Castor bean (Ricinus communis L., 2n = 2X = 20) is a monophyletic species belonging to the Euphorbiaceae family, which includes 280 genera and 8000 species. The origin of R. communis is Africa and then it was taken to India and China. Afterward, the plant spread around the world from temperate to tropical areas1. Castor bean is a dual-purpose medicinal and oil plant2. The medicinal properties of R. communis are due to the presence of some phytochemical compounds like flavonoids, phenolic acids, glycosides, alkaloids, steroids, and terpenoids. The antioxidant activity of phytochemical components of this plant is considered in the treatment of tumors and cancers. In addition, the plant is reported to possess other medicinal properties such as anti-diabetic, anti-microbial, anti-viral, anti-aging, anti-dermatophytic, anti-inflammatory, anti-nociceptive, and anti-hepatotoxic3,4,5,6.
Castor bean seed with 45–55% oil content is one of the most important oilseeds which contains a high percentage of ricinoleic acid. The special physicochemical properties of castor bean oil (including solubility in alcohol, high viscosity, and requiring low heat in the biodiesel production process) make it suitable for various industrial, pharmaceutical, and cosmetic uses. Industrial applications of castor bean oil include biodiesel production, waterproof coating, polymeric materials, lubricant, candle, brake fluid, shoe wax, and carbon paper. In addition, castor bean oil is highly valued in the treatment of diseases such as dry eye, meibomian gland dysfunction, wounds, constipation, and for cosmetic products such as shampoo, soap, and lotion1,5,7.
R. communis can grow in poor soils and stressful climates and is therefore considered as one of the agricultural solutions for areas with limited resources8. This plant is currently grown on a commercial scale in more than 30 countries, including India, Brazil, China, and Thailand9. According to FAO reports, the average global production, area under cultivation, and yield of castor bean seeds in 2021 are estimated to be 1,861,700 tons, 1,296,895 ha, and 1435.5 kg/ha, respectively. The leading producing countries in 2021 included India (1,647,000 tons), Mozambique (72,783 tons), Brazil (35,195 tons), China (21,000 tons), Thailand (12,000), Myanmar (11,696 tons), Ethiopia (11,000 tons), Vietnam (7000 tons), South Africa (6519 tons), and Paraguay (6000 tons).
Castor bean is a single-stemmed plant and its flowers are unisexual, producing male and female flowers on dichasial cymes. Male and female buds are different in terms of size, shape, and location on the inflorescence. The female buds are larger than the male buds, oval shape, and are located at the top of the inflorescence, while the male buds are round and are at the bottom of the inflorescence. As a result of this inflorescence structure, castor bean is reported to be more than 80% open-pollinating and its pollination is done by wind or insects. Also, before the male flowers open, the female flowers produce seeds, which also helps the open-pollination nature of the plant1. Castor bean plants can self-pollinate if isolated by distance or pocketing10. In addition to the reproductive modes mentioned, personal studies on this plant provided evidence of facultative apomixis. This mechanism of reproduction may affect many traits related to genetic diversity and breeding, population survival, and crop production, but no report on this mode of seed production in castor bean has been offered so far.
Sexual reproduction creates genetic diversity among plant species, which is necessary to improve crop quality in agriculture. On the other hand, sexual reproduction segregates advantageous traits in the next generations, which is a weakness of sexual reproduction. In some species, in addition to sexual reproduction, they can reproduce asexually, a process known as apomixis11. Apomixis is the mechanism of seed formation without fertilization and has been observed in more than 400 species of flowering plants (32 plant families). Events of sexual reproduction (meiosis and fertilization) do not happen in apomixis. In apomixis, the progression of meiosis is interrupted in the first or second cycle. As a result, oocyte fertilization does not occur in apomixis development, but the fertilization of polar cells causes endosperm formation12.
Most of the apomictic plants are facultative ones and can reproduce in both sexual and apomixis forms. Apomixis can be divided into two types, gametophytic and sporophytic. In sporophytic, the clonal embryo arises from a somatic cell in the tissue around the ovule. In this type of plants, sexual reproduction also takes place and inside the seed, there is a sexual embryo and one or more asexual embryos (polyembryony). Sporophytic apomixis is found in Citrus species, mango, and orchids. Gametophytic apomixis is known by two mechanisms: apospory and diplospory. In diplospory, the embryonic sac is made by mitosis or after disrupted meiosis by the megaspore mother cell. Diplospory has been reported in Tripsacum, Eragrostis, and Taraxacum. Also, in apospory, one or more ovule somatic cells are distinguished by a structure that is similar in shape and function to that of the megaspore. Apospory is the most common mechanism of apomixis in higher plants and is common in different genera including Beta, Brachiaria, Cenchrus, Chloris, Eriochloa, Heteropogon, Hieracium, Hyparrhenia, Hypericum, Panicum, Paspalum, Pennisetum, Ranunculus, Sorghum, Themeda, and Urochloa13,14,15,16.
Apomixis reproduction is not common in cultivated species and is commonly observed in wild plant species16. Although there is a prominent connection between polyploidy and gametophytic apomixis, the discovery of apomixis in diploids and the prevalence of diploidy in sporophytic apomictic plants rejects the hypothesis of the necessity of polyploidy in the development of apomixis17,18. Also, the evidence obtained from research on model plant species shows that apomixis is the result of epigenetic changes that occur in plants due to incorrect regulation of reproductive pathways and transcriptional modifications, resulting from the processes of polyploidization and distant hybridization19.
Studies on apomixis reproduction describe this process as a form of reoriented sexual reproduction. Apomixis emerges from the deregulated expression of sex-related genes as a result of asynchronous gene expression, gene duplication, and hybridization in sexual species and the evidence is studies conducted on Tripsacum sp. and Boechera sp.20,21.
The studies conducted on apomictic and sexual plants have shown that apomixis is a heritable dominant trait controlled by a single locus22,23. Apospory and diplospory in grasses are controlled by a single dominant locus24. Also, a simple dominant gene for apomixis has been reported in Panicum maximum and Hieracium aurantiacum25,26. However, few studies reported apomixis as an oligogenic trait controlled by a few genes27. In addition, other studies indicate an independent dominant control for different components of apomixis (apomeiosis, parthenogenesis, and endosperm development)28. In past studies, pleiotropy was reported for apomeiosis and parthenogenesis, but studies in the genera Allium, Poa, Erigeron, Taraxacum, Hieracium, Hypericum, and Potentilla showed independent control of these traits29. Furthermore, polygenic control on each apomixis component has been identified in Poa pratensis30.
Apomixis combines the benefits of seed propagation (high propagation rate, easy storage and planting, suitability for planting by machine, less seed use, and less disease spread) with vegetative cloning (preservation of genetic structure and fixation of superior genotypes after crossing). Apomixis has many benefits and applications such as propagation of superior genotypes including hybrid plants in the form of seeds, reduction of hybrid seed prices, farmers’ self-sufficiency in seed production, no need for a male sterile system in hybrid production, production of new varieties with a single cross without creating homozygosity, and increasing survival of interspecific crosses16. In addition, after identifying the genes involved in asexual reproduction, apomixis and its benefits can be transmitted through gene transformation to other plants31.
As mentioned, no research has been reported yet on the ability of castor bean for the asexual mode of seed production by apomixis. Sexual reproduction changes excellent traits in later generations, while asexual reproduction by seed (apomixis) transmits superior traits (phytochemical properties and oil content) to the offspring without any change. In this research, seed production in castor bean was performed in two ways: apomixis and open-pollination. Additionally, yield components, agronomic traits, and phytochemical properties were measured to assess variations among different genotypes and compare seed production using the two reproductive modes. We hypothesized that apomixis ability in castor bean is connected to some morpho-physiological characteristics in a world collection of the plant (Table 1).
