Flavonoid deficiency disrupts redox homeostasis and terpenoid biosynthesis in glandular trichomes of tomato

CbMYB108 integrates the regulation of diterpene biosynthesis and trichome development in Conyza blinii against UV-B

Plants synthesize abundant terpenes through glandular trichomes (GTs), thereby protecting themselves from environmental stresses and increasing the economic value in some medicinal plants. However, the potential mechanisms for simultaneously regulating terpenes synthesis and GTs development remain unclear. Here, we showed that terpenes in Conyza blinii could be synthesized through capitate GTs. By treating with appropriate intensity of UV-B, the density of capitate GTs and diterpene content can be increased. Through analyzing corresponding transcriptome, we identified a MYB transcription factor CbMYB108 as a positive regulator of both diterpene synthesis and capitate GT density. Transiently overexpressing/silencing CbMYB108 on C. blinii leaves could increase diterpene synthesis and capitate GT density. Further verification showed that CbMYB108 upregulated CbDXS and CbGGPPS expression in diterpene synthesis pathway. Moreover, CbMYB108 could also upregulated the expression of CbTTG1, key WD40 protein confirmed in this study to promote GT development, rather than through interaction between CbMYB108 and CbTTG1 proteins. Thus, results showed that the UV-B-induced CbMYB108 owned dual-function of simultaneously improving diterpene synthesis and GT development. Our research lays a theoretical foundation for cultivating C. blinii with high terpene content, and broadens the understanding of the integrated mechanism on terpene synthesis and GT development in plants.

Physiological and biochemical mechanisms underlying the role of anthocyanin in acquired tolerance to salt stress in peanut (Arachis hypogaea L.)

Anthocyanin is an important pigment that prevents oxidative stress and mediates adaptation of plants to salt stress. Peanuts with dark red and black testa are rich in anthocyanin. However, correlation between salt tolerance and anthocyanin content in black and dark red testa peanuts is unknown. In this study, three peanut cultivars namely YZ9102 (pink testa), JHR1 (red testa) and JHB1 (black testa) were subjected to sodium chloride (NaCl) stress. The plant growth, ion uptake, anthocyanin accumulation, oxidation resistance and photosynthetic traits were comparatively analyzed. We observed that the plant height, leaf area and biomass under salt stress was highly inhibited in pink color testa (YZ9102) as compare to black color testa (JHB1). JHB1, a black testa colored peanut was identified as the most salt-tolerance cultivar, followed by red (JHR1) and pink(YZ9102). During salt stress, JHB1 exhibited significantly higher levels of anthocyanin and flavonoid accumulation compared to JHR1 and YZ9102, along with increased relative activities of antioxidant protection and photosynthetic efficiency. However, the K+/Na+ and Ca2+/Na+ were consistently decreased among three cultivars under salt stress, suggesting that the salt tolerance of black testa peanut may not be related to ion absorption. Therefore, we predicted that salt tolerance of JHB1 may be attributed to the accumulation of the anthocyanin and flavonoids, which activated antioxidant protection against the oxidative damage to maintain the higher photosynthetic efficiency and plant growth. These findings will be useful for improving salt tolerance of peanuts.

Multifaceted roles of WRKY transcription factors in abiotic stress and flavonoid biosynthesis

Increasing biotic and abiotic stresses are seriously impeding the growth and yield of staple crops and threatening global food security. As one of the largest classes of regulators in vascular plants, WRKY transcription factors play critical roles governing flavonoid biosynthesis during stress responses. By binding major W-box cis-elements (TGACCA/T) in target promoters, WRKYs modulate diverse signaling pathways. In this review, we optimized existing WRKY phylogenetic trees by incorporating additional plant species with WRKY proteins implicated in stress tolerance and flavonoid regulation. Based on the improved frameworks and documented results, we aim to deduce unifying themes of distinct WRKY subfamilies governing specific stress responses and flavonoid metabolism. These analyses will generate experimentally testable hypotheses regarding the putative functions of uncharacterized WRKY homologs in tuning flavonoid accumulation to enhance stress resilience.

Flavonoid production in tomato mediates both direct and indirect plant defences against whiteflies in tritrophic interactions

BACKGROUND

The role of plant flavonoids in direct defences against chewing and sap-sucking herbivorous insects has been extensively characterized. However, little is known about flavonoid-mediated tritrophic interactions between plants, herbivorous insects and natural enemies. In this study, we investigated how flavonoids modulate plant-insect interactions in a tritrophic system involving near-isogenic lines (NILs) of cultivated tomato (Solanum lycopersicum) with high (line NIL-purple hypocotyl [PH]) and low (line NIL-green hypocotyl [GH]) flavonoid levels, with a generalist herbivore whitefly (Bemisia tabaci) and its predatory bug (Orius sauteri).

RESULTS

By contrasting levels of tomato flavonoids (direct defence) while manipulating the presence of predators (indirect defence), we found that high production of flavonoids in tomato was associated with a higher inducibility of direct defences and a stronger plant resistance to whitefly infestation and stimulated the emissions of induced volatile organic compounds, thereby increasing the attractiveness of B. tabaci-infested plants to the predator O. sauteri. Furthermore, suppression of B. tabaci population growth and enhancement of plant growth were mediated directly by the high production of flavonoids and indirectly by the attraction of O. sauteri, and the combined effects were larger than each effect individually.

CONCLUSION

Our results show that high flavonoid production in tomato enhances herbivore-induced direct and indirect defences to better defend against herbivores in tritrophic interactions. Thus, the development of transgenic plants may present an opportunity to utilize the beneficial role of flavonoids in integrated pest management, while simultaneously maintaining or improving resistance against other pests and pathogens. © 2023 Society of Chemical Industry.

Tomato defences modulate not only insect performance but also their gut microbial composition

Plants protect their tissues from insect herbivory with specialized structures and chemicals, such as cuticles, trichomes, and metabolites contained therein. Bacteria inside the insect gut are also exposed to plant defences and can potentially modify the outcome of plant-insect interactions. To disentangle this complex multi-organism system, we used tomato mutants impaired in the production of plant defences (odorless-2 and jasmonic acid-insensitive1) and two cultivars (Ailsa Craig and Castlemart), exposed them to herbivory by the cabbage looper (Trichoplusia ni H.) and collected the insect frass for bacterial community analysis. While the epicuticular wax and terpene profiles were variable, the leaf fatty acid composition remained consistent among genotypes. Moreover, larval weight confirmed the negative association between plant defences and insect performance. The distinctive frass fatty acid profiles indicated that plant genotype also influences the lipid digestive metabolism of insects. Additionally, comparisons of leaf and insect-gut bacterial communities revealed a limited overlap in bacterial species between the two sample types. Insect bacterial community abundance and diversity were notably reduced in insects fed on the mutants, with Enterobacteriaceae being the predominant group, whereas putatively pathogenic taxa were found in wildtype genotypes. Altogether, these results indicate that plant defences can modulate insect-associated bacterial community composition.

Homeostasis of flavonoids and triterpenoids most likely modulates starch metabolism for pollen tube penetration in rice

In angiosperms, the timely delivery of sperm cell nuclei by pollen tube (PT) to the ovule is vital for double fertilization. Penetration of PT into maternal stigma tissue is a critical step for sperm cell nuclei delivery, yet little is known about the process. Here, a male-specific and sporophytic mutant xt6, where PTs are able to germinate but unable to penetrate the stigma tissue, is reported in Oryza sativa. Through genetic study, the causative gene was identified as Chalcone synthase (OsCHS1), encoding the first enzyme in flavonoid biosynthesis. Indeed, flavonols were undetected in mutant pollen grains and PTs, indicating that the mutation abolished flavonoid biosynthesis. Nevertheless, the phenotype cannot be rescued by exogenous application of quercetin and kaempferol as reported in maize and petunia, suggesting a different mechanism exists in rice. Further analysis showed that loss of OsCHS1 function disrupted the homeostasis of flavonoid and triterpenoid metabolism and led to the accumulation of triterpenoid, which inhibits significantly α-amylase activity, amyloplast hydrolysis and monosaccharide content in xt6, these ultimately impaired tricarboxylic acid (TCA) cycle, reduced ATP content and lowered the turgor pressure as well. Our findings reveal a new mechanism that OsCHS1 modulates starch hydrolysis and glycometabolism through modulating the metabolic homeostasis of flavonoids and triterpenoids which affects α-amylase activity to maintain PT penetration in rice, which contributes to a better understanding of the function of CHS1 in crop fertility and breeding.

New insight into phytometabolism and phytotoxicity mechanism of widespread plasticizer di (2-ethylhexyl) phthalate in rice plants

Di-(2-ethylhexyl) phthalate (DEHP) as widely utilized plasticizer has aroused increasing concerns since its endocrine disrupting effects and continuous accumulation in biota. To date, the interaction mechanism between DEHP and rice plants has not been clearly illustrated at molecular level. Here, we investigated biological transformation and response of rice plants (Oryza sativa L.) to DEHP at realistic exposure concentrations. Nontargeted screening by UPLC-QTOF-MS was used to verify 21 transformation products derived from phase I metabolism (hydroxylation and hydrolysis) and phase II metabolism (conjugation with amino acids, glutathione, and carbohydrates) in rice. MEHHP-asp, MEHHP-tyr, MEHHP-ala, MECPP-tyr and MEOHP-tyr as the conjugation products with amino acids are observed for the first time. Transcriptomics analyses unraveled that DEHP exposure had strong negative effects on genes associated with antioxidative components synthesis, DNA binding, nucleotide excision repair, intracellular homeostasis, and anabolism. Untargeted metabolomics revealed that metabolic network reprogramming in rice roots was induced by DEHP, including nucleotide metabolism, carbohydrate metabolism, amino acid synthesis, lipid metabolism, synthesis of antioxidant component, organic acid metabolism and phenylpropanoid biosynthesis. The integrated analyses of interaction between differentially expressed genes (DEGs) and differentially expressed metabolites (DEMs) endorsed that metabolic network regulated by DEGs was significantly interfered by DEHP, resulting in cell dysfunction of roots and visible growth inhibition. Overall, these finding generated fresh perspective for crops security caused by plasticizer pollution and enhanced the public focus on dietary risk.

Cell-type-specific metabolism in plants

Every plant organ contains tens of different cell types, each with a specialized function. These functions are intrinsically associated with specific metabolic flux distributions that permit the synthesis of the ATP, reducing equivalents and biosynthetic precursors demanded by the cell. Investigating such cell-type-specific metabolism is complicated by the mosaic of different cells within each tissue combined with the relative scarcity of certain types. However, techniques for the isolation of specific cells, their analysis in situ by microscopy, or modeling of their function in silico have permitted insight into cell-type-specific metabolism. In this review we present some of the methods used in the analysis of cell-type-specific metabolism before describing what we know about metabolism in several cell types that have been studied in depth; (i) leaf source and sink cells; (ii) glandular trichomes that are capable of rapid synthesis of specialized metabolites; (iii) guard cells that must accumulate large quantities of the osmolytes needed for stomatal opening; (iv) cells of seeds involved in storage of reserves; and (v) the mesophyll and bundle sheath cells of C4 plants that participate in a CO₂ concentrating cycle. Metabolism is discussed in terms of its principal features, connection to cell function and what factors affect the flux distribution. Demand for precursors and energy, availability of substrates and suppression of deleterious processes are identified as key factors in shaping cell-type-specific metabolism.

Trichome-Specific Analysis and Weighted Gene Co-Expression Correlation Network Analysis (WGCNA) Reveal Potential Regulation Mechanism of Artemisinin Biosynthesis in Artemisia annua

Trichomes are attractive cells for terpenoid biosynthesis and accumulation in Artemisia annua. However, the molecular process underlying the trichome of A. annua is not yet fully elucidated. In this study, an analysis of multi-tissue transcriptome data was performed to examine trichome-specific expression patterns. A total of 6646 genes were screened and highly expressed in trichomes, including artemisinin biosynthetic genes such as amorpha-4,11-diene synthase (ADS) and cytochrome P450 monooxygenase (CYP71AV1). Mapman and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis showed that trichome-specific genes were mainly enriched in lipid metabolism and terpenoid metabolism. These trichome-specific genes were analyzed by a weighted gene co-expression network analysis (WGCNA), and the blue module linked to terpenoid backbone biosynthesis was determined. Hub genes correlated with the artemisinin biosynthetic genes were selected based on TOM value. ORA, Benzoate carboxyl methyltransferase (BAMT), Lysine histidine transporter-like 8 (AATL1), Ubiquitin-like protease 1 (Ulp1) and TUBBY were revealed as key hub genes induced by methyl jasmonate (MeJA) for regulating artemisinin biosynthesis. In summary, the identified trichome-specific genes, modules, pathways and hub genes provide clues and shed light on the potential regulatory mechanisms of artemisinin biosynthesis in trichomes in A. annua.

Flavonols modulate plant development, signaling, and stress responses

Flavonols are plant-specialized metabolites with important functions in plant growth and development. Isolation and characterization of mutants with reduced flavonol levels, especially the transparent testa mutants in Arabidopsis thaliana, have contributed to our understanding of the flavonol biosynthetic pathway. These mutants have also uncovered the roles of flavonols in controlling development in above- and below-ground tissues, notably in the regulation of root architecture, guard cell signaling, and pollen development. In this review, we present recent progress made towards a mechanistic understanding of flavonol function in plant growth and development. Specifically, we highlight findings that flavonols act as reactive oxygen species (ROS) scavengers and inhibitors of auxin transport in diverse tissues and cell types to modulate plant growth and development and responses to abiotic stresses.

Natural variation in the transcription factor REPLUMLESS contributes to both disease resistance and plant growth in Arabidopsis

When attacked by pathogens, plants need to reallocate energy from growth to defense to fend off the invaders, frequently incurring growth penalties. This phenomenon is known as the growth-defense tradeoff and is orchestrated by a hardwired transcriptional network. Altering key factors involved in this network has the potential to increase disease resistance without growth or yield loss, but the mechanisms underlying such changes require further investigation. By conducting a genome-wide association study (GWAS) of leaves infected by the hemi-biotrophic bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000, we discovered that the Arabidopsis transcription factor REPLUMLESS (RPL) is necessary for bacterial resistance. More importantly, RPL functions in promoting both disease resistance and growth. Transcriptome analysis revealed a cluster of genes in the GRETCHEN HAGEN 3 (GH3) family that were significantly upregulated in rpl mutants, leading to the accumulation of indole-3-acetic acid-aspartic acid (IAA-Asp). Consistent with this observation, transcripts of virulence effector genes were activated by IAA-Asp accumulated in the rpl mutants. We found that RPL protein could directly bind to GH3 promoters and repress their expression. RPL also repressed flavonol synthesis by directly repressing CHI expression and thus activated the auxin transport pathway, which promotes plant growth. Therefore, RPL plays an important role in plant immunity and functions in the auxin pathway to optimize Arabidopsis growth and defense.

Identification and characterization of a novel gene involved in glandular trichome development in Nepeta tenuifolia

Nepeta tenuifolia is a medicinal plant rich in terpenoids and flavonoids with antiviral, immunoregulatory, and anti-inflammatory activities. The peltate glandular trichome (PGT) is a multicellular structure considered to be the primary storage organ for monoterpenes; it may serve as an ideal model for studying cell differentiation and the development of glandular trichomes (GTs). The genes that regulate the development of GTs have not yet been well studied. In this study, we identified NtMIXTA1, a GT development-associated gene from the R2R3 MYB SBG9 family. NtMIXTA1 overexpression in tobacco resulted in the production of longer and denser GTs. Virus-induced gene silencing of NtMIXTA1 resulted in lower PGT density, a significant reduction in monoterpene concentration, and the decreased expression of genes related to monoterpene biosynthesis. Comparative transcriptome and widely targeted metabolic analyses revealed that silencing NtMIXTA1 significantly influenced the expression of genes, and the production of metabolites involved in the biosynthesis of terpenoids, flavonoids, and lipids. This study provides a solid foundation describing a mechanism underlying the regulation of GT development. In addition, this study further deepens our understanding of the regulatory networks involved in GT development and GT development-associated metabolite flux, as well as provides valuable reference data for studying plants with a high medicinal value without genetic transformation.

Mutations that alter Arabidopsis flavonoid metabolism affect the circadian clock

Flavonoids are a well-known class of specialized metabolites that play key roles in plant development, reproduction, and survival. Flavonoids are also of considerable interest from the perspective of human health, as both phytonutrients and pharmaceuticals. RNA sequencing analysis of an Arabidopsis null allele for chalcone synthase (CHS), which catalyzes the first step in flavonoid metabolism, has uncovered evidence that these compounds influence the expression of genes associated with the plant circadian clock. Analysis of promoter-luciferase constructs further showed that the transcriptional activity of CCA1 and TOC1, two key clock genes, is altered in CHS-deficient seedlings across the day/night cycle. Similar findings for a mutant line lacking flavonoid 3'-hydroxylase (F3'H) activity, and thus able to synthesize mono- but not dihydroxylated B-ring flavonoids, suggests that the latter are at least partially responsible; this was further supported by the ability of quercetin to enhance CCA1 promoter activity in wild-type and CHS-deficient seedlings. The effects of flavonoids on circadian function were also reflected in photosynthetic activity, with chlorophyll cycling abolished in CHS- and F3'H-deficient plants. Remarkably, the same phenotype was exhibited by plants with artificially high flavonoid levels, indicating that neither the antioxidant potential nor the light-screening properties of flavonoids contribute to optimal clock function, as has recently also been demonstrated in animal systems. Collectively, the current experiments point to a previously unknown connection between flavonoids and circadian cycling in plants and open the way to better understanding of the molecular basis of flavonoid action.