Heredity and Regulation of a Potential Commercial Crop: Perilla frutescens

Perilla (Perilla frutescens L.) is an annual herbaceous plant in the family Labiatae, which is mainly distributed in East Asian countries, such as China, Japan, and Korea, as well as Southeast Asia, and was eventually introduced for cultivation in Europe and North America [1]. Perilla is an important medicinal and food plant with multiple uses as oilseeds, herbal medicine, vegetables, and flowers [2,3]. Perilla has multi-nutritional values. For example, the seeds of perilla are commonly used as ingredients in soup and dessert. Perilla seeds contain 46% oils, of which the a-linolenic acid content can be as much as 60% [4,5]. The leaves of perilla are also consumed as a vegetable and spice in large quantities in China, Japan, and Korea. Perilla is rich in anthocyanins, flavonoids, and phenolic acids as well as other efficacious constituents. It is a raw material for the manufacture of skin care products in the fine chemical industry [6]. Thus, perilla has great potential for application in the human food supply and functional medicinal development. The recent application of genomics and multi-omics has provided a large amount of basic research data for studying the metabolic regulation, cultivation breeding, and genetic improvement of Chinese herbal plants [7]. Specifically, a great deal of effort has also been devoted to research on the evolution and genetic diversity of perilla, valuable medicinal components, mechanisms for genetic regulation, and selection or breeding of component-oriented varieties adaptive to the changing environment. In this Editorial in Agronomy (Special Issue, “Genetic Evolution and Quality Formation of Crops”), we selected the latest publications in the journals of Agronomy and others for review. We focus on the genomic analysis, ploidy evolution, mechanisms of metabolic regulation, important traits related to nutritional and health values, selection of good seeds, and adaptive cultivation, aiming to provide an update on the latest research advancement and to look at the realities of the crop as a valuable medicinal and food plant.

1. Perilla Genome and Evolutionary Patterns

China is considered to be the origin of Perilla frutescens, with the most abundant biodiversity. It has been cultivated in China for more than 2000 years [1]. Zhang et al. (2021) recently identified the two main ploidy types of perilla, haploid (2n = 2x = 20) and tetraploid (2n = 4x = 40) [7]. The predominant species that are currently cultivated are tetraploid, but the near-wild diploid is cultivated in Northern and South-west China [7]. The polyploidization of perilla resulted from a heterologous polyploidization event that occurred over the last 10,000 years [7]. The rate of base mutation in the tetraploid perilla was increased by about 10% compared to the diploid, and the genes in the AA subgenome had the advantage of higher expression levels, while the sequences of BB subgenes replaced the corresponding AA subgene copies through homologous recombination, thus rapidly increasing their frequency in the population. The accelerated rate of evolution of polyploid genomes and the phenomenon of equilibrium exchanges between the subgenomes help new species adapt to the changing environment. The recent polyploid event provides an unparalleled case for studying the plant polyploid evolution. Bae et al. (2023) reported the genome of Korean Island-Originated Perilla citriodora [8], and Tamura et al. (2023) released the genome of Japanese domesticated red perilla, identifying the covariance and differential loci by comparing the genomes with the above-referenced diploid and quadruploid genomes [9].

2. Research Progress of Population Classification and Genetic Diversity of Perilla

The delimitation of taxa within perilla species has always been controversial. Previously, many scientists used morphological and molecular markers, including AFLP, SSR, EST, SCAR, etc., to study the delineation of perilla [1,10]. Based on genome-wide resequencing analysis, the currently cultivated tetraploid perilla can be divided into Perilla frutescens var. frutescens and Perilla frutescens (var. crispa). Perilla frutescens (var. frutescens) is the early type of tetraploid Perilla frutescens, while the northern and southern taxa of Perilla frutescens var. crispa evolved independently at an early stage [7]. According to the morphology and use of Perilla frutescens, they are divided into Perilla frutescens for seeds (oil) and Perilla frutescens for leaves (medicine). Perilla used for oil purposes is mainly the germplasm of the original varieties with high oil content in larger seeds. Leafy (medicinal) perilla is primarily the group rich in secondary metabolites. The rich diversity of the perilla germplasm has resulted in great variations in plant traits (plant height, number of branches, color of flowers, leaves and stems, size of leaves, and degree of pubescence), seed traits (seed size, color, oil content, protein content, and fatty acid content), and secondary metabolite content of leaves (volatile oils, anthocyanins, flavonoids, and phenolic acids) as well as aroma [7]. China, Japan, Korea, and some other countries established germplasm banks for perilla, providing a material basis for resource conservation and genetic research.

3. Metabolic Regulations in Perilla Oil Seeds

Perilla seeds contain approximately 46% oil, while the unsaturated fatty acids can be up to 90%, with the content of a-linolenic acid being more than 60%, leading to the highest content of a-linolenic acid in the seeds of terrestrial plants [5]. Perilla oil has many pharmacologically active components that can improve optic nerve development in the brain, block fat growth, and protect the liver from hyperlipidemia [2]. The biosynthetic perilla seed oil mainly involves de novo fatty acid synthesis in the plastids and translocation to the endoplasmic reticulum for the acyl-CoA-dependent and acyl-CoA-independent pathways. Recent genome-wide identifications of differentially expressed loci during perilla seed development resulted in the characterization of key genes for fatty acid synthesis, such as WRINKLED, FUSCA3, LEAFY COTYLEDON1, and ABSCISIC ACID INSENSITIVE3 [4,11]. Omega-3 FAD is a major component for catalyzing linolenic acid desaturation. Duan et al. (2021) demonstrated that gene encoding PfFAD3.1 is a key regulator of the synthesis of the a-linolenic acid pathway in Perilla frutescens [5]. Using the transcriptome datasets, PfWRI1, PfABI4, and PfRAV1, which may play important roles in regulating oil accumulation, were further characterized [12].

4. Metabolic Regulation Mechanism of Chemicals in Perilla Leaves

Volatile oils, anthocyanins, flavonoids, and phenolic acids are abundant in perilla leaves. It is worth noting that there is a significant difference in the volatile oils extracted from different varieties of perilla. With regards to the main components, perilla can be further classified into PA (perillaldehyde), PK (perillaketone), PL (perillene), and other chemical types. The PA type of perilla is commonly used in traditional Chinese medicine [13]. Vegetable perilla is also a PA type, but its perilla aldehyde content is lower than that of the corresponding medicinal perilla. The PA-type perilla leaves have been widely used as kimchi in Korea. Different chemical types of perilla leaves provide important genetic material for terpene biosynthesis and also have different pharmacological functions in reality. Zhou et al. (2023) determined the morphology and major constituents, including perillaketone, isoegomaketone, and egomaketone, in peltate glandular trichomes, and then they identified eight reductases that catalyzed the conversion of isoegomaketone and egomaketone to PK [14]. In addition, since perilla leaves accumulate large amounts of anthocyanins and phenolic acids that have potent antioxidant activities, they are important for the health food manufacturing industry. The key genes for anthocyanin synthesis in perilla have long been identified. The transcript levels of anthocyanin synthase are significantly higher in red perilla, while the green perilla contains high levels of unigenes encoding limonene synthase [9]. Perilla anthocyanins are markedly affected by light intensity. Xie et al. (2022) identified several key genes in the anthocyanin biosynthesis pathway (CHI, DFR, and ANS) and 147 transcription factors (MYB, bHLH, bZIP, ERF, and NAC) involved in perilla anthocyanins [6]. The anthocyanin content was controlled by back leaf coloration, and the color change in the upper and lower leaves was correlated with the chlorophyll, carotenoid, flavonoid, and anthocyanin pigments; genes, such as F3’H, F3H, F3’5’H, DFR, and ANS, play an important role in regulating the formation of purple substances in upper and lower leaves [6].

5. Genetic Heredity and Quality Breeding of Perilla

Genetic studies on perilla began in the 1990s. Japanese scientists carried out a series of studies, including fruit color and firmness, different chemical constituents, such as anthocyanins [13]. Based on the level of research at that time, they only identified the above-referenced genes and traits via classical genetics and could not locate and clone the loci. Later, researchers focused on quality breeding according to different needs, thus generating seeds with higher contents of oils, particularly for seeds rich in a-linolenic acid [7]. Meanwhile, the Chinese medicinal perilla was selected for higher leaf yield with a high aldehyde content, while vegetable perilla was selected for flavors, palatability, and nutritional value. Systematic and cross breeding has become the major approach for breeding varieties of perilla in recent years. Due to the limited research on closely associated trait markers, molecular-marker-assisted selection (MAS) breeding is still a popular method [7]. Recently, Kim et al. (2021) identified 12 SSR markers associated with leaf-related traits and 11 SSR markers associated with plant-related traits, which contributed to identifying important genes/QTLs for MAS breeding programs [10].

6. Perspectives

Perilla is widely cropped in many regions of Southern and Northern China and other parts of East Asia. However, the changing environment impacts the traits and quality of perilla [15]. The large variation in the photoperiod between the southern and northern regions results in a great challenge for its growth and development. As a short-day plant, the northern perilla varieties cropped in the south tend to flower early when they are still young, whereas the southern perilla growing in the north often fails to enter the post-reproductive phase. In this case, the composition and quality of perilla will be seriously affected. An investigation of perilla seed traits in different regions of Guizhou Province, China, revealed that a-linolenic acid content in perilla seeds increased with the altitude [5]. It also showed that the aldehyde accumulation in perilla grown in Beijing varied significantly throughout the reproductive period [16]. Nowadays, facility cultivation is a widely used efficient way to improve the perilla yield and quality [17]. In an automated factory, the supply of chitosan oligosaccharide lactate to plants, for example, would help to increase the yield and quality of young leaves of red perilla [3]. The research and commercial production of effective components of medicinal plants have become a hot spot of research. Recently, the evaluation of resources and components, genomics and molecular genetics research, breeding and intelligent cultivation have laid a solid foundation for the exploration of the planting mode, industrialization research and application of Perilla frutescens.