcriptional activation of a set of structural genes encoding enzymes of flavonoid biosynthesis at the leaf sheath base (colored by anthocyanins) and in grains (which accumulate proanthocyanidins). Besides its role in flavonoid biosynthesis, pleiotropic effects of this gene in plant growth were revealed. The mutants deficient in the Ant13 locus showed similar germination rates but a decreased rate of root and shoot growth and yield-related parameters in comparison to the parental cultivars. This is the seventh Ant locus (among 30) for which molecular functions in flavonoid biosynthesis regulation have been determined.

Transcriptional regulation of oil biosynthesis in seed plants: Current understanding, applications, and perspectives

Plants produce and accumulate triacylglycerol (TAG) in their seeds as an energy reservoir to support the processes of seed germination and seedling development. Plant seed oils are vital not only for the human diet but also as renewable feedstocks for industrial use. TAG biosynthesis consists of two major steps: de novo fatty acid biosynthesis in the plastids and TAG assembly in the endoplasmic reticulum. The latest advances in unraveling transcriptional regulation have shed light on the molecular mechanisms of plant oil biosynthesis. We summarize recent progress in understanding the regulatory mechanisms of well-characterized and newly discovered transcription factors and other types of regulators that control plant fatty acid biosynthesis. The emerging picture shows that plant oil biosynthesis responds to developmental and environmental cues that stimulate a network of interacting transcriptional activators and repressors, which in turn fine-tune the spatiotemporal regulation of the pathway genes.

Transcriptional Regulation of Anthocyanin Synthesis by MYB-bHLH-WDR Complexes in Kiwifruit (Actinidia chinensis)

The anthocyanin synthetic pathway is regulated centrally by an MYB-bHLH-WD40 (MBW) complex. Anthocyanin pigmentation is an important fruit quality trait in red-fleshed kiwifruit; however, the underlying regulatory mechanisms involving the MBW complex are not well understood. In this study, one R2R3MYB (AcMYBF110 expressed in fruit characteristically), one bHLH (AcbHLH1), two upstream regulators of AcbHLH1 (AcbHLH4 and AcbHLH5), and one WDR (AcWDR1) are characterized as being involved in the regulation of anthocyanin synthesis in kiwifruit. AcMYBF110 plays an important role in the regulation of anthocyanin accumulation by specifically activating the promoters of several anthocyanin pathway genes including AcCHS, AcF3'H, AcANS, AcUFGT3a, AcUFGT6b, and AcGST1. Coexpression of AcbHLH1, AcbHLH4, or AcbHLH5 together with AcMYBF110 induces much greater anthocyanin accumulation in both tobacco leaves and in Actinidia arguta fruit compared with AcMYBF110 alone. Moreover, this activation is further enhanced by adding AcWDR1. We found that both AcMYBF110 and AcWDR1 interact with all three AcbHLH factors, while AcMYBF110 also interacts with AcWDR1 to form three different MBW complexes that have different regulatory roles in anthocyanin accumulation of kiwifruit. The AcMYBF110-AcbHLH1-AcWDR1 complex directly targets the promoters of anthocyanin synthetic genes. Other features of the regulatory pathways identified include promotion of AcMYBF110, AcbHLH1,and AcWDR1 activities by this MBW complex, providing for both reinforcement and feedback regulation, whereas the AcMYBF110-AcbHLH4/5-AcWDR1 complex is indirectly involved in the regulation of anthocyanin synthesis by activating the promoters of AcbHLH1 and AcWDR1 to amplify the regulation signals of the first MBW complex.

Genome-Wide Identification of the TIFY Family in Salvia miltiorrhiza Reveals That SmJAZ3 Interacts With SmWD40-170, a Relevant Protein That Modulates Secondary Metabolism and Development

Salvia miltiorrhiza Bunge (S. miltiorrhiza), a traditional Chinese medicinal herb, contains numerous bioactive components with broad range of pharmacological properties. By increasing the levels of endogenous jasmonate (JA) in plants or treating them with methyl jasmonate (MeJA), the level of tanshinones and salvianolic acids can be greatly enhanced. The jasmonate ZIM (JAZ) proteins belong to the TIFY family, and act as repressors, releasing targeted transcriptional factors in the JA signaling pathway. Herein, we identified and characterized 15 TIFY proteins present in S. miltiorrhiza. Quantitative reverse transcription PCR analysis indicated that the JAZ genes were all constitutively expressed in different tissues and were induced by MeJA treatments. SmJAZ3, which negatively regulates the tanshinones biosynthesis pathway in S. miltiorrhiza and the detailed molecular mechanism is poorly understood. SmJAZ3 acts as a bait protein to capture and identify a WD-repeat containing the protein SmWD40-170. Further molecular and genetic analysis revealed that SmWD40-170 is a positive regulator, promoting the accumulation of secondary metabolites in S. miltiorrhiza. Our study systematically analyzed the TIFY family and speculated a module of the JAZ-WD40 complex provides new insights into the mechanisms regulating the biosynthesis of secondary metabolites in S. miltiorrhiza.

SmMYB111 Is a Key Factor to Phenolic Acid Biosynthesis and Interacts with Both SmTTG1 and SmbHLH51 in Salvia miltiorrhiza

Transcription factors that include myeloblastosis (MYB), basic helix-loop-helix (bHLH), and tryptophan-aspartic acid (WD)-repeat protein often form a ternary complex to regulate the phenylpropanoid pathway. However, only a few MYB and bHLH members involved in the biosynthesis of salvianolic acid B (Sal B) have been reported, and little is known about Sal B pathway regulation by the WD40 protein transparent testa glabra 1 (TTG1)-dependent transcriptional complexes in Salvia miltiorrhiza. We isolated SmTTG1 from that species for detailed functional characterization. Enhanced or reduced expression of SmTTG1 was achieved by gain- or loss-of-function assays, respectively, revealing that SmTTG1 is necessary for Sal B biosynthesis. Interaction partners of the SmTTG1 protein were screened by yeast two-hybrid (Y2H) assays with the cDNA library of S. miltiorrhiza. A new R2R3-MYB transcription factor, SmMYB111, was found through this screening. Transgenic plants overexpressing or showing reduced expression of SmMYB111 upregulated or deregulated, respectively, the yields of Sal B. Both Y2H and bimolecular fluorescent complementation experiments demonstrated that SmMYB111 interacts with SmTTG1 and SmbHLH51, a positive regulator of the phenolic acid pathway. Our data verified the function of SmTTG1 and SmMYB111 in regulating phenolic acid biosynthesis in S. miltiorrhiza. Furthermore, ours is the first report of the potential ternary transcription complex SmTTG1-SmMYB111-SmbHLH51, which is involved in the production of Sal B in that species.

Molecular analysis of anthocyanin-related genes in ornamental cabbage

Ornamental cabbage (Brassica oleracea var. acephala) is a winter-grown and important decorative plant of the family Brassicaceae, which displays an exceptional coloration in the central leaves of the rosette. Anthocyanins are the key determinant of the red, purple, and blue colors of vegetative and reproductive parts of many plant species including ornamental cabbage. Total anthocyanin content was measured spectrophotometrically, and the highest anthocyanin content was detected in the red followed by light-red and white ornamental cabbage lines. Anthocyanin biosynthesis is controlled by members of three different transcription factor (TF) families, such as MYB, basic helix-loop-helix (bHLH), and WD40 repeats (WDR), which function as a MBW complex. We identified three MYB, six bHLH, and one WDR TFs that regulate anthocyanin biosynthesis in ornamental cabbage. The expression of the regulatory and biosynthetic genes for anthocyanin synthesis was determined by qPCR. The tested structural genes of the anthocyanin pathway were shown to be up-regulated in the red followed by light-red ornamental cabbage lines; however, the expression levels of the late biosynthetic genes were barely detected in the white ornamental cabbage lines. Among the regulatory genes, BoPAP2 (MYB), BoTT8, BoEGL3.1, and BoMYC1.2 (bHLH), and BoTTG1 (WDR) were identified as candidates for the regulation of anthocyanin biosynthesis. This work could be useful for the breeding of novel colorful ornamental cabbage cultivars.

Expansion and Function of Repeat Domain Proteins During Stress and Development in Plants

The recurrent repeats having conserved stretches of amino acids exists across all domains of life. Subsequent repetition of single sequence motif and the number and length of the minimal repeating motifs are essential characteristics innate to these proteins. The proteins with tandem peptide repeats are essential for providing surface to mediate protein-protein interactions for fundamental biological functions. Plants are enriched in tandem repeat containing proteins typically distributed into various families. This has been assumed that the occurrence of multigene repeats families in plants enable them to cope up with adverse environmental conditions and allow them to rapidly acclimatize to these conditions. The evolution, structure, and function of repeat proteins have been studied in all kingdoms of life. The presence of repeat proteins is particularly profuse in multicellular organisms in comparison to prokaryotes. The precipitous expansion of repeat proteins in plants is presumed to be through internal tandem duplications. Several repeat protein gene families have been identified in plants. Such as Armadillo (ARM), Ankyrin (ANK), HEAT, Kelch-like repeats, Tetratricopeptide (TPR), Leucine rich repeats (LRR), WD40, and Pentatricopeptide repeats (PPR). The structure and functions of these repeat proteins have been extensively studied in plants suggesting a critical role of these repeating peptides in plant cell physiology, stress and development. In this review, we illustrate the structural, functional, and evolutionary prospects of prolific repeat proteins in plants.

A white mutant of Malay apple fruit (Syzygium malaccense) lacks transcript expression and activity for the last enzyme of anthocyanin synthesis, and the normal expression of a MYB transcription factor

The fruit skin of the mature Malay apple (Syzygium malaccense (L.) Merr. & L.M. Perry) is initially glossy red, then changes to purple. A mutant having mature fruits with white skin has been identified. The skin of wild-type fruit contained five glucose-based anthocyanins (cyanidin-3-O-glucoside, pelargonidin-3-O-glucoside, peonidin-3-O-glucoside, cyanidin-3,5-O-diglucoside and peonidin-3,5-O-diglucoside). Cyanidin-3-O-glucoside accounted for a large proportion of the total anthocyanin content. The accumulation cyanidin-3-O-glucoside during fruit maturation was correlated with increased activities of phenylalanine ammonia lyase (PAL) and UDPglucose:flavonoid 3-O-glucosyltransferase (UF3GlucT, F3GT). In the wild-type fruit skin, transcripts of seven genes that encode enzymes in the anthocyanin biosynthetic pathway were detected. No anthocyanins were found in the white mutant fruit skin. The skin of the white mutant fruit contained transcripts of all seven genes identified, except F3GT. It also showed no F3GT activity. The data indicate that the lack of anthocyanins in the mutant is due to lack of F3GT expression. In addition, the transcript of a MYB transcription factor, highly homologous to three Arabidopsis MYBs involved in anthocyanin synthesis, was virtually absent in the mutant but very high in the wild-type fruit. It is suggested that the lack of MYB expression is part of the cause of the lack of F3GT expression and anthocyanin synthesis during fruit maturation.

At5PTase13 modulates cotyledon vein development through regulating auxin homeostasis

Phosphatidylinositol signaling pathway and the relevant metabolites are known to be critical to the modulation of different aspects of plant growth, development, and stress responses. Inositol polyphosphate 5-phosphatase is a key enzyme involved in phosphatidylinositol metabolism and is encoded by an At5PTase gene family in Arabidopsis thaliana. A previous study shows that At5PTase11 mediates cotyledon vascular development probably through the regulation of intracellular calcium levels. In this study, we provide evidence that At5PTase13 modulates the development of cotyledon veins through its regulation of auxin homeostasis. A T-DNA insertional knockout mutant, At5pt13-1, showed a defect in development of the cotyledon vein, which was rescued completely by exogenous auxin and in part by brassinolide, a steroid hormone. Furthermore, the mutant had reduced auxin content and altered auxin accumulation in seedlings revealed by the DR5:beta-glucuronidase fusion construct in seedlings. In addition, microarray analysis shows that the transcription of key genes responsible for auxin biosynthesis and transport was altered in At5pt13-1. The At5pt13-1 mutant was also less sensitive to auxin inhibition of root elongation. These results suggest that At5PTase13 regulates the homeostasis of auxin, a key hormone controlling vascular development in plants.

Sucrose-specific induction of anthocyanin biosynthesis in Arabidopsis requires the MYB75/PAP1 gene

Sugar-induced anthocyanin accumulation has been observed in many plant species. We observed that sucrose (Suc) is the most effective inducer of anthocyanin biosynthesis in Arabidopsis (Arabidopsis thaliana) seedlings. Other sugars and osmotic controls are either less effective or ineffective. Analysis of Suc-induced anthocyanin accumulation in 43 Arabidopsis accessions shows that considerable natural variation exists for this trait. The Cape Verde Islands (Cvi) accession essentially does not respond to Suc, whereas Landsberg erecta is an intermediate responder. The existing Landsberg erecta/Cvi recombinant inbred line population was used in a quantitative trait loci analysis for Suc-induced anthocyanin accumulation (SIAA). A total of four quantitative trait loci for SIAA were identified in this way. The locus with the largest contribution to the trait, SIAA1, was fine mapped and using a candidate gene approach, it was shown that the MYB75/PAP1 gene encodes SIAA1. Genetic complementation studies and analysis of a laboratory-generated knockout mutation in this gene confirmed this conclusion. Suc, in a concentration-dependent way, induces MYB75/PAP1 mRNA accumulation. Moreover, MYB75/PAP1 is essential for the Suc-mediated expression of the dihydroflavonol reductase gene. The SIAA1 locus in Cvi probably is a weak or loss-of-function MYB75/PAP1 allele. The C24 accession similarly shows a very weak response to Suc-induced anthocyanin accumulation encoded by the same locus. Sequence analysis showed that the Cvi and C24 accessions harbor mutations both inside and downstream of the DNA-binding domain of the MYB75/PAP1 protein, which most likely result in loss of activity.

The Arabidopsis ATR1 Myb transcription factor controls indolic glucosinolate homeostasis

Plants derive a number of important secondary metabolites from the amino acid tryptophan (Trp), including the growth regulator indole-3-acetic acid (IAA) and defense compounds against pathogens and herbivores. In previous work, we found that a dominant overexpression allele of the Arabidopsis (Arabidopsis thaliana) Myb transcription factor ATR1, atr1D, activates expression of a Trp synthesis gene as well as the Trp-metabolizing genes CYP79B2, CYP79B3, and CYP83B1, which encode enzymes implicated in production of IAA and indolic glucosinolate (IG) antiherbivore compounds. Here, we show that ATR1 overexpression confers elevated levels of IAA and IGs. In addition, we show that an atr1 loss-of-function mutation impairs expression of IG synthesis genes and confers reduced IG levels. Furthermore, the atr1-defective mutation suppresses Trp gene dysregulation in a cyp83B1 mutant background. Together, this work implicates ATR1 as a key homeostatic regulator of Trp metabolism and suggests that ATR1 can be manipulated to coordinately control the suite of enzymes that synthesize IGs.

Molecular and biochemical analysis of two cDNA clones encoding dihydroflavonol-4-reductase from Medicago truncatula

Dihydroflavonol-4-reductase (DFR; EC1.1.1.219) catalyzes a key step late in the biosynthesis of anthocyanins, condensed tannins (proanthocyanidins), and other flavonoids important to plant survival and human nutrition. Two DFR cDNA clones (MtDFR1 and MtDFR2) were isolated from the model legume Medicago truncatula cv Jemalong. Both clones were functionally expressed in Escherichia coli, confirming that both encode active DFR proteins that readily reduce taxifolin (dihydroquercetin) to leucocyanidin. M. truncatula leaf anthocyanins were shown to be cyanidin-glucoside derivatives, and the seed coat proanthocyanidins are known catechin and epicatechin derivatives, all biosynthesized from leucocyanidin. Despite high amino acid similarity (79% identical), the recombinant DFR proteins exhibited differing pH and temperature profiles and differing relative substrate preferences. Although no pelargonidin derivatives were identified in M. truncatula, MtDFR1 readily reduced dihydrokaempferol, consistent with the presence of an asparagine residue at a location known to determine substrate specificity in other DFRs, whereas MtDFR2 contained an aspartate residue at the same site and was only marginally active on dihydrokaempferol. Both recombinant DFR proteins very efficiently reduced 5-deoxydihydroflavonol substrates fustin and dihydrorobinetin, substances not previously reported as constituents of M. truncatula. Transcript accumulation for both genes was highest in young seeds and flowers, consistent with accumulation of condensed tannins and leucoanthocyanidins in these tissues. MtDFR1 transcript levels in developing leaves closely paralleled leaf anthocyanin accumulation. Overexpression of MtDFR1 in transgenic tobacco (Nicotiana tabacum) resulted in visible increases in anthocyanin accumulation in flowers, whereas MtDFR2 did not. The data reveal unexpected properties and differences in two DFR proteins from a single species.

New perspectives on proanthocyanidin biochemistry and molecular regulation

Our understanding of proanthocyanidin (syn. condensed tannin) synthesis has been recently extended by substantial developments concerning both structural and regulatory genes. A gene encoding leucoanthocyanidin reductase has been obtained from the tropical forage, Desmodium uncinatum, with the latter enzyme catalyzing formation of (+)-catechin. The BANYULS gene in Arabidopsis thaliana, previously proposed to encode leucoanthocyanidin reductase or to regulate proanthocyanidin biosynthesis, has been shown instead to encode anthocyanidin reductase, which in turn converts anthocyanidins (pelargonidin, cyanidin, or delphinidin) into 2,3-cis-2R,3R-flavan-3-ols (respectively, (-)-epiafzelechin, (-)-epicatechin and (-)-epigallocatechin). However, the enzyme which catalyzes the polymerization reaction remains unknown. Nevertheless, a vacuolar transmembrane protein TT12, defined by the Arabidopsis tt12 mutant, is involved in transport of proanthocyanidin polymer into the vacuole for accumulation. Six different types of regulatory elements, e.g. TFIIIA-like, WD-40-like, WRKY-like, MADS-box-like, myb-like, and bHLH (myc-like), have been cloned and identified using mutants from Arabidopsis (tt1, ttg1, ttg2, tt2, tt16, tt2, tt8) and two other species (Hordeum vulgare [ant13] and Lotus spp [tan1]). Accordingly, increases in proanthocyanidin levels have been induced in the the world's major forage, alfalfa. These advances may now lead to a detailed understanding of how PA synthesis is controlled and to useful alterations in proanthocyanidin concentration for the improvement of forage species, pulses, and other crop plants.