Molecular analysis of proanthocyanidins related to pigmentation in brown cotton fibre (Gossypium hirsutum L.)

The Multidrug and toxin compound extrusion gene GhTT12 promotes the accumulation of both proanthocyanidins and anthocyanins in Gossypium hirsutum

The pigments present in the fibers of naturally colored cotton provide excellent antibacterial and environmentally friendly properties, making these colored fibers increasingly favored by the textile industry and consumers. Proanthocyanidins (PAs), the critical pigments responsible for the color of brown cotton fiber, are produced on the endoplasmic reticulum and subsequently transported to the vacuole for polymerization and/or storage. Previous studies have identified GhTT12 as a potential transmembrane transporter of PAs in Gossypium hirsutum, with GhTT12 being a homolog of Arabidopsis Transparent Testa 12 (TT12). Here, we analyzed the spatiotemporal expression pattern of GhTT12, silenced and transiently overexpressed GhTT12 in cotton to confirm its biological function. The GhTT12 protein contains two Multidrug and toxic compound extrusion (MATE) domains and 12 transmembrane helices, and the GhTT12 gene displayed predominant expressions in flowers and fibers of cotton that had higher contents of PAs, particularly in brown cotton, suggesting that GhTT12 may play a role in the transport of PAs in cotton. Silencing or transient overexpression of GhTT12 in cotton resulted in decreased or increased accumulation levels of PAs and anthocyanins (Ans), respectively, accompanied by correspondingly down- or up-regulation of genes involved in PAs synthesis (GhANR) and oxidative polymerization (GhTT10). These findings indicate that GhTT12 may also participate in the biosynthesis of PAs and Ans. Moreover, the silencing of GhTT12 led to a lightening of the color of brown cotton fibers, probably due to the reductions in both PAs content and PAs oxidation. Overall, this study, along with previous research, provides compelling evidence to support the hypothesis that GhTT12 transports PAs and Ans while also regulating their biosynthesis and oxidative polymerization, thereby promoting the accumulation of PAs and Ans in cotton and ultimately affecting the fiber coloration.

Metabolic analysis of polymeric proanthocyanidins related to red pigmentation in Camellia drupifera cv. 'Hongrou No.1' mesocarps

Red mesocarp, characterized as a unique pigment trait of newly identified Camellia drupifera cv. 'Hongrou No.1'('HR'), is believed to act as the plant's protective shield against diverse adversities. Comprehensive metabolic profiling revealed that the ectopic deposition of polymeric insoluble proanthocyanidins (PAs) in cell walls is responsible for the "red" pigmentation of 'HR' mesocarps. Furthermore, structural equation modeling and variation partitioning analysis demonstrated that a molybdenum-dependent aldehyde oxidase, encoded by CdGLOX1, was induced in 'HR' mesocarps and deemed to be a dominant determinant of polymeric insoluble PA accumulation through the putative oxidative condensation of PA subunits. This study provides a background for an in-depth understanding of the mechanisms of unperceived pigmentation in fruits.

Comparative Analysis of Floral Transcriptomes in Gossypium hirsutum (Malvaceae)

Organ-specific transcriptomes provide valuable insight into the genes involved in organ identity and developmental control. This study investigated transcriptomes of floral organs and subtending bracts in wild and domesticated Gossypium hirsutum, focusing on MADS-box genes critical for floral development. The expression profiles of A, B, C, D, and E class genes were analyzed, confirming their roles in floral organ differentiation. Hierarchical clustering revealed similar expression patterns between bracts and sepals, as well as between petals and stamens, while carpels clustered with developing cotton fibers, reflecting their shared characteristics. Beyond MADS-box genes, other transcription factors were analyzed to explore the genetic basis of floral development. While wild and domesticated cotton showed similar expression patterns for key genes, domesticated cotton exhibited significantly higher expression in carpels compared to wild cotton, which aligns with the increased number of ovules in the carpels of domesticated cotton. Functional enrichment analysis highlighted organ-specific roles: genes upregulated in bracts were enriched for photosynthesis-related GO terms, while diverse functions were enriched in floral organs, supporting their respective functions. Notably, A class genes were not significantly expressed in petals, deviating from the ABCDE model, which warrants further analysis. Lastly, the ABCDE class genes exhibited differential homoeolog expression bias toward each subgenome between two accessions, suggesting that the domestication process has influenced homoeolog utilization despite functional constraints in floral organogenesis.

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.

Comparative Lipidomics Analysis Provides New Insights into the Metabolic Basis of Color Formation in Green Cotton Fiber

Green fiber (GF) is a naturally colored fiber. A limited understanding of its color formation mechanism restricts the improvement of colored cotton quality. This experiment used upland cotton green fiber germplasm 1-4560 and genetic inbred line TM-1; the lipid profiles of green fibers at 30 (white stage) and 35 days post-anthesis (DPA) (early greening stage), as well as those of TM-1 at the same stages, were revealed. Among the 109 differential types of lipids (DTLs) unique to GF, the content of phosphatidylserine PS (16:0_18:3) was significantly different at 30 and 35 DPA. It is speculated that this lipid is crucial for the pigment accumulation and color formation process of green fibers. The 197 DTLs unique to TM-1 may be involved in white fiber (WF) development. Among the shared DTLs in GF35 vs. GF30 and WF35 vs. WF30, sulfoquinovosyldiacyl-glycerol SQDG (18:1_18:1) displays a significant difference in the content change between green fibers and white fibers, potentially affecting color formation through changes in content. The enriched metabolic pathways in both comparison groups are relatively conserved. In the most significantly enriched glycerophospholipid metabolic pathway, 1-acyl-sn-glycero-3-phosphocholine (C04230) only appears in white cotton. This indicates differences in the metabolic pathways between white and green fibers, potentially related to different mechanisms of color formation and fiber development. These findings provide a new theoretical basis for studying cotton fiber development and offer important insights into the specific mechanism of green fiber color formation.

Integrative Transcriptomic and Metabolic Analyses Reveal That Flavonoid Biosynthesis Is the Key Pathway Regulating Pigment Deposition in Naturally Brown Cotton Fibers

Brown cotton is a major cultivar of naturally colored cotton, and brown cotton fibers (BCFs) are widely utilized as raw materials for textile industry production due to their advantages of being green and dyeing-pollution-free. However, the mechanisms underlying the pigmentation in fibers are still poorly understood, which significantly limits their extensive applications in related fields. In this study, we conducted a multidimensional comparative analysis of the transcriptomes and metabolomes between brown and white fibers at different developmental periods to identify the key genes and pathways regulating the pigment deposition. The transcriptomic results indicated that the pathways of flavonoid biosynthesis and phenylpropanoid biosynthesis were significantly enriched regulatory pathways, especially in the late development periods of fiber pigmentation; furthermore, the genes distributed in the pathways of PAL, CHS, F3H, DFR, ANR, and UFGT were identified as significantly up-regulated genes. The metabolic results showed that six metabolites, namely (-)-Epigallocatechin, Apiin, Cyanidin-3-O-glucoside, Gallocatechin, Myricetin, and Poncirin, were significantly accumulated in brown fibers but not in white fibers. Integrative analysis of the transcriptomic and metabolomic data demonstrated a possible regulatory network potentially regulating the pigment deposition, in which three MYB transcription factors promote the expression levels of flavonoid biosynthesis genes, thereby inducing the content increase in (-)-Epigallocatechin, Cyanidin-3-O-glucoside, Gallocatechin, and Myricetin in BCFs. Our findings provide new insights into the pigment deposition mechanism in BCFs and offer references for genetic engineering and breeding of colored cotton materials.

The functions of laccase gene GhLAC15 in fiber colouration and development in brown-colored cotton

The monotonicity of color type in naturally colored cottons (NCCs) has become the main limiting factor to their widespread use, simultaneously coexisting with poor fiber quality. The synchronous improvement of fiber quality and color become more urgent and crucial as the demand for sustainable development increases. The homologous gene of wild cotton Gossypium stocksii LAC15 in G. hirsutum, GhLAC15, was also dominantly expressed in the developing fibers of brown cotton XC20 from 5 DPA (day post anthesis) to 25 DPA, especially at the secondary cell wall thickening stage (20 DPA and 25 DPA). In XC20 plants with downregulated GhLAC15 (GhLAC15i), a remarkable reduction in proanthocyanidins (PAs) and lignin contents was observed. Some of the key genes in the phenylpropane and flavonoid biosynthesis pathway were down-regulated in GhLAC15i plants. Notably, the fiber length of GhLAC15i plants showed an obvious increase and the fiber color was lightened. Moreover, we found that the thickness of cotton fiber cell wall was decreased in GhLAC15i plants and the fiber surface became smoother compared to that of WT. Taken together, this study revealed that GhLAC15 played an important role in PAs and lignin biosynthesis in naturally colored cotton fibers. It might mediate fiber color and fiber quality by catalyzing PAs oxidation and lignin polymerization, ultimately regulating fiber colouration and development.

Metabolic and molecular mechanisms of spine color formation in Chinese red chestnut

The spines of Chinese red chestnut are red and the depth of their color gradually increases with maturity. To identify the anthocyanin types and synthesis pathways in red chestnut and to identify the key genes regulating the anthocyanin biosynthesis pathway, we obtained and analyzed the transcriptome and anthocyanin metabolism of red chestnut and its control variety with green spines at 3 different periods. GO and KEGG analyses revealed that photosynthesis was more highly enriched in green spines compared with red spines, while processes related to defense and metabolism regulation were more highly enriched in red spines. The analysis showed that the change in spine color promoted photoprotection in red chestnut, especially at the early growth stage, which resulted in the accumulation of differentially expressed genes involved in the defense metabolic pathway. The metabolome results revealed 6 anthocyanins in red spines. Moreover, red spines exhibited high levels of cyanidin, peonidin and pelargonidin and low levels of delphinidin, petunidin and malvidin. Compared with those in the control group, the levels of cyanidin, peonidin, pelargonidin and malvidin in red spines were significantly increased, indicating that the cyanidin and pelargonidin pathways were enriched in the synthesis of anthocyanins in red spines, whereas the delphinidin pathways were inhibited and mostly transformed into malvidin. During the process of flower pigment synthesis, the expression of the CHS, CHI, F3H, CYP75A, CYP75B1, DFR and ANS genes clearly increased, that of CYP73A decreased obviously, and that of PAL, 4CL and LAR both increased and decreased. Notably, the findings revealed that the synthesized anthocyanin can be converted into anthocyanidin or epicatechin. In red spines, the upregulation of BZ1 gene expression increases the corresponding anthocyanidin content, and the upregulation of the ANR gene also promotes the conversion of anthocyanin to epicatechin. The transcription factors involved in color formation included 4 WRKYs.

Variation in pigments in pecan testa during kernel development and storage

The pecan (Carya illinoinensis) is an important tree nut worldwide. Browning of the testa during storage considerably reduces its quality. However, the pigments that cause browning and their accumulation patterns are poorly understood. We analyzed the color changes in the testa during the five developmental stages of the kernel after storage at room temperature to compare differences in their color and identify the pigments. Samples exhibiting different colors along with their corresponding -80 °C storage samples were selected for metabolomic analysis. A total of 591 phenolic compounds were detected, 52 phenolics showed regulatory effects on testa discoloration, and 59 metabolites were identified as possible precursors of the pigments. This study revealed the most thorough phenolic composition of pecan to date. Further, the findings provide new insights into the mechanisms of testa browning, deepens our understanding of the bioactive value of pecans, and contributes to the breeding of less browning-susceptible varieties.

Naturally colored cotton for wearable applications

Naturally colored cotton (NCC) offers an environmentally friendly fiber for textile applications. Processing white cotton fiber into textiles requires extensive energy, water, and chemicals, whereas processing of NCC skips the most polluting activity, scouring-bleaching and dyeing; therefore, NCC provides an avenue to minimize the harmful impacts of textile production. NCC varieties are suitable for organic agriculture since they are naturally insect and disease-resistant, salt and drought-tolerant. Various fiber shades, ranging from light green to tan and brown, are available in the cultivated NCC (Gossypium hirsutum L.) species. The pigments responsible for the color of brown cotton fiber are proanthocyanidins or their derivatives synthesized by the flavonoid pathway. Due to pigments, the NCC has excellent ultraviolet protection properties. Some brown cotton varieties exhibited superior thermal resistance of fiber that can be used to make fabrics with enhanced flame retardancy. Here, we review molecular mechanisms involved in the pigment production of brown cotton and challenges in breeding NCC varieties with a wide range of colors but without penalty in fiber quality. Also, we discuss opportunities for NCC with flame-retarding properties in textile applications.

GhmiR858 Inhibits the Accumulation of Proanthocyanidins by Targeting GhTT2L in Cotton (Gossypium hirsutum)

Proanthocyanidins (PAs) are predominantly regulated at the transcriptional level by sophisticated regulatory networks. In cotton, the role of miRNAs as key regulatory factors at the post-transcriptional level is still unclear. Here, we demonstrated that GhmiR858 negatively regulates PA accumulation in cotton leaves and calli by targeting GhTT2L. Excessive expression of GhmiR858 restrained the expression of GhTT2L, resulting in a significant decrease in PA abundance. Conversely, a reduction in GhmiR858 activity upregulated GhTT2L, which increased PA accumulation. Additionally, GhTT2L was found to positively regulate PA accumulation in both cotton and Arabidopsis. Further analyses showed that GhTT2L interacted with transcription factor GhTTG1, which directly binds to the GhANR promoter, to facilitate its transcription. This study provides new information to guide future studies of the PA regulatory mechanisms affected by miRNAs as well as the breeding of novel varieties of colored cotton with rich PAs.

A comprehensive overview of cotton genomics, biotechnology and molecular biological studies

Cotton is an irreplaceable economic crop currently domesticated in the human world for its extremely elongated fiber cells specialized in seed epidermis, which makes it of high research and application value. To date, numerous research on cotton has navigated various aspects, from multi-genome assembly, genome editing, mechanism of fiber development, metabolite biosynthesis, and analysis to genetic breeding. Genomic and 3D genomic studies reveal the origin of cotton species and the spatiotemporal asymmetric chromatin structure in fibers. Mature multiple genome editing systems, such as CRISPR/Cas9, Cas12 (Cpf1) and cytidine base editing (CBE), have been widely used in the study of candidate genes affecting fiber development. Based on this, the cotton fiber cell development network has been preliminarily drawn. Among them, the MYB-bHLH-WDR (MBW) transcription factor complex and IAA and BR signaling pathway regulate the initiation; various plant hormones, including ethylene, mediated regulatory network and membrane protein overlap fine-regulate elongation. Multistage transcription factors targeting CesA 4, 7, and 8 specifically dominate the whole process of secondary cell wall thickening. And fluorescently labeled cytoskeletal proteins can observe real-time dynamic changes in fiber development. Furthermore, research on the synthesis of cotton secondary metabolite gossypol, resistance to diseases and insect pests, plant architecture regulation, and seed oil utilization are all conducive to finding more high-quality breeding-related genes and subsequently facilitating the cultivation of better cotton varieties. This review summarizes the paramount research achievements in cotton molecular biology over the last few decades from the above aspects, thereby enabling us to conduct a status review on the current studies of cotton and provide strong theoretical support for the future direction.

Shades of Fine Dark Chocolate Colors: Polyphenol Metabolomics and Molecular Networking to Enlighten the Brown from the Black

High-quality dark chocolates (70% cocoa content) can have shades from light to dark brown color. This work aimed at revealing compounds that discriminate black and brown chocolates. From 37 fine chocolate samples from years 2019 and 2020 provided by Valrhona,8 dark black samples and 8 light brown samples were selected. A non-targeted metabolomics study was performed based on ultra-high performance liquid chromatography-high resolution mass spectrometry/mass spectrometry experiments, univariate, multivariate, and feature-based molecular networking analyses. Twenty-seven overaccumulated discriminating compounds were found for black chocolates. Among them, glycosylated flavanols including monomers and glycosylated A-type procyanidin dimers and trimers were highly representative. Fifty overaccumulated discriminating compounds were found for brown chocolates. Most of them were B-type procyanidins (from trimers to nonamers). These phenolic compounds may be partially related to the chocolate colors as precursors of colored compounds. This study increases the knowledge on the chemical diversity of dark chocolates by providing new information about the phenolic profiles of black and brown chocolates.

Cloning and functional analysis of GhDFR1, a key gene of flavonoid synthesis pathway in naturally colored cotton

BACKGROUND

The naturally colored brown cotton fiber is the most widely used environmentally friendly textile material, which primarily contains proanthocyanidins and their derivatives. Many structural genes in the flavonoid synthesis pathway are known to improve the genetic resources of naturally colored cotton. Among them, DFR is a crucial late enzyme to synthesis both anthocyanins and proanthocyanidins in the plant flavonoid pathway.

METHODS

The protein sequences of GhDFRs were analyzed using bioinformatic tools. The expression levels of GhDFRs in various tissues and organs of upland cotton Zongxu1 (ZX1), were analyzed by quantitative real-time PCR, and the expression pattern of GhDFR1 during fiber development of white cotton and brown cotton was analyzed further. The function of GhDFR1 in NCC ZX1 was preliminarily analyzed by virus induced gene silencing (VIGS) technology.

RESULTS

Bioinformatic analysis revealed that GhDFRs sequences in upland cotton genome were extremely conserved. Furthermore, evolutionary tree analysis revealed that the functions of GhDFR1 and GhDFR2, and GhDFR3 and GhDFR4, presented different and shared some similarities. Our study showed GhDFR1 and GhDFR2 were specifically expressed in fibers, while GhDFR3 and GhDFR4 were specifically expressed in petals. GhDFR1 was exclusively expressed in brown cotton fiber at various stages of development and progressively increased with the growth of fiber, but the trend of expression in white cotton was quite the opposite. We silenced GhDFR1 expression in brown cotton fiber using VIGS technology, and observed the VIGS-interference plants. After reducing the expression level of GhDFR1, the period for significant GhDFR1 expression in the developing fibers changed, reducing the content of anthocyanins, and lightening the color of mature cotton fibers.

CONCLUSION

GhDFR1 was preferentially expressed in brown cotton during fiber development. The timing of GhDFR1 expression for flavonoid synthesis altered, resulting in anthocyanin contents reduced and the fiber color of the GhDFR1i lines lightened. These findings showed the role of GhDFR1 in fiber coloration of NCC and provided a new candidate for NCC genetic improvement.

Identification and Functional Analysis of the Promoter of a Leucoanthocyanidin Reductase Gene from Gossypium hirsutum

Leucoanthocyanidin reductase (LAR) is the critical enzyme in the synthesis pathway of proanthocyanidins, which are the primary pigments in brown cotton fibers. Our previous study has revealed significant differences in the expression levels of GhLAR1 between white and brown cotton fibers at 10 DPA. In this work, the expression pattern of the GhLAR1 gene was further studied, and the promoter of GhLAR1 (1780 bp) was isolated and characterized. Bioinformatic analysis indicated that GhLAR1 promoter contained many known light response elements and several defenses related to transcriptional factor-binding boxes, which may partially explain the response of the GhLAR1 to temperature, NaCl, and PEG treatments. Furthermore, GhLAR1 was preferentially and strongly expressed in fibers and flowers of cotton, and the expression levels in all tested tissues (especially fibers) of brown cotton were significantly higher than those in white cotton. Consistent with the expression analysis, the GhLAR1 promoter mainly drove GUS expression in epidermal trichomes and floral organs.

The complexities of proanthocyanidin biosynthesis and its regulation in plants

Proanthocyanidins (PAs) are natural flavan-3-ol polymers that contribute protection to plants under biotic and abiotic stress, benefits to human health, and bitterness and astringency to food products. They are also potential targets for carbon sequestration for climate mitigation. In recent years, from model species to commercial crops, research has moved closer to elucidating the flux control and channeling, subunit biosynthesis and polymerization, transport mechanisms, and regulatory networks involved in plant PA metabolism. This review extends the conventional understanding with recent findings that provide new insights to address lingering questions and focus strategies for manipulating PA traits in plants.

Transcriptome and Metabolome Profiling Unveil Pigment Formation Variations in Brown Cotton Lines (Gossypium hirsutum L.)

Naturally brown colored cotton (NBCC) is becoming increasingly popular due to its natural properties of coloration. However, poor fiber quality and color fading are key issues that are hindering the cultivation of naturally colored cotton. In this study, based on transcriptome and metabolome of 18 days post-anthesis (DPA), we compared the variations of pigment formation in two brown cotton fibers (DCF and LCF), with white cotton fiber (WCF) belonging to a near-isogenic line. A transcriptome study revealed a total of 15,785 differentially expressed genes significantly enriched in the flavonoid biosynthesis pathway. Furthermore, for flavonoid biosynthesis-related genes, such as flavonoid 3′5′-hydroxylase (F3′5′H), anthocyanidin synthase (ANS), anthocyanidin reductase (ANR), chalcone synthase (CHS), dihydroflavonol 4-reductase (DFR), and chalcone isomerase (CHI), their expressions significantly increased in LCF compared with DCF and WCF. Moreover, transcription factors MYB and bHLH were significantly expressed in LCF and DCF. Most flavonoid-related metabolites (myricetin naringenin, catechin, epicatechin-epiafzelechin, and epigallocatechin) were found to be more highly up-regulated in LCF and DCF than WCF. These findings reveal the regulatory mechanism controlling different brown pigmentation in cotton fibers and elucidate the need for the proper selection of high-quality brown cotton fiber breeding lines for promising fiber quality and durable brown color pigmentation.

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

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

Function deficiency of GhOMT1 causes anthocyanidins over-accumulation and diversifies fibre colours in cotton (Gossypium hirsutum)

Naturally coloured cotton (NCC) fibres need little or no dyeing process in textile industry to low-carbon emission and are environment-friendly. Proanthocyanidins (PAs) and their derivatives were considered as the main components causing fibre coloration and made NCCs very popular and healthy, but the monotonous fibre colours greatly limit the wide application of NCCs. Here a G. hirsutum empurpled mutant (HS2) caused by T-DNA insertion is found to enhance the anthocyanidins biosynthesis and accumulate anthocyanidins in the whole plant. HPLC and LC/MS-ESI analysis confirmed the anthocyanidins methylation and peonidin, petunidin and malvidin formation are blocked. The deficiency of GhOMT1 in HS2 was associated with the activation of the anthocyanidin biosynthesis and the altered components of anthocyanidins. The transcripts of key genes in anthocyanidin biosynthesis pathway are significantly up-regulated in HS2, while transcripts of the genes for transport and decoration were at similar levels as in WT. To investigate the potential mechanism of GhOMT1 deficiency in cotton fibre coloration, HS2 mutant was crossed with NCCs. Surprisingly, offsprings of HS2 and NCCs enhanced PAs biosynthesis and increased PAs levels in their fibres from the accumulated anthocyanidins through up-regulated GhANR and GhLAR. As expected, multiple novel lines with improved fibre colours including orange red and navy blue were produced in their generations. Based on this work, a new strategy for breeding diversified NCCs was brought out by promoting PA biosynthesis. This work will help shed light on mechanisms of PA biosynthesis and bring out potential molecular breeding strategy to increase PA levels in NCCs.

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

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

Transcriptomic and Metabolomic Responses in Cotton Plant to Apolygus lucorum Infestation

With the wide-scale adoption of transgenic Bacillus thuringiensis (Bt) cotton, Apolygus lucorum (Meyer-Dür) has become the most serious pest and has caused extensive yield loss in cotton production. However, little is known about the defense responses of cotton at the seedling stage to A. lucorum feeding. In this study, to elucidate the cotton defense mechanism, cotton leaves were damaged by A. lucorum for 0, 4, 12 and 24 h. The transcriptomic results showed that A. lucorum feeding elicits a rapid and strong defense response in gene expression during the whole infestation process in cotton plants. Further analysis revealed that at each assessment time, more differentially expressed genes were up-regulated than down-regulated. The integrated analysis of transcriptomic and metabolic data showed that most of the genes involved in jasmonic acid (JA) biosynthesis were initially up-regulated, and this trend continued during an infestation. Meanwhile, the content levels of JA and its intermediate products were also significantly increased throughout the whole infestation process. The similar trend was displayed in condensed tannins biosynthesis. This research proved that, after plants are damaged by A. lucorum, the JA pathway mediates the defense mechanisms in cotton plants by promoting the accumulation of condensed tannins as a defense mechanism against A. lucorum. These results will help us to discover unknown defensive genes and improve the integrated pest management of A. lucorum.

Effects of Natural Brown Cotton Bleached Gauze on Wound Healing

Natural brown cotton has favorable antibacterial and antioxidant properties. In this study, we explored the effect of gauze made from natural brown cotton after scouring and bleaching on wound healing in rats. In this work, a control experiment was adopted. The control group used absorbent cotton gauze, and the experimental group utilized natural brown cotton bleached gauze. The materials were applied to rat models to explore the effects of the two dressings on wound healing. By analyzing the wound healing state of rats, calculating the healing rate, and combining the pathological HE staining, Masson staining, and CD31 immunohistochemical staining, the results showed that both gauzes have positive effects on the wound healing of the rats. Moreover, compared with the control group, the wound healing rate of rats in the experimental group increased by 14.81%, the number of inflammatory cells decreased by 12.93%, the number of new blood vessels increased by 6.88%, the growth rate of the granulation tissue area was 10.76%, the step-up rate of the area occupied by collagen was 33.71%, and the increase rate of optical density value was 10.00%. This study found that natural brown cotton bleached gauze has a better effect on wound healing than ordinary absorbent cotton gauze, and can be used as medical dressings.

Transcriptome Co-expression Network and Metabolome Analysis Identifies Key Genes and Regulators of Proanthocyanidins Biosynthesis in Brown Cotton

Brown cotton fiber (BCF) is a unique raw material of naturally colored cotton (NCC). But characteristics of the regulatory gene network and metabolic components related to the proanthocyanidins biosynthesis pathway at various stages of its fiber development remain unclear. Here, the dynamic changes in proanthocyanidins biosynthesis components and transcripts in the BCF variety "Zong 1-61" and its white near-isogenic lines (NILs) "RT" were characterized at five fiber developmental stages (0, 5, 10, 15, and 20 days post-anthesis; DPA). Enrichment analysis of differentially expressed genes (DEGs), comparison of metabolome differences, and pathway enrichment analysis of a weighted gene correlation network analysis together revealed the dominant gene expression of flavonoid biosynthesis (FB), phenylpropanoid metabolisms, and some carbohydrate metabolisms at 15 or 20 DPA than white cotton. Eventually, 63 genes were identified from five modules putatively related to FB. Three R2R3-MYB and two bHLH transcription factors were predicted as the core genes. Further, GhANS, GhANR1, and GhUFGT2 were preliminarily regulated by GhMYB46, GhMYB6, and GhMYB3, respectively, according to yeast one-hybrid assays in vitro. Our findings provide an important transcriptional regulatory network of proanthocyanidins biosynthesis pathway and dynamic flavonoid metabolism profiles.

Biochemical and Expression Analyses Revealed the Involvement of Proanthocyanidins and/or Their Derivatives in Fiber Pigmentation of Gossypium stocksii

The wild cotton species Gossypium stocksii produces a brown fiber that provides a valuable resource for the color improvement of naturally colored cotton (NCC) fiber. However, the biochemical basis and molecular mechanism of its fiber pigmentation remain unclear. Herein, we analyzed the dynamics of proanthocyanidins (PAs) accumulation in developing the fiber of G. stocksii, which suggested a similar role of PAs and/or their derivatives in the fiber coloration of G. stocksii. In addition, comparative transcriptomics analyses revealed that the PA biosynthetic genes were expressed at higher levels and for a longer period in developing fibers of G. stocksii than G. arboreum (white fiber), and the transcription factors, such as TT8, possibly played crucial regulatory roles in regulating the PA branch genes. Moreover, we found that the anthocyanidin reductase (ANR) was expressed at a higher level than the leucoanthocyanidin reductases (LARs) and significantly upregulated during fiber elongation, suggesting a major role of ANR in PA synthesis in G. stocksii fiber. In summary, this work revealed the accumulation of PAs and the expression enhancement of PA biosynthetic genes in developing fibers of G. stocksii. We believe this work will help our understanding of the molecular mechanisms of cotton fiber coloration and further promote the future breeding of novel NCCs.

Breeding Next-Generation Naturally Colored Cotton

Use of naturally colored cotton (NCC) in the textile industry is limited due to its inferior fiber quality. Recent advances in understanding pigmentation in NCC fiber and the genetic relationship between pigmentation and fiber development may overcome this hindrance and can guide developing high fiber quality and yield NCC.

GhGSTF12, a glutathione S-transferase gene, is essential for anthocyanin accumulation in cotton (Gossypium hirsutum L.)

Anthocyanins are flavonoid pigments providing plants a range of colors from red, pink, orange to blue. Anthocyanins are synthesized in the cytosol but accumulate predominantly in the vacuoles through vacuolar sequestration involving glutathione S-transferases (GSTs) and multidrug and toxic compound extrusion (MATE) and the ATP binding cassette (ABC) transporters. However, little is known about anthocyanin-related GSTs in Upland cotton (Gossypium hirsutum L.). In this study, we performed genome-wide identification of GST genes in Upland cotton and identified GST genes functioning in accumulation of anthocyanins. We demonstrated that GhGSTF12 was able to complement the defective leaf color phenotypes of the Arabidopsis tt19 mutant caused by mutation in a GSTF gene. Virus-induced silencing of GhGSTF12 in the red leaf cultivar turned its red color to green and transient overexpression of GhGSTF12 accelerated anthocyanin accumulation in the red leaf cultivar but not in the green leaf cultivar. Collectively, GhGSTF12 may be involved in transport of anthocyanins from cytosol to vacuoles in cotton. These results also demonstrated a conserved function of plant GSTF genes in anthocyanin accumulation and provide a candidate gene for manipulating pigmentation in cotton tissues.

Quantitative metabolome and transcriptome analysis reveals complex regulatory pathway underlying photoinduced fiber color formation in cotton

As an important plant single cell model and textile application materials, poorly known about fiber color formation in cotton, which is sensitively regulated by environmental signals. Our studies underline the importance of photo signal on sensitive fiber color formation and characterize fiber color early initiation (15 DPA) and late accumulated metabolites (45 DPA) in different lighting condition. The results revealed 236 differential metabolites between control and shading, of which phenylpropanoids metabolites accounted for 20%, including uncharacterized novel metabolites and pathways. Furthermore, the early initiation specific genes respond to the absence of light are highly correlated with phenylpropanoid metabolites related to pigmentation. The current study reveals the complex pathways involving early initiation regulation and late metabolic pathways. In addition, the collection composed of uncharacterized photoinduced metabolites and early initiation signaling/regulatory genes were identified, which are important resources for understanding fiber color formation. This report provides new insight into molecular regulatory and biochemical basis underlying photoinduced fiber color formation in cotton.

Functional analysis of GhCHS, GhANR and GhLAR in colored fiber formation of Gossypium hirsutum L

BACKGROUND

The formation of natural colored fibers mainly results from the accumulation of different anthocyanidins and their derivatives in the fibers of Gossypium hirsutum L. Chalcone synthase (CHS) is the first committed enzyme of flavonoid biosynthesis, and anthocyanidins are transported into fiber cells after biosynthesis mainly by Anthocyanidin reductase (ANR) and Leucoanthocyanidin reductase (LAR) to present diverse colors with distinct stability. The biochemical and molecular mechanism of pigment formation in natural colored cotton fiber is not clear.

RESULTS

The three key genes of GhCHS, GhANR and GhLAR were predominantly expressed in the developing fibers of colored cotton. In the GhCHSi, GhANRi and GhLARi transgenic cottons, the expression levels of GhCHS, GhANR and GhLAR significantly decreased in the developing cotton fiber, negatively correlated with the content of anthocyanidins and the color depth of cotton fiber. In colored cotton Zongxu1 (ZX1) and the GhCHSi, GhANRi and GhLARi transgenic lines of ZX1, HZ and ZH, the anthocyanidin contents of the leaves, cotton kernels, the mixture of fiber and seedcoat were all changed and positively correlated with the fiber color.

CONCLUSION

The three genes of GhCHS, GhANR and GhLAR were predominantly expressed early in developing colored cotton fibers and identified to be a key genes of cotton fiber color formation. The expression levels of the three genes affected the anthocyanidin contents and fiber color depth. So the three genes played a crucial part in cotton fiber color formation and has important significant to improve natural colored cotton quality and create new colored cotton germplasm resources by genetic engineering.

Transcriptome Sequencing and Metabolome Analysis Reveal Genes Involved in Pigmentation of Green-Colored Cotton Fibers

Green-colored fiber (GCF) is the unique raw material for naturally colored cotton textile but we know little about the pigmentation process in GCF. Here we compared transcriptomes and metabolomes of 12, 18 and 24 days post-anthesis (DPA) fibers from a green fiber cotton accession and its white-colored fiber (WCF) near-isogenic line. We found a total of 2047 non-redundant metabolites in GCF and WCF that were enriched in 80 pathways, including those of biosynthesis of phenylpropanoid, cutin, suberin, and wax. Most metabolites, particularly sinapaldehyde, of the phenylpropanoid pathway had a higher level in GCF than in WCF, consistent with the significant up-regulation of the genes responsible for biosynthesis of those metabolites. Weighted gene co-expression network analysis (WGCNA) of genes differentially expressed between GCF and WCF was used to uncover gene-modules co-expressed or associated with the accumulation of green pigments. Of the 16 gene-modules co-expressed with fiber color or time points, the blue module associated with G24 (i.e., GCF at 24 DPA) was of particular importance because a large proportion of its genes were significantly up-regulated at 24 DPA when fiber color was visually distinguishable between GCF and WCF. A total of 56 hub genes, including the two homoeologous Gh4CL4 that could act in green pigment biosynthesis, were identified among the genes of the blue module that are mainly involved in lipid metabolism, phenylpropanoid biosynthesis, RNA transcription, signaling, and transport. Our results provide novel insights into the mechanisms underlying pigmentation of green fibers and clues for developing cottons with stable green colored fibers.

Zinc Finger-Homeodomain Transcriptional Factors (ZHDs) in Upland Cotton (Gossypium hirsutum): Genome-Wide Identification and Expression Analysis in Fiber Development

Zinc finger-homeodomain (ZHD) genes encode a family of plant-specific transcription factors that not only participate in the regulation of plant growth and development but also play an important role in the response to abiotic stress. The ZHD gene family has been studied in several model plants, including Solanum lycopersicum, Zea mays, Oryza sativa, and Arabidopsis thaliana. However, a comprehensive study of the genes of the ZHD family and their roles in fiber development and pigmentation in upland cotton has not been completed. To address this gap, we selected a brown fiber cultivar for our study; brown color in cotton is one of the most desired colors in the textile industry. The natural colored fibers require less processing and little dying, thereby eliminating dye costs and chemical residues. Using bioinformatics approaches, we identified 37 GhZHD genes from Gossypium hirsutum and then divided these genes into seven groups based on their phylogeny. The GhZHD genes were mostly conserved in each subfamily with minor variations in motif distribution and gene structure. These genes were largely distributed on 19 of the 26 upland cotton chromosomes. Among the Gossypium genomes, the paralogs and orthologs of the GhZHD genes were identified and further characterized. Furthermore, among the paralogs, we observed that the ZHD family duplications in Gossypium genomes (G. hirsutum, G. arboreum, and G. raimondii) were probably derived from segmental duplication or genome-wide duplication (GWD) events. Through a combination of qRT-PCR and proanthocyanidins (PA) accumulation analyses in brown cotton fibers, we concluded that the candidate genes involved in early fiber development and fiber pigment synthesis include the following: GhZHD29, GhZHD35, GhZHD30, GhZHD31, GhZHD11, GhZHD27, GhZHD18, GhZHD15, GhZHD16, GhZHD22, GhZHD6, GhZHD33, GhZHD13, GhZHD5, and GhZHD23. This study delivers insights into the evolution of the GhZHD genes in brown cotton, serves as a valuable resource for further studies, and identifies the conditions necessary for improving the quality of brown cotton fiber.

Up-regulation of GhTT2-3A in cotton fibres during secondary wall thickening results in brown fibres with improved quality

Brown cotton fibres are the most widely used naturally coloured raw materials for the eco-friendly textile industry. Previous studies have indicated that brown fibre pigments belong to proanthocyanidins (PAs) or their derivatives, and fibre coloration is negatively associated with cotton productivity and fibre quality. To date, the molecular basis controlling the biosynthesis and accumulation of brown pigments in cotton fibres is largely unknown. In this study, based on expressional and transgenic analyses of cotton homologs of ArabidopsisPA regulator TRANSPARENT TESTA 2 (TT2) and fine-mapping of the cotton dark-brown fibre gene (Lc1), we show that a TT2 homolog, GhTT2-3A, controls PA biosynthesis and brown pigmentation in cotton fibres. We observed that GhTT2-3A activated GhbHLH130D, a homolog of ArabidopsisTT8, which in turn synergistically acted with GhTT2-3A to activate downstream PA structural genes and PA synthesis and accumulation in cotton fibres. Furthermore, the up-regulation of GhTT2-3A in fibres at the secondary wall-thickening stage resulted in brown mature fibres, and fibre quality and lint percentage were comparable to that of the white-fibre control. The findings of this study reveal the regulatory mechanism controlling brown pigmentation in cotton fibres and demonstrate a promising biotechnological strategy to break the negative linkage between coloration and fibre quality and/or productivity.

Linkage and association mapping reveals the genetic basis of brown fibre (Gossypium hirsutum)

Brown fibre cotton is an environmental-friendly resource that plays a key role in the textile industry. However, the fibre quality and yield of natural brown cotton are poor, and fundamental research on brown cotton is relatively scarce. To understand the genetic basis of brown fibre cotton, we constructed linkage and association populations to systematically examine brown fibre accessions. We fine-mapped the brown fibre region, Lc1 , and dissected it into 2 loci, qBF-A07-1 and qBF-A07-2. The qBF-A07-1 locus mediates the initiation of brown fibre production, whereas the shade of the brown fibre is affected by the interaction between qBF-A07-1 and qBF-A07-2. Gh_A07G2341 and Gh_A07G0100 were identified as candidate genes for qBF-A07-1 and qBF-A07-2, respectively. Haploid analysis of the signals significantly associated with these two loci showed that most tetraploid modern brown cotton accessions exhibit the introgression signature of Gossypium barbadense. We identified 10 quantitative trait loci (QTLs) for fibre yield and 19 QTLs for fibre quality through a genome-wide association study (GWAS) and found that qBF-A07-2 negatively affects fibre yield and quality through an epistatic interaction with qBF-A07-1. This study sheds light on the genetics of fibre colour and lint-related traits in brown fibre cotton, which will guide the elite cultivars breeding of brown fibre cotton.

Characterization of two TT2-type MYB transcription factors regulating proanthocyanidin biosynthesis in tetraploid cotton, Gossypium hirsutum

Two TT2-type MYB transcription factors identified from tetraploid cotton are involved in regulating proanthocyanidin biosynthesis, providing new strategies for engineering condensed tannins in crops. Proanthocyanidins (PAs), also known as condensed tannins, are important secondary metabolites involved in stress resistance in plants, and are health supplements that help to reduce cholesterol levels. As one of the most widely grown crops in the world, cotton provides the majority of natural fabrics and is a supplemental food for ruminant animals. The previous studies have suggested that PAs present in cotton are a major contributor to fiber color. However, the biosynthesis of PAs in cotton still remains to be elucidated. AtTT2 (transparent testa 2) is a MYB family transcription factor from Arabidopsis that initiates the biosynthesis of PAs by inducing the expression of multiple genes in the pathway. In this study, we isolated two R2R3-type MYB transcription factors from Gossypium hirsutum that are homologous to AtTT2. Expression analysis showed that both genes were expressed at different levels in various cotton tissues, including leaf, seed coat, and fiber. Protoplast transactivation assays revealed that these two GhMYBs were able to activate promoters of genes encoding enzymes in the PA biosynthesis pathway, namely anthocyanidin reductase and leucoanthocyanidin reductase. Complementation experiments showed that both of the GhMYBs were able to recover the transparent testa seed coat phenotype of the Arabidopsis tt2 mutant by restoring PA biosynthesis. Ectopic expression of either of the two GhMYBs in Medicago truncatula hairy roots increased the contents of anthocyanins and PAs compared to control lines expressing the GUS gene, and expression levels of MtDFR, MtLAR, and MtANR were also elevated in lines expressing GhMYBs. Together, these data provide new insights into engineering condensed tannins in cotton.

Molecular analysis of caffeoyl residues related to pigmentation in green cotton fibers

The pigment components in green cotton fibers were isolated and identified as 22-O-caffeoyl-22-hydroxymonodocosanoin and 22-O-caffeoyl-22-hydroxydocosanoic acid. The concentration of 22-O-caffeoyl-22-hydroxymonodocosanoin correlated positively with the degree of colour in the green fibers, indicating a role for caffeoyl derivatives in the pigmentation of green cotton fibers. Upland cotton (Gossypium hirsutum L.) contains four genes, Gh4CL1-Gh4CL4, encoding 4-coumarate:CoA ligases (4CLs), key enzymes in the phenylpropanoid biosynthesis pathway. In 15-24-day post-anthesis fibers, the expression level of Gh4CL1 was very low, Gh4CL3 had a similar expression level in both white and green cottons, Gh4CL2 had a significantly higher expression level in green fibers than in white fibers, while Gh4CL4 had a higher expression level in white fibers than in green fibers. According to enzyme kinetics analysis, Gh4CL1 displayed a preference for 4-coumarate, Gh4CL3 and Gh4CL4 exhibited a somewhat low but still prominent activity towards ferulate, while Gh4CL2 had a strong preference for caffeate and ferulate. These results suggest that Gh4CL2 might be involved in the metabolism of caffeoyl residues and related to pigment biosynthesis in green cotton fibers. Our findings provide insights for understanding the biochemical and molecular mechanisms of pigmentation in green cotton fibers.

Asymmetric subgenome selection and cis-regulatory divergence during cotton domestication

Comparative population genomics offers an excellent opportunity for unraveling the genetic history of crop domestication. Upland cotton (Gossypium hirsutum) has long been an important economic crop, but a genome-wide and evolutionary understanding of the effects of human selection is lacking. Here, we describe a variation map for 352 wild and domesticated cotton accessions. We scanned 93 domestication sweeps occupying 74 Mb of the A subgenome and 104 Mb of the D subgenome, and identified 19 candidate loci for fiber-quality-related traits through a genome-wide association study. We provide evidence showing asymmetric subgenome domestication for directional selection of long fibers. Global analyses of DNase I-hypersensitive sites and 3D genome architecture, linking functional variants to gene transcription, demonstrate the effects of domestication on cis-regulatory divergence. This study provides new insights into the evolution of gene organization, regulation and adaptation in a major crop, and should serve as a rich resource for genome-based cotton improvement.

The GhTT2_A07 gene is linked to the brown colour and natural flame retardancy phenotypes of Lc1 cotton (Gossypium hirsutum L.) fibres

Some naturally coloured brown cotton fibres from accessions of Gossypium hirsutum L. can be used to make textiles with enhanced flame retardancy (FR). Several independent brown fibre loci have been identified and mapped to chromosomes, but the underlying genes have not yet been identified, and the mechanism of lint fibre FR is not yet fully understood. In this study, we show that both the brown colour and enhanced FR of the Lc1 lint colour locus are linked to a 1.4Mb inversion on chromosome A07 that is immediately upstream of a gene with similarity to Arabidopsis TRANSPARENT TESTA 2 (TT2). As a result of the alternative upstream sequence, the transcription factor GhTT2_A07 is highly up-regulated in developing fibres. In turn, genes in the phenylpropanoid metabolic pathway are activated, leading to biosynthesis of proanthocyanidins and accumulation of inorganic elements. We show that enhanced FR and anthocyanin precursors appear in developing brown fibres well before the brown colour is detectible, demonstrating for the first time that the polymerized proanthocyanidins that constitute the brown colour are not the source of enhanced FR. Identifying the particular colourless metabolite that provides Lc1 cotton with enhanced FR could help minimize the use of synthetic chemical flame retardant additives in textiles.