Georgianna K. Oguis, Edward K. Gilding, Mark A. Jackson, and David J. Craik^*

• Published online 2019 May 28
• doi: 10.3389/fpls.2019.00645

Abstract

The perennial leguminous herb Clitoria ternatea (butterfly pea) has attracted significant interest based on its agricultural and medical applications, which range from use as a fodder and nitrogen fixing crop, to applications in food coloring and cosmetics, traditional medicine and as a source of an eco-friendly insecticide. In this article we provide a broad multidisciplinary review that includes descriptions of the physical appearance, distribution, taxonomy, habitat, growth and propagation, phytochemical composition and applications of this plant. Notable amongst its repertoire of chemical components are anthocyanins which give C. ternatea flowers their characteristic blue color, and cyclotides, ultra-stable macrocyclic peptides that are present in all tissues of this plant. The latter are potent insecticidal molecules and are implicated as the bioactive agents in a plant extract used commercially as an insecticide. We include a description of the genetic origin of these peptides, which interestingly involve the co-option of an ancestral albumin gene to produce the cyclotide precursor protein. The biosynthesis step in which the cyclic peptide backbone is formed involves an asparaginyl endopeptidase, of which in C. ternatea is known as butelase-1. This enzyme is highly efficient in peptide ligation and has been the focus of many recent studies on peptide ligation and cyclization for biotechnological applications. The article concludes with some suggestions for future studies on this plant, including the need to explore possible synergies between the various peptidic and non-peptidic phytochemicals.

Keywords: peptides, forage crop, anthocyanins, organic pesticide, butelase, medicinal plant

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Introduction

Clitoria ternatea, commonly known as butterfly pea, is a perennial herbaceous plant from the Fabaceae family. It has recently attracted a lot of interest as it has potential applications both in modern medicine and agriculture, and as a source of natural food colorants and antioxidants. C. ternatea has long been cultivated as a forage and fodder crop, and early studies assessed the plant for these purposes (Reid and Sinclair, 1980; Barro and Ribeiro, 1983; Hall, 1985). Numerous field trials in Queensland, Australia, eventually led to the registry of C. ternatea cv. ‘Milgarra’ (Oram, 1992), the only cultivar in Australia that was released for grazing purposes (Conway and Doughton, 2005). Additionally, C. ternatea has been widely used in traditional medicine, particularly as a supplement to enhance cognitive functions and alleviate symptoms of numerous ailments including fever, inflammation, pain, and diabetes (Mukherjee et al., 2008).

In as early as the 1950s, studies on C. ternatea sought to elucidate its pharmacological activities, phytochemical composition and active constituents (Grindley et al., 1954; Piala et al., 1962; Kulshreshtha and Khare, 1967; Morita et al., 1976). The novel C. ternatea anthocyanins termed “ternatins” which render C. ternatea flowers with their vivid blue color, were first isolated in 1985 (Saito et al., 1985). Following further isolation and structural characterization of numerous other ternatins, the ternatin biosynthetic pathway was postulated a decade later (Terahara et al., 1998). In 2003, comparison of C. ternatea lines bearing different floral colors provided insights into the role of acylation on C. ternatea floral color determination (Kazuma et al., 2003a). The abundance of these unique anthocyanins alongside other secondary metabolites in C. ternatea makes the plant an ideal source of natural additives that can enhance the appearance and nutritive values of consumer products (Pasukamonset et al., 2016, 2017, 2018; Siti Azima et al., 2017). Although a number of recent studies has endeavored to elucidate the pharmacological activities of C. ternatea (Adhikary et al., 2017; Kavitha, 2018; Singh et al., 2018), the contribution of individual extract components on any bioactivity measured remains unknown.

Figure 1 summarizes some of the key agricultural and biochemical studies conducted on C. ternatea from the 1950s to the present, providing a convenient timeline of discoveries. The corresponding references to the key studies and milestones are listed in Table 1. In recent years, the small circular defense molecules called cyclotides, in C. ternatea (Nguyen et al., 2011; Poth et al., 2011a,b; Nguyen et al., 2014) have fueled scientific innovations that may have impact in modern agriculture, biotechnology and medicine. In 2017, Sero-X^®, a cyclotide-containing eco-friendly pesticide made from extracts of C. ternatea, was approved for commercial use in Australia¹. In addition, the C. ternatea cyclotide processing enzyme, butelase-1, which is the fastest ligase known to date and is capable of ligating peptides across a vast range of sizes (26 to >200 residues), can potentially be used in the large scale synthesis of macrocycle libraries and peptide-based pharmaceuticals (Nguyen et al., 2014, 2015).

FIGURE 1

Timeline of the key studies and milestones on Clitoria ternatea research from the 1950s to the present. The biological (blue) and biochemical (purple) studies pursued from the 1950s to early 1970s characterized the properties of roots and seeds. Toward the end of the 1970s, researchers began to isolate and characterize the phytochemical compounds from C. ternatea. Ternatins, the anthocyanins that render C. ternatea its vivid blue color, were first isolated in 1985; and the structure of the largest of the ternatins, ternatin A1, was characterized in 1989. Further isolation and characterization of the ternatins in C. ternatea led to the elucidation of the ternatin biosynthetic pathway in 1998. Parallel to the studies that characterized the phytochemical composition of C. ternatea, were agricultural studies that evaluated C. ternatea as a forage and fodder crop. A series of field studies in Queensland, Australia lead the development and eventual release of the C. ternatea Milgarra cultivar in 1991. From 2001 to the present, studies have been determining the pharmacological activities and biological activities of C. ternatea extracts. In 2011, cyclotides, the circular insecticidal molecules which can also be used as scaffolds for peptide-based therapeutics, were discovered in C. ternatea. While cyclotides had previously been characterized in other angiosperm species, C. ternatea is to date, the only legume that is known to produce them. In 2014, butelase-1, the ligase that facilitates cyclization in C. ternatea cyclotides, was discovered and characterized. Cyclotides and the auxiliary enzymes, have applications both in modern medicine and agriculture. In 2017, Sero-X^® an eco-friendly insecticide made from C. ternatea extracts was registered for commercial use in Australia.

References:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6546959/