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Cotton is the most critical fiber crop and one of the top three oilseed crops in the world. One pronounced feature of cotton is that it is rich in secondary metabolites, mainly including terpenoids, flavonoids, and phenolic acids. These secondary metabolites have various ecological roles, such as defense and signal transmission. With the concept of plant secondary metabolites becoming more and more evident in the mid-twentieth century, cotton secondary metabolites as natural phytoalexins were also established. Terpenoids are stored in pigment glands that are distributed almost all cotton plant surfaces or subsurfaces and defend cotton plants from chewing insects, pathogens, and other herbivores. Flavonoids are relevant to fiber quality and color and also play a role in mechanism in insect and pathogen resistance. Phenolic acids play a role in weed suppression and insect and pathogen resistance. There are several reviews on cotton secondary metabolites, and the most recent one was five years ago. They all focus on the metabolites themselves. None of them focus on applications in cotton production. This review started from browsing the abundant literature on cotton secondary metabolites, and then analyzing their potential application in cotton production. Finally, our recent findings were discussed in this chapter.
Mississippi State University, Mississippi State, USA
Te-Ming Tseng
Mississippi State University, Mississippi State, USA
K. Raja Reddy
Mississippi State University, Mississippi State, USA
Natraj Krishnan
Mississippi State University, Mississippi State, USA
Shien Lu
Mississippi State University, Mississippi State, USA
*Address all correspondence to: ziming.yue@gmail.com
1. Introduction
Cotton is the world’s most important natural fiber crop and one of the top oilseed crops among soybean, rapeseed, canola, sunflower seed, and peanut. Another by-product is the protein-rich meal from cotton seeds. Cotton is grown in 75 countries worldwide [1]. Different from other crops, cultivated cotton includes four species in the genus Gossypium: G. hursutum, G. barbadense, G. arboretum, and G. herbaceum. The first two are allotetraploid and distributed in the New World (Americas), and the latter are diploid and distributed in the Old World (Asia and Africa). The genus Gossypium belongs to the Malvaceae family, and it has approximately 50 diploid species among eight genomes (A–G and K genomes) and seven allotetraploid species [2, 3].
Beyond its economic value, cotton also possesses a rich array of secondary metabolites and organic compounds synthesized by plants for various ecological functions. These secondary metabolites play a crucial role in the plant’s defense against pests, pathogens, and environmental stresses. There are at least three reviews on cotton secondary metabolites: [4, 5, 6]. Ref. [4] is comprehensive and expected the cotton secondary metabolites to be utilized to defend against pests in cotton production, but data accumulation was not as abundant as in the present day, and that was published before GMO cotton appearance; Ref. [5] focused more on cotton terpenoids; and Ref. [6] focused on cotton flavonoids. This review will follow the coverage of Ref. [4] and review the logical development of the utilization of secondary metabolites in cotton production.
In recent years, there has been a growing interest in harnessing the potential of cotton secondary metabolites for applications in agronomy. Part of the reason is that cotton is still the “dirtiest” crop as it consumes 10.24% of the insecticides and 4.71% of pesticides in 2019 with only 2.5% of the arable land according to the International Cotton Advisory Committee [7], and it was one of the early crops that widely adopted GMO technology [8].
The key distinction between the eras of Refs. [4, 5], and the current period lies in the acknowledgment of secondary metabolites in plants as an innovative foundation for potential biopesticides. This recognition has opened avenues for their application in sustainable agriculture, as highlighted by Ref. [9]. Researchers and agronomists have identified the diverse properties of secondary metabolites, including antimicrobial, insecticidal, antioxidant, and allelopathic activities. These bioactive compounds have the potential to revolutionize agricultural practices by offering environmentally friendly alternatives to synthetic pesticides, fertilizers, and growth regulators.
The use of cotton secondary metabolites in agronomy presents several advantages. Firstly, it provides an opportunity to reduce the reliance on conventional chemical inputs, thereby minimizing the environmental impact of their use. Secondly, these natural compounds may offer a sustainable solution to combat pests while maintaining crop productivity. Moreover, using cotton secondary metabolites can lead to the development of novel bio-based products and promote the concept of integrated pest management (IPM) and sustainable agriculture.
This review explores the current knowledge and research advancements in using cotton secondary metabolites in agronomy. It will delve into the compounds’ chemical diversity, modes of action, and potential applications in crop protection, nutrient management, and plant growth promotion. Furthermore, the challenges and opportunities associated with their practical implementation will be discussed, along with future perspectives for utilizing cotton secondary metabolites in sustainable agricultural systems.
By critically evaluating the existing literature and highlighting key findings, this review intends to provide a comprehensive overview of the potential benefits and limitations of incorporating cotton secondary metabolites into agronomic practices. Ultimately, such insights can contribute to developing innovative strategies that harness the power of these bioactive compounds to promote sustainable and environmentally conscious approaches to crop production.
As the name initially meant, plant secondary metabolites used to be considered as waste in metabolism without primary growth, development, and reproduction functions [10]. Nowadays, plant secondary metabolites usually refer to small molecules produced in metabolism with ecological functions, such as defense and survival [10]. Plant secondary metabolites have a long history of being used as herbal medicine; their use in agriculture is a recent development. They are classified into terpenoids, phenolic compounds (flavonoids and phenylpropanoids), and N-containing compounds (cyanogenic glycosides, alkaloids, and glucosinolates) based on their biosynthetic pathways [11].
2.1 Cotton secondary metabolites
Cotton secondary metabolites are flavonoids (including tannins), terpenoids, phenolic acids, and fatty acids. Alkaloids and sulfur-containing secondary metabolites have not been emphasized due to their number so far.
2.1.1 Flavanoids
Flavonoids have a backbone diphenylpropane structure (C6-C3-C6) where two aromatic rings are linked via a three-carbon chain (Figure 1). The A ring is typically formed from a molecule of resorcinol or phloroglucinol synthesized via the acetate pathway with a characteristic hydroxylation pattern at positions 5 and 7 [12]. The B ring comes from the shikimate pathway with a characteristic hydroxylation pattern at positions 4′-, 3′, 4′-, or 3′, 4′, 5′-. One impressive phenomenon is that the number of characterized cotton flavonoids has significantly increased from 23 in 1994 [13] to 52 in 2017 [6], and up to 190 in 2023 [14]. With the MS/MS technique, it is easier to identify more and more flavonoids, while the functions of the newly discovered flavonoids in cotton plants largely remain to be investigated. Of the reported ecological functions, signaling, defense, and coloring functions have been well studied as listed in Table 1.
Tannins are polyphenolic substances that have molecular weights ranging from 500 to 3000 [47]. They can cause proteins to precipitate. Tannins possess numerous phenolic hydroxy groups that enable them to establish multiple hydrogen bonds with proteins and other large molecules with -NH-, -NH2, or -OH groups [47]. This interaction results in the formation of complexes that can dissociate under normal pH conditions and are resistant to enzymatic degradation. The term “tannins” initially emerged from the leather industry, referring to substances capable of converting animal hides into leather [48]. Tannins can be categorized into two groups based on their biological origin. Hydrolyzable tannins are complex polyphenols that can be broken down through hydrolysis to yield gallic or ellagic acid and a sugar unit, typically glucose [47].
On the other hand, nonhydrolyzable or condensed tannins are formed by the polymerization of hydroxy flavans, resulting in the formation of dimers, trimers, or oligomers. Certain condensed tannins can become insoluble during extraction due to strong interactions with proteins, nucleic acids, or polysaccharides, leading to precipitate formation. Condensed tannins are also procyanidins since they produce anthocyanidins upon acid heating [48].
Together with phenolic acids [49, 50], gossypol [51, 52, 53, 54], flavonoids [27], and tannins [47] are also listed in the groups of compounds that have been investigated and utilized in host resistance-breeding programs in cotton.
Condensed tannins have been documented as host plant resistance mechanisms for cotton bollworm (Helicoverpa zea Boddie) and tobacco budworm (Heliothis virescens F.). As a cotton resistance mechanism, condensed tannins were also found to antagonize the effects of the B.t. toxin [55]. The resistance imparted by condensed tannins deterred feeding by the larvae, which ultimately reduced the amount of B.t. toxin ingested by the insect; as a result, larvae feeding on high-tannin, B.t.-toxin-containing diets had lower mortality rates than insects feeding on diets containing condensed tannins or B.t. toxin alone. Condensed tannins could have also directly reduced the availability and activity of the B.t. toxin protein since tannins can precipitate proteins. Hence, the report proposed that insect management strategies utilizing B.t. toxin, whether as a topical insecticide or in genetically modified cotton crops, may not be suitable when combined with high-tannin plants.
Therefore, the report suggests that insect control tactics employing the B.t. toxin, either as a topically applied microbial insecticide or in transgenic cotton plants, may not be compatible with plants with high tannin concentrations.
Ref. [25] reported that condensed tannins in cotton leaves were up to 20% of dry weight. Their concentrations in individual leaves increased successively until about the tenth leaf. The upper leaves maintained the concentrations until early fall. The tannins resisted spider mites and could be hydrolyzed into digests consisting of one cyanidin and four delphinidin.
Tannins contribute to disease resistance. Resistant young leaves of cotton contain condensed tannins in higher concentration than do older susceptible leaves [56].
2.1.3 Lignins
Besides the above bioactive secondary metabolites, some structural metabolites such as lignins and cellulosic also belong to secondary metabolites [57], constituting a significant part of the biomass. For example, the cotton plant stalk comprises about 68% holocellulose and 26% lignin. These two classes of secondary metabolites also combine to be called lignocellulosic [1]. Lignins have protective functions, such as preventing lodging, etc.
2.1.4 Terpenoids
Terpenoids, or isoprenoids, are the largest group of plant secondary metabolites derived from the five-carbon compound isoprene, and its derivatives called terpenes. Terpenoids contain additional functional groups, usually oxygen, such as terpene aldehyde. Compared to flavonoids, cotton terpenoids do not have so many compounds, but the naked eye can see their existence as pigment glands. Cotton plants produce a variety of monoterpene and sesquiterpenoids. Monoterpenes include α-pinene, β -pinene, limonene, β-ocimene, β-myrcene, α-terpenene, γ-terpenene, etc. They are volatile oils, often as semiochemicals, attracting or deterring insects or parasitoids [34, 58]. Cotton plants produce both volatile and nonvolatile sesquiterpenoids. The former includes α-humulene, α-copaene, β-carophyllene, β-carophyllene oxide, α-aromadendrene, α-selinene, β-selinene, γ-bisabolene, β-bisabolol, δ-cadinene, 12-hydroxy-β-caryophyllene, gossonorol, spathulenol, 12-hydroxy-β-caryophyllene acetate, 12-hydroxy-β-caryophyllene oxide acetate, and guaia-1(10), 11-diene, etc. The latter includes hemigossypol, hemigossypolone, desoxyhemigossypol, gossypol, and heliocides H1, H2, H3, and H4 [59], as well as 6-methoxy and 6, 6′-dimethoxy gossypol derivatives [60]. They are nonvolatile. Almost all sesquiterpenoids and part of monoterpenes (the remaining are released to the air) are stored in the pigment glands, which appear oval and spherical and as black, orange, yellow-brown (yellowish-brown), green (red-brown), or purple depending on the species in all the tissues of cotton plants except pollen and seed coat [61]. Some monoterpenes (β-ocimene and myrcene) and sesquiterpenoids (hemigossypolone) form heliocides (C-25) H1, H2, H3, and H4) by direls alder reaction in the glands. Different genotypes impacted the distribution of the pigment glands [62, 63]. Figure 2 shows the structures of some common terpenoids in cotton plants.
Figure 2.
Structures of some common terpenoids in cotton plants.
Gossypol is a dimeric sesquiterpenoid stored in the pigment glands of cotton plants. Gossypol was first extracted from cotton seeds by Marchlewski in 1899 and characterized by Adams et al. in 1938 [64, 65]. It constitutes 20–40% of the pigment gland weight and accounts for 0.4–1.7% of the whole cotton seed kernel. From the gossypol history, gossypol was first discovered from cotton seeds (cotyledons) because the toxicity of gossypol to humans and animals blocked the efficient use of cottonseed oil and protein. However, [66] found the importance of cotton roots in gossypol production in cotton plants. Recent progress [61] showed that cotyledons and roots are primary sources of gossypol in cotton plants. These two plant parts contain higher gossypol than other parts. The former is responsible to provide gossypol during germination, after which the developing roots become the primary source for gossypol production in cotton plants.
The defensive role of cotton pigment glands and gossypol was first recognized by Cook [67] in 1906. It turned out that gossypol and other terpenoids in the cotton pigment glands are associated closely with resistance mechanism to insects and pathogens [68, 69], including monoterpenes, sesquiterpenoids, and heliocides.
Despite that the pigment glands are distributed almost throughout the whole plant, the gland content compositions are different from different parts. The glands of cottonseeds contain mostly gossypol with traces of deoxyhemigossypol, and the glands of cotton leaves contain hemigossypolone, a gossypol derivative, and heliocides [70]. On fresh weight basis, cotton seed (cotyledons) usually has similar gossypol content of around 1% in roots at the harvesting stage (Yue, unpublished data); on dry weight basis, cotton root usually has the highest gossypol content [66] among different plant parts, which is consistent with the recent progresses that cotyledon (seeds) and roots are the source of gossypol for the whole plants [61, 71].
2.1.5 Phenolic acids
Phenolic acids in cotton include benzoic acid, cinnamic acid, and their derivatives (Table 2). They are water-soluble. They include vanillic acid, benzoic acid, ferulic acid, sinapic acid, cinnamic acid, syringic acid, p-coumaric acid, gentisic acid, caffeic acid, chlorogenic acid, gentisic acid, p-hydroxybenzoic acid, p-hydroxybenzoic acid, 3, 4-dihydroxybenzoic (protocatechic) acid, etc. Their functions in cotton plants were thought to be related to resistance mechanism to pests and pathogens [50]. They also played a role in cotton allelopathy [74].
Cotton fatty acids are mainly in the seeds. Cottonseed oil has a fatty acid profile that is composed of 52.89% linoleic acid, 25.39% palmitic acid, 16.35% oleic acids, together with small amounts of 2.33% stearic acid, 1% myristic acid, and 0.6% palmitoleic acid, as well as 0.17% linolenic acid [75]. Ref. [76] reported malvalic, sterculic, and dihydrosterculic acids in the hypocotyl of both glanded and glandless cotton seeds. Malvalic and sterculic acids represent 0.3% and 0.5% of the total fatty acids in the analyzed varieties.
2.1.7 Miscellaneous secondary compounds
Beyond these main classes of secondary metabolites, the following secondary compounds were also identified in the cotton plant (Table 3).
Additional secondary compounds identified in the cotton plant.
2.2 Secondary metabolite compositions of different cotton species
Compositions of secondary metabolites vary among cotton species. From 1-week-old roots of Gossypium hirsutum and G. barbadense, the disesquiterpenoid aldehydes, gossypol, 6-methoxygossypol, and 6, 6’-dimethoxygossypol, and the sesquiterpenoid aldehydes, hemigossypol and methoxyhemigossypol were isolated and identified. While the compounds 6-methoxygossypol and 6, 6′-dimethoxygossypol constituted 30% of the total terpenoid aldehydes in the seeds of the cultivar of G. barbadense but occurred only in trace quantities in those of G. hirsutum [60].
As mentioned above, the cotton genus contains around 50 species, and four are domesticated independently [2, 78]. These different cotton species have other secondary metabolite profiles, which are representative of biodiversity. For example, myrcene is a major monoterpene presented in G. hirsutum but not present in G. barbadense. In contrast, G. barbadense contains copaene, β-carophyllene oxide, and (−)-δ-cadinene at 14.3, 8.0, and 7.8% in leaf oil compared to only 0.7, 0.4, and 0.3 respectively in oils from G. hirsutum. The variations between the two species could potentially impact insect-host relationships [79].
Different biotypes in the same species also have different secondary metabolite compositions. Ref. [80] found high levels of intraspecific diversity in the terpene profiles of the plants. Two distinct chemotypes were identified: one chemotype contained higher levels of the monoterpenes γ-terpinene, limonene, α-thujene, α-terpinene, terpinolene, and p-cymene, while the other chemotype was distinguished by higher levels of α- and β-pinene. The distribution of chemotypes followed a geographic gradient from west to east with an increasing frequency of the former chemotype. Such differences in secondary metabolites are the foundation for a cotton breeding program for a high level of one or several secondary metabolites.
2.3 Inductive vs. constitutive secondary metabolites
A secondary metabolite can be constitutive or inducive. A constitutive secondary metabolite means the plant produces the secondary metabolite as a routine without environmental influence. An inductive secondary metabolite is produced or increased under one or more environmental factors, such as pest infestation or abiotic stresses. Most secondary metabolites are both constitutive and inductive. For example, cotton flavonoids and gossypol are constitutive as they are routinely produced, and they are inductive as their production increases when there are abiotic stresses, such as salinity stress [81]. In addition, increased terpenoid accumulation in cotton foliage was observed as a general wound response upon mechanical damage, Spodoptera littoralis caterpillar infestation, and jasmonic acid treatment [82] generally increased secondary metabolites production with E-β-ocimene, heliocide H1 and H4, showing the highest increases. In response to aphid attack, cotton plants synthesized and released callose outside the cell plasma membrane [83].
2.4 Cotton secondary metabolites and abiotic and biotic stresses
2.4.1 Abiotic stresses
Cotton production can be affected by various abiotic stresses, which include drought, salinity, nutrient deficiency, chemical burns, and ultraviolet radiation stress. They all turned out to have interactions with secondary metabolites. Abiotic stresses can cause secondary metabolite content and pest resistance change, influencing fiber and seed quality.
Ref. [84] reported total phenol and tannin content under salinity stress increased significantly after third water stress exposure. Evidence also showed that secondary metabolite increase resulted from abiotic stresses, increased pest resistance, and led to higher fiber quality. Such cotton traits can be applied in cotton production directly.
Nutrient deficiency is another kind of abiotic stress, and Ref. [85] showed that K deficiency reduced the antioxidant capacity of cotton seedlings and resulted in a metabolic disorder characterized by elevated levels of primary metabolites and reduced production of secondary metabolites. The results were from an analysis of xylem sap. How much degree the xylem sap represents the defense response of cotton plants to K deficiency is of concern. We compared root gossypol concentrations of cotton seedlings from irrigated under tap water and Hoagland solution for three weeks after emergence. The root gossypol concentrations of tap water irrigated seedlings (subjected to nutrient deficiency) were higher than the Hoagland solution irrigated seedling (Yue, unpublished data).
Cotton plants suffer from ultraviolet radiation damage, but many cotton secondary metabolites can protect against ultraviolet radiation. This partly originates from the fluorescence properties of some secondary metabolites that can convert ultraviolet radiation to lower-energy light. It partly results from the solid antioxidative properties of cotton secondary metabolites [86].
2.4.2 Biotic stresses
Biotic stresses in cotton production include insects and other herbivores, pathogens, weeds, etc. As discussed in 2.3, the secondary metabolite production was a defense response to the insects and salinity stresses; pathogen inoculation/infection also induced secondary metabolite biosynthesis. Ref. [28] used dimethoxybenzaldehyde (DMB) histochemical reagent to reveal induced biosynthesis of flavanols (catechin, gallocatechin, and their condensed proanthocyanidins) as red color in the cotton stem section upon Verticillium dahliae inoculation. The Verticillium wilt-resistant cultivar showed much more intense flavanol synthesis than the susceptible cultivar.
Among the different stresses for cotton plant growth, development, and reproduction such as insects, pathogens, and abiotic stresses (drought, salinity, and nutrient deficiency, etc.), only weed infestation has not been reported to induce plant defense or immune response or change of secondary metabolites concentrations, which is the basis for potential utilization. Phenolic acids have been explored for their weed-suppressive effects in cotton [74]. Flavonoids, however, have not been examined. Cotton has a rich array of flavonoids, some of which, such as quercetin, have been reported to be allelopathic [87].
2.5 Glandless cotton
Since McMichael bred a glandless cotton cultivar in 1959 [88], which was devoid of toxic gossypol, the utilization of cotton secondary metabolites has another aspect: reduction of secondary metabolites so that cotton seeds can be less toxic and more valuable. Recently, Ref. [89] reported the use of RNAi to selectively silence the δ-cadinene synthase gene to reduce gossypol levels in cotton seed by 97% while not affecting levels of gossypol and other terpenoids in the rest of the plants.
3. Utilization of secondary metabolites in cotton production
3.1 Secondary metabolites being extracted and applied directly in cotton production
Cotton secondary metabolites have been recognized as natural pesticides [79]. The simplest way to utilize them is to extract them and apply the extract directly in cotton production.
Ref. [90] extracted secondary metabolites by water from different plant parts of G. hirsutum plant and used them to suppress weed. Among the various water extracts of the plant parts, leaf extract was found to impart maximum inhibitory effect on the germination indices of the wheat (T. aestivum) followed by the stem extracts. The concentrated extract led to wheat germination percentage and seedling vigor index reduction of 46% and 62%, respectively. The research did not reach into the active ingredients of the cotton leaf extract. As discussed earlier, cotton leaf is rich in secondary metabolites such as condensed tannins (up to 20% dry weight), terpenoids, non-tannin flavonoids, and phenolic acids. As terpenoids are not soluble in water, they might not be the main active ingredients of the extract except the extract contained a significant number of solid particles (our HPLC analysis showed leaf water leachate contained less than 1 mg/L gossypol). Hence, the most probable active ingredients are non-tannin flavonoids and phenolic acids. Phenolic acids have been explored for cotton allelopathy [74]. Flavonoids, however, have not been investigated for cotton allelopathy. As listed in Table 1, cotton contains a rich array of flavonoids, some of which have been reported to be allelopathic. For example, Ref. [87] reported quercetin from Fagopyrum esculentum roots to inhibit the radicle growth of Phelipanche ramose. The data also suggest that two ortho-free hydroxy groups of the C (B?) ring could be essential to impart allelopathic activity. In contrast, the carbon skeleton of the B ring and substituents of both the A and B rings are unnecessary. According to this rule, lots of flavonoids in Table 1 are allelopathic such as Gossypetin, Gossypin, Gossypitrin, Gossypetin 8-O-rhamnoside, Catechin, Luteolin, Luteolin 8-C-hexosyl-O-hexoside, Hyperoside, Quercetin-3-sophoroside, Quercemeritrin, Gallocatchin, Delphinidin, Cyanidin, Hirsutrin, Butin, Isoquercitrin Hyperoside, Isotrifoliin, Rhamnetin, etc. Hence, allelopathy spread by cotton flavonoids is expected to be confirmed soon [91].
In this way of extraction and application, we can extract secondary metabolites from other allelopathic plants and apply them to cotton production. For example, Ref. [92] used water extracts from sorghum, sunflower, and brassica for preemergent application in the cotton field. It was observed that the best treatment produced dry biomass reduction by 40% of weeds Trianthema portulacastrum and Cyperus rotundus and an increase in seed cotton yield (12%). Similar extraction and application can be extended to different allelopathic plants. For example, we used sicklepod seed methanol extract (replaced with water after extraction) and applied it to soybean to prevent deer browsing [93].
3.2 Screening and breeding for high secondary metabolites cultivars
Since Cook [67] observed the better resistance to insect herbivores of Guatemalan cotton, numerous efforts have been made to increase U.S. cotton resistance to various biotic and abiotic stresses in cotton production [6]. So far, secondary metabolites were reported to contribute cotton resistance include condensed tannins [47, 89], terpenoids/gossypol [51, 52, 53, 54, 63, 94], phenolic acids [49, 50, 74], and bacterial endotoxins [95, 96]. All these secondary metabolites have been screened and bred for higher secondary metabolite levels. Their common basis is that selection among cultivars or species for higher secondary metabolite levels is an attainable goal [63, 97].
3.3 Cotton secondary metabolite regulators or elicitors
As discussed before, cotton secondary metabolites (gossypol and flavonoids) were shown to increase under salinity stress [81]. The salinity stress was imposed by salt water as irrigation; foliar application of the salt water also increased root gossypol concentrations in our lab. The saltwater can be regarded as a secondary metabolite regulator or elicitor to increase gossypol and flavonoid production for defense. In addition, Ref. [43] showed that oligosaccharides filtrates of Fusarium oxysporum f. sp. vasinfectum could stimulate phenolic compounds production in cotton plants. Furthermore, some herbicides [98, 99] were reported to cause an increase in gossypol concentration in cotton plants. Since the introduction of GMO cotton in the mid-1990s, herbicide application has become a routine and indispensable practice in cotton production. Tank-mix of herbicide with secondary metabolites regulators or elicitors, such as the salt, as mentioned earlier solution or pretreatment of the cotton seeds by the herbicides as mentioned above can increase the gossypol concentration of the seedling roots, a potential practice to prevent nematode damage to cotton plants.
3.4 Metabolic engineering method to modify cotton
Contrary to the exogenous metabolite regulator, metabolic engineering optimizes genetic and regulatory processes within cells to increase the cell’s production of a specific substance. Compared to general metabolic engineering, where the target substance is commercial, we would increase specific substances (secondary metabolites) in situ for cotton plant defense or profit.
Cotton is a multibillion-dollar industry where engineering is often employed to improve profitability. In a study conducted by John and Keller in 1996 [100], they utilized particle bombardment to introduce phbB and phbC genes into cotton. This genetic modification led to the development of cotton plants with fibers containing poly-(3R)-hydroxybutanoate (PHB), thereby enhancing the insulating properties of the fibers. Genetic mapulations of production of trienoic fatty acids in cotton seeds have improved low-temperature seed germination, plant photosynthesis, and fiber quality [101].
Most recently, Ref. [102] used a selective gene editing method to remove the toxic enantiomer (−)-gossypol while the (+)-gossypol and other structurally related terpenoids remained, keeping their resistance to insect herbivores and pathogens. The modified genes (including the current widely accepted GMO cotton genes) may flow into wild species in nature, which is a concern.
3.5 Cotton secondary metabolites as a potential stress assessment tool for field crop management
Although the general concept of plant secondary metabolites and their ecological functions became evident around the mid-twentieth century, gossypol was first extracted from cotton seeds in 1899 [103], and cotton pigment glands were correctly interpreted in 1906 [67]. Hence, we have accumulated over 124 years of knowledge of cotton secondary metabolites. The knowledge of cotton secondary metabolites has benefitted cotton breeding and human and animal health regarding contraception, byssinosis, and utilization of cotton seeds. Now, we have known draught [84], salinity [81], chilling [104], insect [86], pathogen [59], nutrient deficiency (Yue, unpublished data), herbicides [98, 99], and even mechanical damage [82] of the cotton plants can induce production of cotton terpenoids in cotton plants, especially root gossypol concentration to increase. In addition, the root system is the main organ to synthesize gossypol [61]. The root system also increased its gossypol concentration while supplying gossypol to aboveground parts except cotyledons. Hence, cotton root gossypol concentration is sensitive to all the above factors and is a cumulative index of cotton plant stress and defense status. In other words, cotton root gossypol concentrations are the common currency in cotton plant stress and defense response.
Furthermore, our ongoing projects showed that adjacent plants can increase cotton root gossypol concentration compared to isolated cotton plants without adjacent plants [105]. Nutrient deficiency stress increased cotton root gossypol concentration comparing to cotton plants fertilized by Hoagland solution (Yue, unpublished data). By analyzing all the previous data on cotton root gossypol concentrations, it is found cotton roots have the highest gossypol concentration on a dry matter basis (6.3% in Ref. [66]) among different cotton plant parts. Cotton roots have a similar gossypol concentration as seeds at the harvesting stage on a fresh weight basis, around 1% (Yue, unpublished data).
Cotton root gossypol concentration analysis is potentially a stress assessment tool for field crop management, just like body temperature measurement in medical examinations. The basis is that we set up a series of standard values of cotton root gossypol concentrations at different ages or sizes without those stresses.
It is time to push our understanding of the cotton secondary metabolites into productivity. Direct application of cotton secondary metabolites in crop management will convert the traditional knowledge accumulation pattern of cotton secondary metabolites from funding agency publication to application data production, making us acquire knowledge of cotton secondary metabolites faster. A specific farmer might not be able to analyze these secondary metabolites, but our research and extension centers should be able to do it.
3.6 Employment or avoidance of effects of secondary metabolites in cotton stubble on soil health and cotton crop next season as another field of application of cotton secondary metabolites in cotton production
As a cumulative stress index, cotton root gossypol concentration increases with age and size. At harvesting stage, cotton root gossypol concentration is high and around 1% (Yue unpublished data) fresh weight, each cotton stubble is estimated as 10 grams, and cotton plant density is estimated as 40,000 plants/acre [106]. The phytoalexin gossypol dose is estimated as 4000 g/acre, this dose is significantly heavier comparing to other herbicides and insecticides. For example, glyphosate burndown dose is 1 lb./acre [107]. Despite constrained in the dead cotton stubbles, the root gossypol and its glands are expected to have significant impact on soil health and possibly cotton crop next season. Employment of their effects on soil health and cotton plants next season and avoidance of the effects are another field of application of secondary metabolites in cotton production.
Cotton plants are rich in secondary metabolites, which mainly include terpenoids, flavonoids, and phenolic acids. Terpenoids are not soluble in water, stored in pigment glands, including monoterpenes, sesquiterpenoids, and heliocides. They can deter insects and pathogens. One representative is gossypol, whose main sources are cotyledons and living roots. Cotton root gossypol concentration is a cumulative index for cotton plant stresses and defense response, and potentially used as a stress assessment tool for cotton crop management. It also increases with cotton plant age and size, reaching around 1% root fresh weight at harvesting stage and representing a heavier dose than other herbicides and insecticides. The impact of the high-dose gossypol in cotton roots after harvest on soil health and cotton plants next season is expected to be confirmed in the near future. Flavonoids used to be thought only as pigments but recent literature showed they can also deter insects and pathogens. A key difference of flavonoids from terpenoids is that they are soluble in water and stored in vacuoles. The allelopathic properties of flavonoids are expected to be confirmed in the near future. Tannins are a special group of flavonoids as they can bond to and precipitate protein and are also involved in deterring insects and pathogens. Phenolic acids are often precursors in flavonoids synthesis and soluble in water. Literature shows they are active to deter insects and pathogen and also allelopathic to suppress weeds.
Traditionally, all the three classes of cotton secondary metabolites terpenoids (gossypol), flavonoids, including tannins and phenolic acids, are integrated in cotton screening and breeding programs. Recent progresses have expanded their application in cotton production; direct extraction of the secondary metabolites and application; use of chemical regulators or elicitors of cotton secondary metabolites to change their concentrations in situ to achieve the functions of the target secondary metabolites, and change of the genetic bases of the secondary metabolites to produce or not to produce one or a set of secondary metabolites in crop plants in situ to achieve some benefits, which include reduction of the toxic gossypol in cotton seeds to increase their value in utilizing their oil and protein.
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Written By
Ziming Yue, Te-Ming Tseng, K. Raja Reddy, Natraj Krishnan and Shien Lu
Submitted: 30 October 2023Reviewed: 12 December 2023Published: 06 February 2024
Open access peer-reviewed chapter
