Crop Wild Relatives as Germplasm Resource for Cultivar Improvement in Mint (Mentha L.)

Morphological and genetic evidence suggest gene flow among native and naturalized mint species

PREMISE

Cultivation and naturalization of plants beyond their natural range can bring previously geographically isolated taxa together, increasing the opportunity for hybridization, the outcomes of which are not predictable. Here, we explored the phenotypic and genomic effects of interspecific gene flow following the widespread cultivation of Mentha spicata (spearmint), M. longifolia, and M. suaveolens.

METHODS

We morphologically evaluated 155 herbarium specimens of three Mentha species and sequenced the genomes of a subset of 93 specimens. We analyzed the whole genomes in a population and the phylogenetic framework and associated genomic classifications in conjunction with the morphological assessments.

RESULTS

The allopolyploid M. spicata, which likely evolved in cultivation, had altered trichome characters, that is possibly a product of human selection for a more palatable plant or a byproduct of selection for essential oils. There were signs of genetic admixture between mints, including allopolyploids, indicating that the reproductive barriers between Mentha species with differences in ploidy are likely incomplete. Still, despite gene flow between species, we found that genetic variants associated with the cultivated trichome morphology continue to segregate.

CONCLUSIONS

Although hybridization, allopolyploidization, and human selection during cultivation can increase species richness (e.g., by forming hybrid taxa), we showed that unless reproductive barriers are strong, these processes can also result in mixing of genes between species and the potential loss of natural biodiversity.

A Comprehensive Review of the Key Characteristics of the Genus Mentha, Natural Compounds and Biotechnological Approaches for the Production of Secondary Metabolites

Context

The genus Mentha is one of the most aromatic and well-known members of the Lamiaceae family. A wide range of bioactive compounds has been reported in mints. Regarding the high economic importance of Mentha plants due to the presence of valuable metabolites, the demand for their products is growing exponentially. Therefore, to supply such demand, new strategies should be adopted to improve the yield and medicinal quality of the products.

Evidence Acquisition

The current review is written based on scientific literature obtained from online databases, including Google Scholar, PubMed, Scopus, and Web of Science regarding the characteristic features of some species of the genus Mentha, their distribution and cultivation, main uses and benefits, phytochemical composition, biotechnological approaches for the production of secondary metabolites, and strategies for enhanced production of mints secondary metabolites.

Results

In this article, we offer an overview of the key characteristics, natural compounds, biological properties, and medicinal uses of the genus Mentha. Current research describes biotechnological techniques such as in vitro culture methods for the production of high-value secondary metabolites. This review also highlights the strategies such as elicitation, genetic, and metabolic engineering to improve the secondary compounds production level in mint plants. Overall, it can be concluded that identifying the biosynthetic pathways, leading to the accumulation of pharmaceutically important bioactive compounds, has paved the way for developing highly productive mint plants with improved phytochemical profiles.

Multiomics comparison among populations of three plant sources of Amomi Fructus

Amomi Fructus (Sharen, AF) is a traditional Chinese medicine (TCM) from three source species (or varieties), including Wurfbainia villosa var. villosa (WVV), W. villosa var. xanthioides (WVX), or W. longiligularis (WL). Among them, WVV has been transplanted from its top-geoherb region, Guangdong, to its current main production area, Yunnan, for >50 years in China. However, the genetic and transcriptomic differentiation among multiple AF source species (or varieties) and between the origin and transplanted populations of WVV is unknown. In our study, the observed overall higher expression of terpenoid biosynthesis genes in WVV than in WVX provided possible evidence for the better pharmacological effect of WVV. We also screened six candidate borneol dehydrogenases (BDHs) that potentially catalyzed borneol into camphor in WVV and functionally verified them. Highly expressed genes at the P2 stage of WVV, Wv05G1424 and Wv05G1438, were capable of catalyzing the formation of camphor from (+)-borneol, (-)-borneol and DL-isoborneol. Moreover, the BDH genes may experience independent evolution after acquiring the ancestral copies, and the following tandem duplications might account for the abundant camphor content in WVV. Furthermore, four populations of WVV, WVX, and WL are genetically differentiated, and the gene flow from WVX to WVV in Yunnan contributed to the greater genetic diversity in the introduced population (WVV-JH) than in its top-geoherb region (WVV-YC), which showed the lowest genetic diversity and might undergo genetic degradation. In addition, terpene synthesis (TPS) and BDH genes were selected among populations of multiple AF source species (or varieties) and between the top- and non-top-geoherb regions, which might explain the difference in metabolites between these populations. Our findings provide important guidance for the conservation, genetic improvement, and industrial development of the three source species (or varieties) and for identifying top-geoherbalism with molecular markers, and proper clinical application of AF.

A chromosome-level genome assembly reveals that a bipartite gene cluster formed via an inverted duplication controls monoterpenoid biosynthesis in Schizonepeta tenuifolia

Biosynthetic gene clusters (BGCs) are regions of a genome where genes involved in a biosynthetic pathway are in proximity. The origin and evolution of plant BGCs as well as their role in specialized metabolism remain largely unclear. In this study, we have assembled a chromosome-scale genome of Japanese catnip (Schizonepeta tenuifolia) and discovered a BGC that contains multiple copies of genes involved in four adjacent steps in the biosynthesis of p-menthane monoterpenoids. This BGC has an unprecedented bipartite structure, with mirrored biosynthetic regions separated by 260 kilobases. This bipartite BGC includes identical copies of a gene encoding an old yellow enzyme, a type of flavin-dependent reductase. In vitro assays and virus-induced gene silencing revealed that this gene encodes the missing isopiperitenone reductase. This enzyme evolved from a completely different enzyme family to isopiperitenone reductase from closely related Mentha spp., indicating convergent evolution of this pathway step. Phylogenomic analysis revealed that this bipartite BGC has emerged uniquely in the S. tenuifolia lineage and through insertion of pathway genes into a region rich in monoterpene synthases. The cluster gained its bipartite structure via an inverted duplication. The discovered bipartite BGC for p-menthane biosynthesis in S. tenuifolia has similarities to the recently described duplicated p-menthane biosynthesis gene pairs in the Mentha longifolia genome, providing an example of the convergent evolution of gene order. This work expands our understanding of plant BGCs with respect to both form and evolution, and highlights the power of BGCs for gene discovery in plant biosynthetic pathways.

Mentha: Nutritional and Health Attributes to Treat Various Ailments Including Cardiovascular Diseases

A poor diet, resulting in malnutrition, is a critical challenge that leads to a variety of metabolic disorders, including obesity, diabetes, and cardiovascular diseases. Mentha species are famous as therapeutic herbs and have long served as herbal medicine. Recently, the demand for its products, such as herbal drugs, medicines, and natural herbal formulations, has increased significantly. However, the available literature lacks a thorough overview of Mentha phytochemicals' effects for reducing malnutritional risks against cardiovascular diseases. In this context, we aimed to review the recent advances of Mentha phytochemicals and future challenges for reducing malnutritional risks in cardiovascular patients. Current studies indicated that Mentha species phytochemicals possess unique antimicrobial, antidiabetic, cytotoxic, and antioxidant potential, which can be used as herbal medicine directly or indirectly (such as food ingredients) and are effective in controlling and curing cardiovascular diseases. The presence of aromatic and flavor compounds of Mentha species greatly enhance the nutritional values of the food. Further interdisciplinary investigations are pivotal to explore main volatile compounds, synergistic actions of phytochemicals, organoleptic effects, and stability of Mentha sp. phytochemicals.

Direct and Indirect Effects of Essential Oils for Sustainable Crop Protection

Plant essential oils (EOs) are gaining interest as biopesticides for crop protection. EOs have been recognized as important ingredients of plant protection products including insecticidal, acaricidal, fungicidal, and nematicidal agents. Considering the growing importance of EOs as active ingredients, the domestication and cultivation of Medicinal and Aromatic Plants (MAPs) to produce chemically stable EOs contributes to species conservation, provides the sustainability of production, and decreases the variations in the active ingredients. In addition to these direct effects on plant pests and diseases, EOs can induce plant defenses (priming effects) resulting in better protection. This aspect is of relevance considering that the EU framework aims to achieve the sustainable use of new plant protection products (PPPs), and since 2020, the use of contaminant PPPs has been prohibited. In this paper, we review the most updated information on the direct plant protection effects of EOs, focusing on their modes of action against insects, fungi, and nematodes, as well as the information available on EOs with plant defense priming effects.

Genetic Manipulation of Biosynthetic Pathways in Mint

In recent years, the study of aromatic plants has seen an increase, with great interest from industrial, academic, and pharmaceutical industries. Among plants attracting increased attention are the Mentha spp. (mint), members of the Lamiaceae family. Mint essential oils comprise a diverse class of molecules known as terpenoids/isoprenoids, organic chemicals that are among the most diverse class of naturally plant derived compounds. The terpenoid profile of several Mentha spp. is dominated by menthol, a cyclic monoterpene with some remarkable biological properties that make it useful in the pharmaceutical, medical, cosmetic, and cleaning product industries. As the global market for Mentha essential oils increases, the desire to improve oil composition and yield follows. The monoterpenoid biosynthesis pathway is well characterised so metabolic engineering attempts have been made to facilitate this improvement. This review focuses on the Mentha spp. and attempts at altering the carbon flux through the biosynthetic pathways to increase the yield and enhance the composition of the essential oil. This includes manipulation of endogenous and heterologous biosynthetic enzymes through overexpression and RNAi suppression. Genes involved in the MEP pathway, the menthol and carvone biosynthetic pathways and transcription factors known to affect secondary metabolism will be discussed along with non-metabolic engineering approaches including environmental factors and the use of plant growth regulators.

Whole Transcriptome Sequencing Unveils the Genomic Determinants of Putative Somaclonal Variation in Mint (Mentha L.)

Mint (Mentha L., Lamiaceae) is a strongly scented herb of the family Lamiaceae that is grown mostly by clonal propagation, making it a valuable species for the study of somaclonal variation and its phenotypic consequences. The recent introduction of a few species of mint in South America, followed by a presumably rampant propagation, make this region particularly ideal for studying the extent of somaclonal genetic diversity. Hence, the objective of this work was to offer a preliminary characterization of somaclonal genetically coding diversity of the mint in the northern Andes in order to address the question of whether somaclonal variants may have emerged despite relatively recent introductions in a region where mint is not native. A total of 29 clonally propagated specimens, collected in mint export farms in the province of Antioquia, a major region for mint production in the northwest Andes of Colombia, were genotyped using RNA sequencing (RNA-Seq). SNP calling was carried out from the leaves’ transcriptome profiles of each plant by combining the GATK4 and TRINITY protocols, obtaining a total of 2033 loci across 912 transcripts with a minimum read depth of 20X and 4% of missing data. Unsupervised machine learning algorithms considered the K-means, AGNES and UPGMA approaches, all of which suggested three genetic clusters for M. spicata and a unique cluster for M. × piperita. The results indicate that at least two different origins of M. spicata reached the eastern region of the Antioquia province, clonally propagated in the locality ever since for local consumption and export. One of these ancestries had more population structure, possibly due to environmental or anthropological pressures that intervened in the fragmentation of this genetic group or to a higher somaclonal mutation rate. This work offers a first step into the study of the accumulation and transmission of presumably quasi-neutral somatic mutations at coding regions in an herbaceous clonally propagated scented species such as mint, likely favored by an expected population expansion after its Andean introduction. These ad hoc hypotheses warrant further study as part of future research.

Spearmint (Mentha spicata L.) Phytochemical Profile: Impact of Pre/Post-Harvest Processing and Extractive Recovery

The purpose of this study was to chemically compare samples of Mentha spicata (marketing byproducts, production byproducts, and export material), cultivated in the open field and under greenhouse, using an integrated approach by HPLC/DAD and GC/MS analysis. The presence of phenolic compounds was higher in the marketing byproducts cultivated in the open field. Marketing byproducts also had the highest amount of carvone. For this reason, this byproduct was selected as a candidate for the development of natural ingredients. With the best selected material, the optimization of simultaneous high-intensity ultrasound-assisted extraction processes was proposed for the recovery of the compounds of interest. This extraction was defined by Peleg’s equation and polynomial regression analysis. Modeling showed that the factors amplitude, time, and solvent were found to be significant in the recovery process (p < 0.005). The maximum amount of compounds was obtained using 90% amplitude for 5 min and ethanol/water mixture (80:20) for extraction to simultaneously obtain phenolic and terpenoid compounds. This system obtained the highest amount of monoterpenoid and sesquiterpenoid compounds from the essential oil of M. spicata (64.93% vs. 84.55%). Thus, with an efficient and eco-friendly method, it was possible to optimize the extraction of compounds in M. spicata as a starting point for the use of its byproducts.

Quality evaluation of wild germplasm of Chinese prickly ash (Zanthoxylum bungeanum Maxim) from Qinling mountains at different elevations based on HPLC-fingerprint

Wild Chinese prickly ash resources provide a valuable genetic resource for Zanthoxylum bungeanum Maxim improvement and breeding. The Qinling Mountains was an abundant source for wild Chinese prickly ash. In this study, the phenolic and flavonoid compounds of wild germplasm resources from different altitudes and six cultivated varieties were analyzed by high performance liquid chromatography (HPLC). The chromatograms of them were essentially consistent, although their chemical composition contents were greatly different. The thirty samples were divided into three categories through the hierarchical clustering analysis. Catechin, hyperoside and quercitrin were considered to be key compounds for the quality evaluation, and by contrast, the wild samples with an altitude of 2300±50 m (Group IV) had the highest content of key compounds, and presented stronger antioxidant activity and antibacterial ability, indicating that these wild samples could be identified as the excellent breeding resources. This is the first time to evaluate the quality of wild Chinese prickly ash at different altitudes in Qinling Mountains. These excellent wild germplasm resources provided substantial potential accessions for use directly in Chinese prickly ash breeding programs.

Diploid mint (M. longifolia) can produce spearmint type oil with a high yield potential

Mint oil is a key source of natural flavors with wide industrial applications. Two unbalanced polyploid cultivars named Native (Mentha Spicata L) and Scotch (M. × gracilis Sole) are the main producers of spearmint type oil, which is characterized by high levels of the monoterpenes (−)-carvone and (−)-limonene. These cultivars have been the backbone of spearmint oil production for decades, while breeding and improvement remained largely unexplored, in part, due to sterility in cultivated lines. Here we show that sexual breeding at the diploid level can be leveraged to develop new varieties that produce spearmint type oil, along with the improvement of other important traits. Using field trials and GC-FID oil analysis we characterized plant materials from a public germplasm repository and identified a diploid accession that exhibited 89.5% increase in oil yield, compared to the industry standard, and another that produces spearmint type oil. Spearmint-type oil was present at high frequency in a segregating F₂ population (32/160) produced from these two accessions. Field-testing of ten of these F₂ lines showed segregation for oil yield and confirmed the production of spearmint-type oil profiles. Two of these lines combined high yield and spearmint-type oil with acceptable analytic and sensory profiles. These results demonstrate that spearmint-type oil can be produced in a diploid background with high yield potential, providing a simpler genetic system for the development of improved spearmint varieties.

Conservation Genomics of Wild Red Sage (Salvia miltiorrhiza) and Its Endangered Relatives in China: Population Structure and Interspecific Relationships Revealed From 2b-RAD Data

Red sage (Salvia miltiorrhiza) is a widely used medicinal plant for treatment of cardiovascular and cerebrovascular diseases. Because of excessive excavation by huge market demand and habitat loss by human activities, the wild population resources of S. miltiorrhiza have reduced drastically in recent years. Meanwhile, population status of two closely related species S. bowleyana and S. paramiltiorrhiza were in a trend of decreasing due to their potential replacement of S. miltiorrhiza. Particularly, S. paramiltiorrhiza was threatened and endemic to a small region in eastern China. However, to date there has been no conservation genetic research reported for wild S. miltiorrhiza population and its endangered relatives. Assess the wild germplasm diversity for S. miltiorrhiza and its related species would provide fundamental genetic background for cultivation and molecular breeding of this medicinally important species. In the present study, we investigated the genetic diversity, population structure, and intra/inter-specific differentiation of S. miltiorrhiza and above two relatives using 2b-RAD genome-wide genotyping method. By investigating 81 individuals of S. miltiorrhiza, 55 individuals of S. bowleyana and 15 individuals of S. paramiltiorrhiza from 23 locations in China, we obtained 23,928 SNPs in total. A comparatively high genetic diversity was observed in S. miltiorrhiza (π = 0.0788, H e = 0.0783 ± 0.0007). The observed and expected heterozygosity in populations of these three species ranged from 0.0297 to 0.1481 and 0.0251 to 0.831, respectively. Two major lineage groups were detected in the examined S. miltiorrhiza populations. The results indicated that Dabie Mountain as a genetic diversity center of S. miltiorrhiza and possible complex inter-specific genetic exchange/hybridization occurred between S. miltiorrhiza and the two relatives. We suggest that strategic conservation and germplasm preservation should be considered not only for wild populations of S. miltiorrhiza, but also for its related S. bowleyana and S. paramiltiorrhiza.

A Revised Phylogeny of the Mentha spicata Clade Reveals Cryptic Species

The genus Mentha is taxonomically and phylogenetically challenging due to complex genomes, polyploidization and an extensive historical nomenclature, potentially hiding cryptic taxa. A straightforward interpretation of phylogenetic relationships within the section Mentha is further hindered by dominant but outdated concepts on historically identified hybrid taxa. Mentha spicata is traditionally considered to be of hybrid origin, but the evidence for this is weak. Here, we aim to understand the phylogenetic relationships within the section Mentha using large sample sizes and to revisit the hybrid status and identity of M. spicata. We show that two of three traditional species in the subsection Spicatae are polyphyletic, as is the subsection as a whole, while the real number of cryptic species was underestimated. Compared to previous studies we present a fundamentally different phylogeny, with a basal split between M. spicata s.s. and M. longifolia s.s. Cluster analyses of morphological and genotypic data demonstrate that there is a dissociation between morphologically and genotypically defined groups of samples. We did not find any evidence that M. spicata is of hybrid origin, and we conclude its taxonomic status should be revised. The combination of genetic and phenotypic information is essential when evaluating hyperdiverse taxonomic groups.

Verticillium Wilt of Mint in the United States of America

Verticillium wilt, caused by the fungus Verticillium dahliae, is the most important and destructive disease of mint (Mentha spp.) in the United States (U.S.). The disease was first observed in commercial mint fields in the Midwestern U.S. in the 1920s and, by the 1950s, was present in mint producing regions of the U.S. Pacific Northwest. Verticillium wilt continues to be a major limiting factor in commercial peppermint (Mentha x piperita) and Scotch spearmint (Mentha x gracilis) production, two of the most important sources of mint oil in the U.S. The perennial aspect of U.S. mint production, coupled with the soilborne, polyetic nature of V. dahliae, makes controlling Verticillium wilt in mint a challenge. Studies investigating the biology and genetics of the fungus, the molecular mechanisms of virulence and resistance, and the role of soil microbiota in modulating host-pathogen interactions are needed to improve our understanding of Verticillium wilt epidemiology and inform novel disease management strategies. This review will discuss the history and importance of Verticillium wilt in commercial U.S. mint production, as well as provide a format to highlight past and recent research advances in an effort to better understand and manage the disease.