Friends in Arms: Flavonoids and the Auxin/Cytokinin Balance in Terrestrialization

Antibacterial and Antifungal Properties of New Synthetic Tricyclic Flavonoids

Background/Objectives: The discovery of new molecules to which bacteria have not yet developed resistance is a significant medical priority. Synthetic flavonoids are good candidates for developing new antimicrobials. Our study investigates a series of newly synthesized tricyclic flavonoids with several different substituents on the flavonoid core. Methods: By varying the nature of the substituents on rings A and B, a structure-activity relationship study using different microbial strains has been performed. The antibacterial and antifungal properties of these compounds have been investigated against Gram-positive and Gram-negative bacteria and several Candida strains. Results: All seven tested compounds have been found to exhibit the highest antimicrobial activity against S. aureus, with an inhibition zone of up to 23 mm. The tricyclic flavonoids 5c, 5e, and 5f showed good antifungal properties against C. parapsilosis, with an inhibition zone of around 17 mm. Conclusions: All the data support the idea that flavonoids 5 are reliable candidates for developing effective antimicrobial agents.

Co-option of plant gene regulatory network in nutrient responses during terrestrialization

Plant responses to nitrate, phosphate and sucrose form a complex molecular network crucial for terrestrial adaptation. However, the origins, functional diversity and evolvability of this network during plant terrestrialization remain scarcely understood. Here we compare the transcriptomic response to these nutrients in the bryophyte Marchantia polymorpha and the streptophyte alga Klebsormidium nitens. We show that the largely species-specific nutrient response pattern is driven by gene regulatory network (GRN) alterations. Intriguingly, while pathways governing the GRNs exhibit modest conservation, M. polymorpha GRNs exhibit more regulatory connections through the redeployment of ancient transcription factor CSD. In M. polymorpha, functional analyses reveal the involvement of pre-existing cytokinin machineries in downstream targets, orchestrating plastic morpho-physiological responses to nutrient status. Our findings implicate the genetic co-option events facilitating successful land plant establishment.

The evolution of flavonoid biosynthesis

The flavonoid pathway is characteristic of land plants and a central biosynthetic component enabling life in a terrestrial environment. Flavonoids provide tolerance to both abiotic and biotic stresses and facilitate beneficial relationships, such as signalling to symbiont microorganisms, or attracting pollinators and seed dispersal agents. The biosynthetic pathway shows great diversity across species, resulting principally from repeated biosynthetic gene duplication and neofunctionalization events during evolution. Such events may reflect a selection for new flavonoid structures with novel functions that enable occupancy of varied ecological niches. However, the biochemical and genetic diversity of the pathway also likely resulted from evolution along parallel trends across land plant lineages, producing variant compounds with similar biological functions. Analyses of the wide range of whole-plant genome sequences now available, particularly for archegoniate plants, have enabled proposals on which genes were ancestral to land plants and which arose within the land plant lineages. In this review, we discuss the emerging proposals for how the flavonoid pathway may have evolved and diversified. This article is part of the theme issue 'The evolution of plant metabolism'.

Cytokinins enhance the metabolic activity of in vitro-grown catmint (Nepeta nuda L.)

The phytohormones cytokinins are essential mediators of developmental and environmental signaling, primarily during cell division and endophytic interactions, among other processes. Considering the limited understanding of the regulatory mechanisms that affect the growth and bioactivity of the medicinal plant Nepeta nuda (Lamiaceae), our study aimed to explore how cytokinins influence the plant's metabolic status. Exogenous administration of active cytokinin forms on in vitro N. nuda internodes stimulated intensive callus formation and de novo shoot regeneration, leading to a marked increase in biomass. This process involved an accumulation of oxidants, which were scavenged by peroxidases using phenolics as substrates. The callus tissue formed upon the addition of the cytokinin 6-benzylaminopurine (BAP) acted as a sink for sugars and phenolics during the allocation of nutrients between the culture medium and regenerated plants. In accordance, the cytokinin significantly enhanced the content of polar metabolites and their respective in vitro biological activities compared to untreated in vitro and wild-grown plants. The BAP-mediated accumulation of major phenolic metabolites, rosmarinic acid (RA) and caffeic acid (CA), corresponded with variations in the expression levels of genes involved in their biosynthesis. In contrast, the accumulation of iridoids and the expression of corresponding biosynthetic genes were not significantly affected. In conclusion, our study elucidated the mechanism of cytokinin action in N. nuda in vitro culture and demonstrated its potential in stimulating the production of bioactive compounds. This knowledge could serve as a basis for further investigations of the environmental impact on plant productivity.

PmLBD3 links auxin and brassinosteroid signalling pathways on dwarfism in Prunus mume

BACKGROUND

Grafting with dwarf rootstock is an efficient method to control plant height in fruit production. However, the molecular mechanism remains unclear. Our previous study showed that plants with Prunus mume (mume) rootstock exhibited a considerable reduction in plant height, internode length, and number of nodes compared with Prunus persica (peach) rootstock. The present study aimed to investigate the mechanism behind the regulation of plant height by mume rootstocks through transcriptomic and metabolomic analyses with two grafting combinations, 'Longyan/Mume' and 'Longyan/Peach'.

RESULTS

There was a significant decrease in brassinolide levels in plants that were grafted onto mume rootstocks. Plant hormone signal transduction and brassinolide production metabolism gene expression also changed significantly. Flavonoid levels, amino acid and fatty acid metabolites, and energy metabolism in dwarf plants decreased. There was a notable upregulation of PmLBD3 gene expression in plant specimens that were subjected to grafting onto mume rootstocks. Auxin signalling cues promoted PmARF3 transcription, which directly controlled this upregulation. Through its binding to PmBAS1 and PmSAUR36a gene promoters, PmLBD3 promoted endogenous brassinolide inactivation and inhibited cell proliferation.

CONCLUSIONS

Auxin signalling and brassinolide levels are linked by PmLBD3. Our findings showed that PmLBD3 is a key transcription factor that regulates the balance of hormones through the auxin and brassinolide signalling pathways and causes dwarf plants in stone fruits.

A Structure-Activity Relationship Study on the Antioxidant Properties of Dithiocarbamic Flavanones

The antioxidant properties of 3-dithiocarbamic flavanones have been investigated. The influence of the halogen substituents on ring A of the flavanones and the nature of the secondary amine from the dithiocarbamic moiety have been accounted. The results indicated that the presence of a halogen substituent at the C-8 position of the benzopyran ring induce better antioxidant properties against DPPH and ABTS than butylated hydroxytoluene (BHT) and ascorbic acid. The presence of a halogen substituent at the mentioned position appears to induce a higher stability for a free radical intermediate at the C-3 position of the benzopyran ring. A free radical enolate is most likely to be involved in the antioxidant activity of this dithiocarbamic flavanone. It is a stable intermediate that supports the influence of dithiocarbamic moiety on the antioxidant properties of the reported flavanones.

Biosynthesis and metabolic engineering of isoflavonoids in model plants and crops: a review

Isoflavonoids, the major secondary metabolites within the flavonoid biosynthetic pathway, play important roles in plant defense and exhibit free radical scavenging properties in mammals. Recent advancements in understanding the synthesis, transport, and regulation of isoflavonoids have identified their biosynthetic pathways as promising targets for metabolic engineering, offering potential benefits such as enhanced plant resistance, improved biomass, and restoration of soil fertility. This review provides an overview of recent breakthroughs in isoflavonoid biosynthesis, encompassing key enzymes in the biosynthetic pathway, transporters influencing their subcellular localization, molecular mechanisms regulating the metabolic pathway (including transcriptional and post-transcriptional regulation, as well as epigenetic modifications). Metabolic engineering strategies aimed at boosting isoflavonoid content in both leguminous and non-leguminous plants. Additionally, we discuss emerging technologies and resources for precise isoflavonoid regulation. This comprehensive review primarily focuses on model plants and crops, offering insights for more effective and sustainable metabolic engineering approaches to enhance nutritional quality and stress tolerance.

Comparative transcriptome and metabolome profiles of the leaf and fruits of a Xianjinfeng litchi budding mutant and its mother plant

Background: Litchi (Litchi chinensis) is an important sub-tropical fruit in the horticulture market in China. Breeding for improved fruit characteristics is needed for satisfying consumer demands. Budding is a sustainable method for its propagation. During our ongoing breeding program, we observed a litchi mutant with flat leaves and sharp fruit peel cracking in comparison to the curled leaves and blunt fruit peel cracking fruits of the mother plant. Methods: To understand the possible molecular pathways involved, we performed a combined metabolome and transcriptome analysis. Results: We identified 1,060 metabolites in litchi leaves and fruits, of which 106 and 101 were differentially accumulated between the leaves and fruits, respectively. The mutant leaves were richer in carbohydrates, nucleotides, and phenolic acids, while the mother plant was rich in most of the amino acids and derivatives, flavonoids, lipids and organic acids and derivatives, and vitamins. Contrastingly, mutant fruits had higher levels of amino acids and derivatives, carbohydrates and derivatives, and organic acids and derivatives. However, the mother plant's fruits contained higher levels of flavonoids, scopoletin, amines, some amino acids and derivatives, benzamidine, carbohydrates and derivatives, and some organic acids and derivatives. The number of differentially expressed genes was consistent with the metabolome profiles. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway-enriched gene expressions showed consistent profiles as of metabolome analysis. Conclusion: These results provide the groundwork for breeding litchi for fruit and leaf traits that are useful for its taste and yield.

Establishment of feijoa (Acca sellowiana) callus and cell suspension cultures and identification of arctigenin - a high value bioactive compound

Feijoa (Acca sellowiana (O. Berg.) Burret), also known as pineapple guava, is a member of the Myrtaceae family and is well known for its fruit. Chemical profiling of the different tissues of the feijoa plant has shown that they generate an array of useful bioactive compounds which have health benefits such as significant antioxidant activities. In this study, an in vitro culture system has been developed, which could be explored to extract high-value bioactive compounds from feijoa. Feijoa tissue culture was initiated by the induction of callus from floral buds. Sections of floral buds were plated on MS medium supplemented with 2,4-D and BAP at 2.0mg/L and 0.2mg/L concentrations, respectively. Cell suspension cultures of feijoa were established using a liquid MS medium with different concentrations of 2,4-D and BAP and cultured on a rotary shaker. The growth of the cell suspension was evaluated with different parameters such as different carbohydrate sources, concentration of MS media, and inoculum density. When the cell suspensions were treated with different concentrations of MeJA at different time points, phytochemicals UPLC - QTOF MS analysis identified extractables of interest. The main compounds identified were secondary metabolites (flavonoids and flavonoid-glucosides) and plant hormones. These compounds are of interest for their potential use in therapeutics or skin and personal care products. This report investigates essential methodology parameters for establishing cell suspension cultures from feijoa floral buds, which could be used to generate in vitro biomass to produce high-value bioactive compounds. This is the first study reporting the identification of arctigenin from feijoa, a high-value compound whose pharmaceutical properties, including anti-tumour, anti-inflammatory and anti-colitis effects, have been widely reported. The ability of feijoa cell cultures to produce such high-value bioactive compounds is extremely promising for its use in pharmaceuticals, cosmeceuticals and nutraceuticals applications.

Understanding AP2/ERF Transcription Factor Responses and Tolerance to Various Abiotic Stresses in Plants: A Comprehensive Review

Abiotic stress is an adverse environmental factor that severely affects plant growth and development, and plants have developed complex regulatory mechanisms to adapt to these unfavourable conditions through long-term evolution. In recent years, many transcription factor families of genes have been identified to regulate the ability of plants to respond to abiotic stresses. Among them, the AP2/ERF (APETALA2/ethylene responsive factor) family is a large class of plant-specific proteins that regulate plant response to abiotic stresses and can also play a role in regulating plant growth and development. This paper reviews the structural features and classification of AP2/ERF transcription factors that are involved in transcriptional regulation, reciprocal proteins, downstream genes, and hormone-dependent signalling and hormone-independent signalling pathways in response to abiotic stress. The AP2/ERF transcription factors can synergise with hormone signalling to form cross-regulatory networks in response to and tolerance of abiotic stresses. Many of the AP2/ERF transcription factors activate the expression of abiotic stress-responsive genes that are dependent or independent of abscisic acid and ethylene in response to abscisic acid and ethylene. In addition, the AP2/ERF transcription factors are involved in gibberellin, auxin, brassinosteroid, and cytokinin-mediated abiotic stress responses. The study of AP2/ERF transcription factors and interacting proteins, as well as the identification of their downstream target genes, can provide us with a more comprehensive understanding of the mechanism of plant action in response to abiotic stress, which can improve plants' ability to tolerate abiotic stress and provide a more theoretical basis for increasing plant yield under abiotic stress.

Uncovering the Interrelation between Metabolite Profiles and Bioactivity of In Vitro- and Wild-Grown Catmint (Nepeta nuda L.)

Nepeta nuda L. is a medicinal plant enriched with secondary metabolites serving to attract pollinators and deter herbivores. Phenolics and iridoids of N. nuda have been extensively investigated because of their beneficial impacts on human health. This study explores the chemical profiles of in vitro shoots and wild-grown N. nuda plants (flowers and leaves) through metabolomic analysis utilizing gas chromatography and mass spectrometry (GC-MS). Initially, we examined the differences in the volatiles' composition in in vitro-cultivated shoots comparing them with flowers and leaves from plants growing in natural environment. The characteristic iridoid 4a-α,7-β,7a-α-nepetalactone was highly represented in shoots of in vitro plants and in flowers of plants from nature populations, whereas most of the monoterpenes were abundant in leaves of wild-grown plants. The known in vitro biological activities encompassing antioxidant, antiviral, antibacterial potentials alongside the newly assessed anti-inflammatory effects exhibited consistent associations with the total content of phenolics, reducing sugars, and the identified metabolic profiles in polar (organic acids, amino acids, alcohols, sugars, phenolics) and non-polar (fatty acids, alkanes, sterols) fractions. Phytohormonal levels were also quantified to infer the regulatory pathways governing phytochemical production. The overall dataset highlighted compounds with the potential to contribute to N. nuda bioactivity.

Plant Hormone Modularity and the Survival-Reproduction Trade-Off

Biological modularity refers to the organization of living systems into separate functional units that interact in different combinations to promote individual well-being and species survival. Modularity provides a framework for generating and selecting variations that can lead to adaptive evolution. While the exact mechanisms underlying the evolution of modularity are still being explored, it is believed that the pressure of conflicting demands on limited resources is a primary selection force. One prominent example of conflicting demands is the trade-off between survival and reproduction. In this review, we explore the available evidence regarding the modularity of plant hormones within the context of the survival-reproduction trade-off. Our findings reveal that the cytokinin module is dedicated to maximizing reproduction, while the remaining hormone modules function to ensure reproduction. The signaling mechanisms of these hormone modules reflect their roles in this survival-reproduction trade-off. While the cytokinin response pathway exhibits a sequence of activation events that aligns with the developmental robustness expected from a hormone focused on reproduction, the remaining hormone modules employ double-negative signaling mechanisms, which reflects the necessity to prevent the excessive allocation of resources to survival.