Continuous cropping (CC) is a common practice in agriculture, and usually causes serious economic losses due to soil degeneration, decreased crop yield and quality, and increased disease incidence, especially in medicinal plants. Continuous cropping obstacles (CCOs) are mainly due to changes in soil microbial communities, nutrient availability, and allelopathic effects. Recently, progressive studies have illustrated the molecular mechanisms of CCOs, and valid strategies to overcome them. Transcriptomic and metabolomics analyses revealed that identified DEGs (differently expressed genes) and metabolites involved in the response to CCOs are involved in various biological processes, including photosynthesis, carbon metabolism, secondary metabolite biosynthesis, and bioactive compounds. Soil improvement is an effective strategy to overcome this problem. Soil amendments can improve the microbial community by increasing the abundance of beneficial microorganisms, soil fertility, and nutrient availability. In this review, we sum up the recent status of the research on CCOs in medicinal plants, the combination of transcriptomic and metabolomics studies, and related control strategies, including uses of soil amendments, crop rotation, and intercropping. Finally, we propose future research trends for understanding CCOs, and strategies to overcome these obstacles and promote sustainable agriculture practices in medicinal plants.

The oil of Camellia spp. has become a well-known high-quality edible oil because of its rich nutrition. It is of great significance to breed fine varieties of Camellia spp. for the sustainable growth of the Camellia spp. industry. This study mainly evaluated the quality and antioxidant capacity of the camellia seed from several sources. The fatty acid composition and main active components of 40 kinds of C. oleifera, C. vietnamensis, C. osmantha, and C. gigantocarpa seeds, and so on, from different regions, were tested using GC–MS and HPLC. The quality of different Camellia spp. germplasm resources was comprehensively evaluated using multiple indices. The unsaturated fatty acid content and the antioxidant capacity of C. vietnamensis from Hainan were higher than those of C. oleifera Abel. In addition, there were a few differences in the fatty acid compositions of Camellia spp. oil from different species. Correlation analysis confirmed that rutin, total saponin, total flavonoids, squalene, and vitamin E were strongly correlated to the antioxidant capacity of Camellia spp. In the comprehensive evaluation, the best quality and strongest antioxidant activity were found for Chengmai Dafeng (C. vietnamensis). These methods in the study were applied for the first time for the quality evaluation of the Camellia spp. species. This study provided new insights into the quality evaluation of the Camellia spp. species, thus facilitating further development of variety breeding along with quality evaluation.

Oil-tea camellia trees, the collective term for a class of economically valuable woody oil crops in China, have attracted extensive attention because of their rich nutritional and pharmaceutical value. This study aimed to analyze the genetic relationship and genetic diversity of oil-tea camellia species using polymorphic SSR markers. One-hundred and forty samples of five species were tested for genetic diversity using twenty-four SSR markers. In this study, a total of 385 alleles were identified using 24 SSR markers, and the average number of alleles per locus was 16.0417. The average Shannon’s information index (I) was 0.1890, and the percentages of polymorphic loci (P) of oil-tea camellia trees were 7.79−79.48%, indicating that oil-tea camellia trees have low diversity. Analysis of molecular variance (AMOVA) showed that the majority of genetic variation (77%) was within populations, and a small fraction (23%) occurred among populations. Principal coordinate analysis (PCoA) results indicated that the first two principal axes explained 7.30% (PC1) and 6.68% (PC2) of the total variance, respectively. Both UPGMA and PCoA divided the 140 accessions into three groups. Camellia oleifera clustered into one class, Camellia vietnamensis and Camellia gauchowensis clustered into one class, and Camellia crapnelliana and Camellia chekiangoleosa clustered into another class. It could be speculated that the genetic relationship of C. vietnamensis and C. gauchowensis is quite close. SSR markers could reflect the genetic relationship among oil-tea camellia germplasm resources, and the results of this study could provide comprehensive information on the conservation, collection, and breeding of oil-tea camellia germplasms.

Pogostemon cablin (patchouli) cultivation is challenged by serious soil sickness, of which autotoxins accumulation is a major cause. p-hydroxybenzoic acid (p-HBA) is one of the main autotoxins of patchouli. However, the molecular mechanism underlying the response of patchouli to p-HBA remains unclear. In this study, RNA-sequencing combined with physiological analysis was used to monitor the dynamic transcriptomic and physiological changes in patchouli seedlings 0, 6, 12, 24, 48, and 96 h after p-HBA treatment. p-HBA stress inhibited root biomass accumulation, induced excessive hydrogen peroxide accumulation and lipid peroxidation, and activated most antioxidant enzymes. Compared with that of the control, the osmotic adjustment substance content was elevated with treatment. Subsequently, 15,532, 8,217, 8,946, 2,489, and 5,843 differentially expressed genes (DEGs) at 6, 12, 24, 48, and 96 h after p-HBA treatment, respectively, were identified in patchouli roots. GO functional enrichment analysis showed that the DEGs were enriched mainly in plasma membrane, defense response, response to chitin, DNA-binding transcription factor activity and abscisic acid-activated signaling pathway. The upregulated genes were involved in glycolysis/gluconeogenesis, cysteine and methionine metabolism, starch and sucrose metabolism, biosynthesis of unsaturated fatty acids, and linoleic acid metabolism. Genes associated with MAPK signaling pathway-plant, plant-pathogen interaction, plant hormone signal transduction were downregulated with p-HBA treatment. These pathways are related to root browning and rotting, leading to plant death.

Tea saponins have high surface-active and biological activities and are widely used in chemicals, food, pharmaceuticals, and pesticides. Tea saponins are usually extracted using ethanol or water, but both methods have their disadvantages, including a negative impact on the environment, high energy consumption, and low purity. In this study, we explored an effective process for extracting tea saponins from tea meal using deep eutectic solvents combined with ultrasonic extraction and enzymatic techniques. The experimental results showed that a high extraction efficiency of 20.93 ± 0.48% could be achieved in 20 min using an ultrasonic power of 40% and a binary DES consisting of betaine and ethylene glycol (with a molar ratio of 1:3) at a material–liquid ratio of 1:35 and that the purity of the tea saponins after purification by a large-pore adsorption resin reached 95.94%, which was higher than that of commercially available standard tea saponin samples. In addition, the extracted tea saponins were evaluated for their antioxidant and bacteriostatic activities using chemical and biological methods; the results showed that the tea saponins extracted using these methods possessed antioxidant properties and displayed significant antibacterial activity. Therefore, the present study developed a method for using deep eutectic solvents as an environmentally friendly technological solution for obtaining high-purity tea saponins from tea meal oil. This is expected to replace the current organic solvent and water extraction process and has great potential for industrial development and a number of possible applications.