Interplay between R2R3 MYB-type activators and repressors regulates proanthocyanidin biosynthesis in banana (Musa acuminata)

DlMYB1 positively regulates anthocyanin biosynthesis and contributes to red exocarp coloration in red-skinned longan

Red-skinned longan (Dimocarpus longan Lour.) varieties exhibit striking exocarp coloration and high potential market value. In this study, we characterized anthocyanin accumulation in the red-skinned longan RP1901 using high performance liquid chromatography (HPLC). The exocarp of RP1901 accumulated 264.2 μg g-1 fresh weight (FW) of petunidin 3-O-glucoside (Pt3G) and 28.2 μg g-1 FW of cyanidin 3-O-glucoside (Cy3G), whereas the common longan cultivar ShiXia showed no detectable levels of these pigments. Preliminary RNA-seq analysis suggested upregulation of anthocyanin pathway genes and DlMYB1 gene. RT-qPCR analysis indicated that F3'H, ANS, GST4, and DlMYB1 were significantly upregulated in the exocarp of RP1901 compared to ShiXia. Bioinformatic analysis revealed that DlMYB1 is structurally conserved across species but diverges by 12 amino acids from ShiXia (same species) and 16 amino acids from LcMYB1 in Litchi chinensis (different genus), indicating significant genetic variation between red-skinned and common longan varieties. Subcellular localization of a DlMYB1-eGFP fusion protein confirmed its nuclear localization, consistent with transcription factor function. Overexpression of DlMYB1 in Arabidopsis thaliana led to upregulated expression of AtPAL1, AtDFR, AtLDOX, and AtGST genes and significant accumulation of anthocyanin in transgenic plants. These findings demonstrate that DlMYB1 acts as a positive regulator of anthocyanin biosynthesis and plays a key role in the pigmentation of red-skinned longan RP1901.

Cytokinin-mediated repression of anthocyanin biosynthesis in banana fruits

Anthocyanins are pigments responsible for vibrant plant colors and play vital roles in plant physiology. This study compares two banana cultivars, Grand Naine (GN) and Red Banana (RB), which exhibit significant differences in anthocyanin pigmentation. Transcriptomic profiling of peel (PL) and pulp (PP) tissues revealed cytokinin-responsive type-B response regulators (RRs), MaRR_B9 and MaRR_B12, as key modulators of anthocyanin biosynthesis. Cytokinin treatment of PP tissues increased the expression of MaRR_B9 and MaRR_B12, while significantly reducing the expression of dihydroflavanol reductase (MaDFR1, MaDFR2) and anthocyanidin synthase (MaANS) genes along with anthocyanin content. Through a combination of physiological, molecular, and biochemical analyses, we demonstrate that MaRR_B9 and MaRR_B12 exert direct regulatory control over key structural genes of anthocyanin biosynthesis, MaDFRs and MaANS. Additionally, a type B-RRs motif (AGATT) was identified in the promoter regions of MaDFR2 and MaANS, suggesting that MaRRs might directly regulate the transcription of MaDFR2 and MaANS. MaRR_B9 and MaRR_B12 interact with the promoters of MaDFR2 and MaANS, repressing these genes in vivo. Overexpression of MaRR_B9 and MaRR_B12 in banana fruits leads to a reduction in anthocyanin content, notably the cyanidin derivative, accompanied by altered expression patterns of MaDFRs and MaANS. Thus, the present study identifies MaRR_B9 and MaRR_B12 as novel regulators of anthocyanin biosynthesis in banana and provides further evidence that the cytokinin regulatory network modifies anthocyanin accumulation in plants. In conclusion, our findings reveal new molecular targets, in the form of MaRRs, for the genetic optimization aimed at enhancing anthocyanin content, stress resilience, and nutritional value in crop plants.

In-Depth Exploration of the Coloration Mechanism of Iris dichotoma Pall. via Transcriptomic and Metabolomic Analyses

Iris dichotoma Pall., renowned for its high ornamental value, is frequently cultivated in flowerbeds and courtyards, endowing garden landscapes with unique allure. Dark-hued flowers are widely regarded as more aesthetically appealing. This study utilized the petals of two distinct Iris dichotoma Pall. phenotypes as research materials to investigate the underlying mechanism of flower color formation. The purple-flowered Iris dichotoma Pall. was designated as Group P, and the white-flowered one as Group W. A comprehensive integrative analysis of the transcriptome and metabolome of the two petal types was carried out. Metabolomic analysis revealed that the contents of several anthocyanin derivatives, including delphinidin, petunidin, malvidin, peonidin, and procyanidin, were significantly higher in purple petals compared to white petals, with delphinidin exhibiting the highest content. The transcriptomic analysis detected 6731 differentially expressed genes (DEGs) between the white and purple petal types. Specifically, 3596 genes showed higher expression levels in purple petals, while 3135 genes exhibited lower expression levels in purple petals compared to white petals. Ten phenylalanine ammonia-lyase (PAL) genes, two chalcone synthase (CHS) genes, one anthocyanidin reductase (ANR) gene, one 4-coumarate-CoA ligase (4CL) gene, one dihydroflavonol 4-reductase (DFR) gene, one flavanone 3'-hydroxylase (F3'H) gene, and one flavonol synthase (FLS) gene were identified; they all had purple petals displaying higher expression levels than white petals. This research uncovers the potential formation mechanism of anthocyanins in the two Iris dichotoma Pall. types, thereby furnishing a theoretical foundation for floral breeding endeavors.

Shaping the future of bananas: advancing genetic trait regulation and breeding in the postgenomics era

Bananas (Musa spp.) are among the top-produced food crops, serving as a primary source of food for millions of people. Cultivated bananas originated primarily from the wild diploid species Musa acuminata (A genome) and Musa balbisiana (B genome) through intra- and interspecific hybridization and selections via somatic variation. Following the publication of complete A- and B-genome sequences, prospects for complementary studies on S- and T-genome traits, key gene identification for yield, ripening, quality, and stress resistance, and advances in molecular breeding have significantly expanded. In this review, latest research progress on banana A, B, S, and T genomes is briefly summarized, highlighting key advances in banana cytoplasmic inheritance, flower and fruit development, sterility, and parthenocarpy, postharvest ripening and quality regulation, and biotic and abiotic stress resistance associated with desirable economic traits. We provide updates on transgenic, gene editing, and molecular breeding. We also explore future directions for banana breeding and genetic improvement.

Heat-responsive MaHSF11 transcriptional activator positively regulates flavonol biosynthesis and flavonoid B-ring hydroxylation in banana (Musa acuminata)

Plant flavonols act primarily as ultraviolet radiation absorbers, reactive oxygen species scavengers, and phytoalexins, and they contribute to biotic and abiotic stress tolerance in plants. Banana (Musa acuminata), an herbaceous monocot and important fruit crop, accumulates flavonol derivatives in different organs, including the edible fruit pulp. Although flavonol content varies greatly in different organs, the molecular mechanisms involving transcriptional regulation of flavonol synthesis in banana are not known. Here, we characterized three SG7-R2R3 MYB transcription factors (MaMYBFA1, MaMYBFA2, and MaMYBFA3) and heat shock transcription factor (MaHSF11), to elucidate the molecular mechanism involved in transcriptional regulation of flavonol biosynthesis in banana. MaMYBFA positively regulates flavonol synthase 2 (MaFLS2) and downregulates MaFLS1. We show these transcription factors to be weak regulators of flavonol synthesis. Overexpression of MaHSF11 enhances flavonol contents, particularly that of myricetin, and promotes flavonol B-ring hydroxylation, which contributes to the diversity of flavonol derivatives. MaHSF11 directly interacts with the MaFLS1 and flavonoid 3',5'-hydroxylase1 (MaF3'5'H1) promoters, both in vitro and in vivo. MaHSF11 activates the expression of MaDREB1 directly, which is known to promote cold and chilling tolerance in banana fruit. Overall, our study elucidates a regulatory mechanism for flavonol synthesis in banana and suggests possible targets for genetic optimization to enhance nutritional value and stress responses in this globally important fruit crop.

The P-type ATPase gene AHA5 is involved in proanthocyanidins accumulation in Medicago truncatula

Proanthocyanidins (PAs) are the second most abundant plant phenolic natural products. The proton membrane H+-ATPase (AHA) is required for PA transportation in vacuoles, but it remains unclear which AHA gene(s) encode tonoplast proton pump in M. truncatula. Here, we identified three Tnt1 mutant lines of MtAHA5, resulting in PAs deficit in seeds. MtAHA5 was preferentially expressed in developing seeds, exhibiting its highest transcript levels at early stages. Although MtAHA3, MtAHA4, and MtAHA9 shared similar transcript patterns with MtAHA5 and other structural genes involved in PA biosynthesis, their mutant lines did not exhibit any PA-deficit phenotypes. Subcellular localization analysis demonstrated that MtAHA5 is targeted to the tonoplast in tobacco leaves; conversely, MtAHA3 and MtAHA9 are localized to the cytoplasm, suggesting that MtAHA5 acts as a tonoplast proton pump but not MtAHA3 or MtAHA9. Further genetic analyses revealed that MtAHA5 could complement the PA-deficit phenotype in mtaha5 mutants and ataha10 mutants. Transient transcription assays indicated that MtAHA5 is activated by the MBW complex to regulate the PA accumulation. Collectively, our findings suggest that MtAHA5 serves as a tonoplast proton pump to generate the driving force for MATE1-mediated transport of PA precursors into vacuoles.

Comprehensive transcriptomic analysis revealed the mechanism of ZjLAR and ZjANR promoting proanthocyanidin biosynthesis in jujube fruit

Jujube (Ziziphus jujuba Mill.) is a traditional fruit tree in China with immense economic and ecological value. Jujube fruits are abundant in polyphenolic secondary metabolites, particularly proanthocyanidins (PAs), which play a crucial role in enhancing the quality of jujube fruits. However, the mechanism underlying the biosynthesis of PAs remains unclear. The PA contents of sour jujube 'Qingjiansuanzao' and cultivated jujube 'Junzao' were compared at different developmental stages to unravel this mechanism. The PA contents of sour jujube were higher than that of cultivated jujube and decreased during fruit development. Combined with transcriptome analysis, a large number of differentially expressed genes related to PA biosynthesis were screened. Correlation analysis showed that ZjLAR and ZjANR played an active role in promoting the biosynthesis of PAs. Transient overexpression of ZjLAR and ZjANR in jujube fruits resulted in higher total PAs and monomeric catechin, but the PAs decreased after transient silencing. Overexpressing ZjLAR and ZjANR in Arabidopsis and tomato increased the content of PAs in Arabidopsis seeds and tomato fruits. These findings provide a new basis for further understanding of the biosynthesis of jujube PAs and are significant for improving the quality of jujube fruit.

A genus-specific R2R3 MYB transcription factor, CsMYB34, regulates galloylated catechin biosynthesis in Camellia sinensis

Galloylated catechins are the dominant polyphenols in Camellia sinensis (L.) O. Kuntze. The mechanisms responsible for accumulation of these specialized metabolites in tea plants remains unclear. This paper presents an extended member of subgroup 5 of transcription factors R2R3-MYB, CsMYB34, as a critical gene specifically regulating galloylated catechin biosynthesis. CsMYB34 has a TT2-type motif [VIRTKATRCSKVFIP]. Its transcription levels were positively correlated with galloylated catechin content in 19 tea varieties, with correlation coefficients ≥0.79. Suppression of CsMYB34 expression caused a significant decrease in galloylated catechin content, as well as reduced expression levels of the key galloylated catechin biosynthesis gene CsSCPL4. Yeast one-hybrid (Y1H), electrophoretic mobile shift assay (EMSA) and dual-luciferase reporter system (DLR) showed that CsMYB34 interacts directly with the promoter region of CsSCPL4, thereby upregulating its transcription. This research indicates that the CsMYB34 transcription factor selectively modulates the biosynthetic pathway of galloylated catechins, thereby offering a plausible rationale for the observed elevated levels of these compounds in tea leaves.

Exploring the differences in traits and genes between brown cotton and white cotton hybrid offspring (Gossypium hirsutum L.)

Brown cotton and white cotton are two important raw materials used in the cotton fiber industry. Clarifying the differences in morphology, agronomic traits, and fiber pigments between these varieties can facilitate the implementation of corresponding cultivation and breeding techniques. Therefore, we obtained F2 generation brown cotton plants through hybridization and compared them with their parents. In terms of agronomic traits, plant morphology and leaf shape were similar, but brown cotton presented more villi on the main stem. The first fruiting branch node was within the range of 4-6 cm, and the first fruiting branch node height was greater than that of TM-1, i.e., between 13.25 cm and 22.79 cm, with no difference compared with that of P26. The plant height was greater than that of the parents, and the number of bolls was essentially the same as that in TM-1 and greater than that in P26. The lint percentage and average fiber length were lower in TM-1 than in P26, and the seed index was greater than that in TM-1 and P26. Pigment measurements revealed that the chlorophyll a content in brown cotton during the boll stage was lower than that in white cotton, and the content of proanthocyanidin in the cotton fibers was greater in brown cotton than in white cotton. At 15 days after pollination, the highest content was 159.8 mg/g. To determine the differences in gene expression levels, we conducted transcriptome sequencing. Gene Ontology (GO) analysis revealed that the differentially expressed genes (DEGs) were enriched in pathways related to the cell wall and enzyme activity, whereas Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that the DEGs were enriched in flavonoid synthesis pathways. Transcription factor analysis revealed that the expression of the MYB3 transcription factor (Ghir_D07G002110) was higher in brown cotton, and bioinformatics analysis revealed that this gene has regulatory effects on the CHS, CHI1, and F3H genes.

CsCPC, an R3-MYB transcription factor, acts as a negative regulator of citric acid accumulation in Citrus

The citric acid accumulation during fruit ripening determines the quality of fleshy fruits. However, the molecular mechanism underlying citric acid accumulation is not clearly understood yet in citrus due to the scarcity of paired germplasm that exhibits significant difference in organic acid accumulation. Two citrus triploid hybrids with distinct citric acid content in their mature fruits were herein identified from a previously conducted interploidy cross in our group, providing an ideal paired material for studying acid accumulation in citrus. Through a comparative transcriptome analysis of the pulps of the above two triploid hybrids, an R3-MYB transcription factor, CAPRICE (CsCPC), was identified to be a regulator of citric acid accumulation in citrus fruits. Through transgenic experiments involving overexpression (in callus and kumquat fruits) and RNAi (in lemon leaves), we demonstrated that CsCPC suppresses citric acid accumulation by negatively regulating the expression of CsPH1 and CsPH5. Moreover, CsCPC competed with an R2R3-MYB CsPH4 for binding to ANTHOCYANIN1 (CsAN1) and thus disturbed the activation of CsPH1 and CsPH5 that encode vacuolar P-ATPase, which eventually led to a decrease in citric acid content. CsPH4 activated the expression of CsCPC and thus formed an activator-repressor feedback loop, which ultimately inhibited citric acid accumulation in citrus fruit. In summary, this study reveals a new regulatory mechanism of CsCPC-mediated inhibition of citric acid accumulation in citrus fruits, which would support the improvement of citrus fruit quality.

TRANSPARENT TESTA 16 collaborates with the MYB-bHLH-WD40 transcriptional complex to produce brown fiber cotton

Naturally colored cotton (NCC; Gossypium spp.) does not require additional chemical dyeing and is an environmentally friendly textile material with great research potential and applications. Our previous study using linkage and association mapping identified TRANSPARENT TESTA 2 (Gh_TT2) as acting on the proanthocyanin synthesis pathway. However, limited information is available about the genetic regulatory network of NCC. Here, we verified the effectiveness of Gh_TT2 and the roles of Gh_TT2 and red foliated mutant gene (Re) in pigment formation and deposition of brown fiber cotton (BFC). Variations in Gh_TT2 derived from interspecific hybridization between Gossypium barbadense acc. Pima 90-53 and Gossypium hirsutum acc. Handan208 resulted in gene expression differences, thereby causing phenotypic variation. Additionally, the MYB-bHLH-WD complex was found to be negatively modulated by TRANSPARENT TESTA 16/ARABIDOPSIS BSISTER (TT16/ABS). RNA-seq suggested that differential expression of homologous genes of key enzymes in the proanthocyanin synthesis pathway strongly contributes to the color rendering of natural dark brown and light brown cotton. Our study proposes a regulatory model in BFC, which will provide theoretical guidance for the genetic improvement of NCC.

Novel Insights into Pigment Composition and Molecular Mechanisms Governing Flower Coloration in Rose Cultivars Exhibiting Diverse Petal Hues

The pigmentation of various components leads to different colors of roses. However, the intricate molecular machinery and metabolic pathways underlying rose pigmentation remain largely unexplored. In this study, we determined that pink and black-red petals contain abundant anthocyanins, reaching concentrations of 800 μg/g and 1400 μg/g, respectively, significantly surpassing those in white and yellow petals. We identified 22 key anthocyanin components, predominantly cyanidin, pelargonidin, delphinidin, peonidin, and petunidin, which were preferentially enriched in pink and black-red petals. Additionally, we confirmed the presence of five carotenoid species-lutein, zeaxanthin, ζ-carotene, α-carotene, and β-carotene-with zeaxanthin and carotenoids notably accumulating in yellow petals at significantly higher levels compared with other colors. Furthermore, RNA-seq and qRT-PCR analyses revealed the association between pigment accumulation and the expression patterns of genes involved in anthocyanin and carotenoid biosynthesis pathways. Through promoter core element prediction and transcriptional metabolic co-expression analyses, we found that the MYB transcription factor likely positively modulates the expressions of key biosynthetic genes such as CHS, F3'H, and DFR, while the NAC transcription factor enhances the transcriptional activities of PSY, ZISO, and LYCB. Overall, this study explores the components of flower color, unravels the synthesis of anthocyanins and carotenoids, identifies regulatory factors, and highlights the prospects of rose breeding.

A 3R-MYB transcription factor is involved in Methyl Jasmonate-Induced disease resistance in Agaricus bisporus and has implications for disease resistance in Arabidopsis

INTRODUCTION

Methyl jasmonate (MeJA) and MYB transcription factors (TFs) play important roles in pathogen resistance in several plants, but MYB TFs in conjunction with MeJA-induced defense against Pseudomonas tolaasii in edible mushrooms remain unknown.

OBJECTIVES

To investigate the role of a novel 3R-MYB transcription factor (AbMYB11) in MeJA-induced disease resistance of Agaricus bisporus and in the resistance of transgenic Arabidopsis to P. tolaasii.

METHODS

Mushrooms were treated with MeJA alone or in combination with phenylpropanoid pathway inhibitors, and the effects of the treatments on the disease-related and physiological indicators of the mushrooms were determined to assess the role of MeJA in inducing resistance and the importance of the phenylpropanoid pathway involved. Subcellular localization, gene expression analysis, dual-luciferase reporter assay, electrophoretic mobility shift assay, and transgenic Arabidopsis experiments were performed to elucidate the molecular mechanism of AbMYB11 in regulating disease resistance.

RESULTS

MeJA application greatly improved mushroom resistance to P. tolaasii infection, and suppression of the phenylpropanoid pathway significantly weakened this effect. MeJA treatment stimulated the accumulation of phenylpropanoid metabolites, which was accompanied by increased the activities of biosynthetic enzymes and the expression of phenylpropanoid pathway-related genes (AbPAL1, Ab4CL1, AbC4H1) and an AbPR-like gene, further confirming the critical role of the phenylpropanoid pathway in MeJA-induced responses to P. tolaasii. Importantly, AbMYB11, localized in the nucleus, was rapidly induced by MeJA treatment under P. tolaasii infection; it transcriptionally activated the phenylpropanoid pathway-related and AbPR-like genes, and AbMYB11 overexpression in Arabidopsis significantly increased the transcription of phenylpropanoid-related genes, the accumulation of total phenolics and flavonoids, and improved resistance to P. tolaasii.

CONCLUSION

This study clarified the pivotal role of AbMYB11 as a regulator in disease resistance by modulating the phenylpropanoid pathway, providing a novel idea for the breeding of highly disease-resistant edible mushrooms and plants.

CaLAP1 and CaLAP2 orchestrate anthocyanin biosynthesis in the seed coat of Cicer arietinum

MAIN CONCLUSION

Our findings shed light on the regulation of anthocyanin and proanthocyanidin biosynthesis in chickpea seed coats. Expression of R2R3-MYB transcription factors CaLAP1 and CaLAP2 enhanced the anthocyanins and proanthocyanidins content in chickpea. The seed coat color is a major economic trait in leguminous crop chickpea (Cicer arietinum). Anthocyanins and proanthocyanidins (PAs) are two classes of flavonoids that mainly contribute to the flower, seed coat and color of Desi chickpea cultivars. Throughout the land plant lineage, the accumulation of anthocyanins and PAs is regulated by MYB and bHLH transcription factors (TFs), which form an MBW (MYB, bHLH, and WD40) complex. Here, we report two R2R3-MYB TFs in chickpea belonging to the anthocyanin-specific subgroup-6, CaLAP1 (Legume Anthocyanin Production 1), and CaLAP2 (Legume Anthocyanin Production 2), which are mainly expressed in the flowers and developmental stages of the seeds. CaLAP1 and CaLAP2 interact with TT8-like CabHLH1 and WD40, forming the MBW complex, and bind to the promoter sequences of anthocyanin- and PA biosynthetic genes CaCHS6, CaDFR2, CaANS, and CaANR, leading to anthocyanins and PA accumulation in the seed coat of chickpea. Moreover, these CaLAPs partially complement the anthocyanin-deficient phenotype in the Arabidopsis thaliana sextuple mutant seedlings. Overexpression of CaLAPs in chickpea resulted in significantly higher expression of anthocyanin and PA biosynthetic genes leading to a darker seed coat color with higher accumulation of anthocyanin and PA. Our findings show that CaLAPs positively modulate anthocyanin and PA content in seed coats, which might influence plant development and resistance to various biotic and abiotic stresses.

The MYB transcription factor OsMYBxoc1 regulates resistance to Xoc by directly repressing transcription of the iron transport gene OsNRAMP5 in rice

Bacterial leaf streak caused by Xanthomonas oryzae pv. oryzicola (Xoc) is a continuous threat to rice cultivation, leading to substantial yield losses with socioeconomic implications. Iron ions are essential mineral nutrients for plant growth, but little information is available on how they influence mechanisms of rice immunity against Xoc. Here, we investigated the role of the myeloblastosis-related (MYB) transcriptional repressor OsMYBxoc1 in modulation of rice resistance through control of iron ion transport. Overexpression of OsMYBxoc1 significantly increased rice resistance, whereas OsMYBxoc1 RNA-interference lines and knockout mutants showed the opposite result. Suppression of OsMYBxoc1 expression dampened the immune response induced by pathogen-associated molecular patterns. We demonstrated that OsMYBxoc1 binds specifically to the OsNRAMP5 promoter and represses transcription of OsNRAMP5. OsNRAMP5, a negative regulator of rice resistance to bacterial leaf streak, possesses metal ion transport activity, and inhibition of OsMYBxoc1 expression increased the iron ion content in rice. Activity of the ion-dependent H2O2 scavenging enzyme catalase was increased in plants with suppressed expression of OsMYBxoc1 or overexpression of OsNRAMP5. We found that iron ions promoted Xoc infection and interfered with the production of reactive oxygen species induced by Xoc. The type III effector XopAK directly inhibited OsMYBxoc1 transcription, indicating that the pathogen may promote its own proliferation by relieving restriction of iron ion transport in plants. In addition, iron complemented the pathogenicity defects of the RS105_ΔXopAK mutant strain, further confirming that iron utilization by Xoc may be dependent upon XopAK. In conclusion, our study reveals a novel mechanism by which OsMYBxoc1 modulates rice resistance by regulating iron accumulation and demonstrates that Xoc can accumulate iron ions by secreting the effector XopAK to promote its own infection.

Identification and functional marker development of SbPLSH1 conferring purple leaf sheath in sorghum

KEY MESSAGE

We identified a SbPLSH1gene conferring purple leaf sheath in sorghum (sorghumbicolor(L.) Moench)and developed a functional markerfor it. The purple leaf sheath of sorghum, a trait mostly related to anthocyanin deposition, is a visually distinguishable morphological marker widely used to evaluate the purity of crop hybrids. We aimed to dissect the genetic mechanism for leaf sheath color to mine the genes regulating this trait. In this study, two F2 populations were constructed by crossing a purple leaf sheath inbred line (Gaoliangzhe) with two green leaf sheath inbred lines (BTx623 and Silimei). Based on the results of bulked-segregant analysis sequencing, bulk-segregant RNA sequencing, and map-based cloning, SbPLSH1 (Sobic.006G175700), which encodes a bHLH transcription factor on chromosome 6, was identified as the candidate gene for purple leaf sheath in sorghum. Genetic analysis demonstrated that overexpression of SbPLSH1 in Arabidopsis resulted in anthocyanin deposition and purple petiole, while two single-nucleotide polymorphism (SNP) variants on the exon 6 resulted in loss of function. Further haplotype analysis revealed that there were two missense mutations and one cis-acting element mutation in SbPLSH1, which are closely associated with leaf sheath color in sorghum. Based on the variations, a functional marker (LSC4-2) for marker-assisted selection was developed, which has a broad-spectrum capability of distinguishing leaf sheath color in natural variants. In summary, this study lays a foundation for analyzing the genetic mechanism for sorghum leaf sheath color.

BrTTG1 regulates seed coat proanthocyanidin formation through a direct interaction with structural gene promoters of flavonoid pathway and glutathione S-transferases in Brassica rapa L

Introduction

Seed coat color is a significant agronomic trait in horticultural crops such as Brassica rapa which is characterized by brown or yellow seed coat coloration. Previous Brassica rapa studies have shown that BrTTG1 is responsible for seed coat proanthocyanidin formation, which is dependent on the MYB-bHLH-WD40 complex, whereas some studies have reported that TRANSPARENT TESTA GLABRA 1 (TTG1) directly interacts with the structural gene promoters of the flavonoid pathway.

Methods

Herein, the brown-seeded inbred B147 and ttg1 yellow-seeded inbred B80 mutants were used as plant materials for gene expression level analysis, gene promoter clone and transient overexpression.

Results

The analysis identified eleven structural genes involved in the flavonoid biosynthesis pathway, which are potentially responsible for BrTTG1- dependent seed coat proanthocyanidin formation. The promoters of these genes were cloned and cis-acting elements were identified. Yeast one-hybrid and dual-luciferase assays confirmed that BrTTG1 directly and independently interacted with proCHS-Bra008792, proDFR-Bra027457, proTT12-Bra003361, proTT19-Bra008570, proTT19-Bra023602 and proAHA10-Bra016610. A TTG1-binding motif (RTWWGTRGM) was also identified. Overexpression of TTG1 in the yellow-seed B. rapa inbred induced proanthocyanidin accumulation by increasing the expression levels of related genes.

Discussion

Our study unveiled, for the first time, the direct interaction between TTG1 and the promoters of the flavonoid biosynthesis pathway structural genes and glutathione S-transferases in Brassica rapa. Additionally, we have identified a novel TTG1-binding motif, providing a basis for further exploration into the function of TTG1 and the accumulation of proanthocyanidins in seed coats.

Quantitative analysis of MBW complex formation in the context of trichome patterning

Trichome patterning in Arabidopsis is regulated by R2R3MYB, bHLH and WDR (MBW) genes. These are considered to form a trimeric MBW protein complex that promotes trichome formation. The MBW proteins are engaged in a regulatory network to select trichome cells among epidermal cells through R3MYB proteins that can move between cells and repress the MBW complex by competitive binding with the R2R3MYB to the bHLHL protein. We use quantitative pull-down assays to determine the relative dissociation constants for the protein-protein interactions of the involved genes. We find similar binding strength between the trichome promoting genes and weaker binding of the R3MYB inhibitors. We used the dissociation constants to calculate the relative percentage of all possible complex combinations and found surprisingly low fractions of those complexes that are typically considered to be relevant for the regulation events. Finally, we predict an increased robustness in patterning as a consequence of higher ordered complexes mediated by GL3 dimerization.

PnMYB4 negatively modulates saponin biosynthesis in Panax notoginseng through interplay with PnMYB1

Saponins are the main triterpenoid ingredients from Panax notoginseng, a well-known Chinese medicine, and are important sources for producing drugs to prevent and treat cerebrovascular and cardiovascular diseases. However, the transcriptional regulatory network of saponin biosynthesis in P. notoginseng is largely unknown. In the present study we demonstrated that one R2R3-MYB transcription factor, designated PnMYB4, acts as a repressor of saponin accumulation. Suppression of PnMYB4 in P. notoginseng calli significantly increased the saponin content and the expression level of saponin biosynthetic genes. PnMYB4 directly bound to the promoters of key saponin biosynthetic genes, including PnSS, PnSE, and PnDS, to repress saponin accumulation. PnMYB4 and the activator PnMYB1 could interacted with PnbHLH, which is a positive regulator of saponin biosynthesis, to modulate the biosynthesis of saponin. PnMYB4 competed with PnMYB1 for binding to PnbHLH, repressing activation of the promoters of saponin structural genes induced by the PnMYB1-PnbHLH complex. Our study reveals that a complex regulatory module of saponin biosynthesis is associated with positive and negative MYB transcriptional regulators and provides a theoretical basis for improving the content of saponins and efficacy of P. notoginseng.

Identification and functional characterization of AcMYB113 in anthocyanin metabolism of Aesculus chinensis Bunge var. chinensis leaves

Anthocyanins can be induced by environmental factors such as low-temperature and play essential roles in plant color formation. In this study, leaves of Aesculus chinensis Bunge var. chinensis with different colors under natural low-temperature in autumn were collected and grouped into green leaf (GL) and red leaf (RL). To reveal the underlying mechanism of color formation in RL, a combined analysis of the metabolome and transcriptome was conducted with GL and RL. Metabolic analyses revealed that total anthocyanin content and primary anthocyanin components were increased RL relative to GL and cyanidin was the main anthocyanin compound in RL. Transcriptome analysis provided a total of 18720 differentially expressed genes (DEGs), of which 9150 DEGs were upregulated and 9570 DEGs were downregulated in RL relative to GL. KEGG analysis showed that DEGs were mainly enriched in flavonoid biosynthesis, phenylalanine metabolism, and phenylpropanoid biosynthesis. Furthermore, co-expression network analysis indicated that 56 AcMYB transcription factors were highly expressed in RL compared with GL, among which AcMYB113 (an R2R3-MYB TF) had a strong correlation with anthocyanins. Overexpression of AcMYB113 in apple resulted in dark-purple transgenic calluses. In addition, the transient expression experiment showed that AcMYB113 enhanced anthocyanin synthesis by activating pathways of anthocyanin biosynthesis in leaves of Aesculus chinensis Bunge var. chinensis. Taken together, our findings reveal new insights into the molecular mechanism of anthocyanin accumulation in RL and provide candidate genes for the breeding of anthocyanin-rich cultivars.

The multifaceted roles of R2R3 transcription factor HlMYB7 in the regulation of flavonoid and bitter acids biosynthesis, development and biotic stress tolerance in hop (Humulus lupulus L.)

Hop (Humulus lupulus) biosynthesizes the highly economically valuable secondary metabolites, which include flavonoids, bitter acids, polyphenols and essential oils. These compounds have important pharmacological properties and are widely implicated in the brewing industry owing to bittering flavor, floral aroma and preservative activity. Our previous studies documented that ternary MYB-bHLH-WD40 (MBW) and binary WRKY1-WD40 (WW) protein complexes transcriptionally regulate the accumulation of bitter acid (BA) and prenylflavonoids (PF). In the present study, we investigated the regulatory functions of the R2R3-MYB repressor HlMYB7 transcription factor, which contains a conserved N-terminal domain along with the repressive motif EAR, in regulating the PF- and BA-biosynthetic pathway and their accumulation in hop. Constitutive expression of HlMYB7 resulted in transcriptional repression of structural genes involved in the terminal steps of biosynthesis of PF and BA, as well as stunted growth, delayed flowering, and reduced tolerance to viroid infection in hop. Furthermore, yeast two-hybrid and transient reporter assays revealed that HlMYB7 targets both PF and BA pathway genes and suppresses MBW and WW protein complexes. Heterologous expression of HlMYB7 leads to down-regulation of structural genes of flavonoid pathway in Arabidopsis thaliana, including a decrease in anthocyanin content in Nicotiana tabacum. The combined results from functional and transcriptomic analyses highlight the important role of HlMYB7 in fine-tuning and balancing the accumulation of secondary metabolites at the transcriptional level, thus offer a plausible target for metabolic engineering in hop.

The R2R3-MYB-SG7 transcription factor CaMYB39 orchestrates surface phenylpropanoid metabolism and pathogen resistance in chickpea

Flavonoids are important plant pigments and defense compounds; understanding the transcriptional regulation of flavonoid biosynthesis may enable engineering crops with improved nutrition and stress tolerance. Here, we characterize R2R3-MYB domain subgroup 7 transcription factor CaMYB39, which regulates flavonol biosynthesis primarily in chickpea trichomes. CaMYB39 overexpression in chickpea was accompanied by a change in flux availability for the phenylpropanoid pathway, particularly flavonol biosynthesis. Lines overexpressing CaMYB39 showed higher isoflavonoid levels, suggesting its role in regulating isoflavonoid pathway. CaMYB39 transactivates the transcription of early flavonoid biosynthetic genes (EBG). FLAVONOL SYNTHASE2, an EBG, encodes an enzyme with higher substrate specificity for dihydrokaempferol than other dihydroflavonols explaining the preferential accumulation of kaempferol derivatives as prominent flavonols in chickpea. Interestingly, CaMYB39 overexpression increased trichome density and enhanced the accumulation of diverse flavonol derivatives in trichome-rich tissues. Moreover, CaMYB39 overexpression reduced reactive oxygen species levels and induced defense gene expression which aids in partially blocking the penetration efficiency of the fungal pathogen, Ascochyta rabiei, resulting in lesser symptoms, thus establishing its role against deadly Ascochyta blight (AB) disease. Overall, our study reports an instance where R2R3-MYB-SG7 member, CaMYB39, besides regulating flavonol biosynthesis, modulates diverse pathways like general phenylpropanoid, isoflavonoid, trichome density, and defense against necrotrophic fungal infection in chickpea.

Oligomeric Proanthocyanidins Confer Cold Tolerance in Rice through Maintaining Energy Homeostasis

Oligomeric proanthocyanidins (OPCs) are abundant polyphenols found in foods and botanicals that benefit human health, but our understanding of the functions of OPCs in rice plants is limited, particularly under cold stress. Two rice genotypes, named Zhongzao39 (ZZ39) and its recombinant inbred line RIL82, were subjected to cold stress. More damage was caused to RIL82 by cold stress than to ZZ39 plants. Transcriptome analysis suggested that OPCs were involved in regulating cold tolerance in the two genotypes. A greater increase in OPCs content was detected in ZZ39 than in RIL82 plants under cold stress compared to their respective controls. Exogenous OPCs alleviated cold damage of rice plants by increasing antioxidant capacity. ATPase activity was higher and poly (ADP-ribose) polymerase (PARP) activity was lower under cold stress in ZZ39 than in RIL82 plants. Importantly, improvements in cold tolerance were observed in plants treated with the OPCs and 3-aminobenzamide (PARP inhibitor, 3ab) combination compared to the seedling plants treated with H₂O, OPCs, or 3ab alone. Therefore, OPCs increased ATPase activity and inhibited PARP activity to provide sufficient energy for rice seedling plants to develop antioxidant capacity against cold stress.

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.