Antioxidant Metabolites in Primitive, Wild, and Cultivated Citrus and Their Role in Stress Tolerance

Abstract

The genus Citrus contains a vast range of antioxidant metabolites, dietary metabolites, and antioxidant polyphenols that protect plants from unfavorable environmental conditions, enhance their tolerance to abiotic and biotic stresses, and possess multiple health-promoting effects in humans. This review summarizes various antioxidant metabolites such as organic acids, amino acids, alkaloids, fatty acids, carotenoids, ascorbic acid, tocopherols, terpenoids, hydroxycinnamic acids, flavonoids, and anthocyanins that are distributed in different citrus species. Among these antioxidant metabolites, flavonoids are abundantly present in primitive, wild, and cultivated citrus species and possess the highest antioxidant activity. We demonstrate that the primitive and wild citrus species (e.g., Atalantia buxifolia and C. latipes) have a high level of antioxidant metabolites and are tolerant to various abiotic and biotic stresses compared with cultivated citrus species (e.g., C. sinensis and C. reticulata). Additionally, we highlight the potential usage of citrus wastes (rag, seeds, fruit peels, etc.) and the health-promoting properties of citrus metabolites. Furthermore, we summarize the genes that are involved in the biosynthesis of antioxidant metabolites in different citrus species. We speculate that the genome-engineering technologies should be used to confirm the functions of candidate genes that are responsible for the accumulation of antioxidant metabolites, which will serve as an alternative tool to breed citrus cultivars with increased antioxidant metabolites.

Keywords: antioxidant metabolites; citrus; flavonoids; stress tolerance.

Figure 1

The role of antioxidant metabolites and reactive oxygen species (ROS) in normal and oxidative stress conditions. (a) High levels of antioxidant metabolites such as flavonoids can protect cellular organelles. (b) Under mild stress condition, the moderate level of antioxidant metabolites can detoxify ROS, which can maintain the balance between the production and scavenging of ROS. (c) Under high oxidative stress, the level of ROS is increased while that of antioxidant metabolites is reduced, which damages the membranes, DNA, proteins, and other cellular organelles, finally leading to cell death.

Figure 2

Distributions of antioxidants and oxidants in different subcellular organelles of the plant cells. APX, ascorbate peroxidase; GR, glutathione reductase; GPX, glutathione peroxidase; SOD, superoxide dismutase; CAT, catalase; GSH, glutathione; NO, nitric acid; ¹O₂, singlet oxygen; O•⁻², superoxide; H₂O₂, hydrogen peroxide.

Figure 3

Flavonoid biosynthesis pathway. Gene abbreviations were taken from KEGG (www.genome.jp/kegg/pathway accessed on 14 August 2021) for plants. PAL, phenylalanine ammonia lyase; C4H, cinnamate 4-hydroxylase; 4CL, 4-coumarate: CoA ligase; CHS, chalcone synthase; CHI, chalcone isomerase; F3H, flavanone 3-hydroxylase; F3′M, flavonoid 3′-monooxygenase; FLS, flavonol synthase; DFR, dihydroflavonol 4-reductase; ANS, anthocyanidin synthase; BAN, banyuls; UGT, UDP-glucosyl transferase 78D3.

Figure 4

Correlation among antioxidant metabolites and stress tolerance in different Citrus species. The primitive citrus species that have a high level of metabolites are tolerant to various abiotic and biotic stresses; the wild citrus species (C. medica and C. latipes) possessing moderate levels of metabolites are semi-tolerant to stresses; and the cultivated citrus species that contain fewer metabolites are more prone to stresses. High levels of metabolites are positively correlated with abiotic [13] and biotic stress tolerance in citrus [15].