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Phenolic compounds are produced by the plants mainly for their growth, development, and protection. These aromatic benzene ring compounds are very much essential during the plant’s biotic and abiotic stress interactions. They constitute an essential part of plant’s secondary metabolites and play a vital role in various physiological and mechanical activities. These diverse plant phenolic compounds act both as attractants and repellents toward various organisms in the environment. They could act as attractants toward the beneficial organisms and as toxicants against the invading pests and pathogens. These metabolite compounds often enhance during a plethora of stress conditions and act as the first line of defense to provide plant disease resistance. They are also known to influence the other plant metabolic pathways, namely phytoalexin biosynthesis and reactive oxygen species generation. These phenolic compounds participate both in the above- and below-ground plant defense systems. They are produced as root exudates and influence the soil diversity and the neighboring plants. The present review provides an overview of the roles of plant phenolic compounds in the plant kingdom as signaling compounds, pigment compounds, antimicrobials, and defense compounds.
CSIR-Indian Institute of Chemical Technology, India
International Crops Research for Semi-Arid Tropics (ICRISAT), India
*Address all correspondence to: s.pratyusha@cgiar.org
1. Introduction
Plants produce an amazing diversity of secondary metabolites, and the most important ones are the phenolic compounds. They are the most stable products in the plant kingdom. Humans have known them for centuries, and their role in plants’ nutrition, fertility, growth, and protection has made them compounds of interest and to understand them completely. These anti-herbivore chemicals produced by plants are one of the most common plant allelochemicals in the ecosystem. These phenolic compounds are characterized by single or more hydroxyl groups bound to a six-carbon aromatic ring. These compounds attained the leading status due to the resistant properties bestowed by them [1]. More than 8000 phenolic structures are currently known, ranging from simple phenolic acids to highly polymerized substances as tannins [2]. They are the most abundant secondary metabolites with wide distribution in the plant kingdom.
Primarily these phenolic compounds are usually involved in the plant defense responses and apart from that, they are were also seen playing a role during crop pollination and camouflage [3, 4]. These compounds are mostly found bound to the sugars. Most of them being aromatic in nature also play an important role in plant communication. At the plant’s rhizosphere, certain phenolic compounds monitor their surroundings through quorum sensing [5]. The plant growth-promoting microbes at the rhizosphere breakdown these phenolics and, in turn, enhance the soil fertility. They also aid in the chelation of the soil minerals and elements, improve the soil porosity, and in turn increase the absorption capacity of the plants [6]. During stress and pathogen invasion, these phenolic compounds are often accumulated in the plant’s subepidermal tissues. The synthesis and concentration of the accumulated phenolics depend on many internal and external factors such as plant physiology, age, development stage, climate, and the type of pathogen attack [7]. The significant nature of the phenolic compounds is their dual function as both attractants and repellent compounds. Depending on the surrounding environment, the plant produces either the attractants phenolic derivatives, namely allelochemicals and chemoattractants, to attract the pollinators, symbiotic microbes [8, 9], or repellent phenolic derivatives to repel the pests and pathogens [10].
Due to present-day environmental challenges, there is a need for eco-friendly agricultural practices, and to ensure the future demand for food, exploitation of sustainable solutions is very much necessary. This brings us to the attention of plant phenolic compounds with diverse beneficial roles as plant growth promoters, crop yield enhancers, and as protectors against varied environmental stresses. Hence, the present chapter explores the diversity of plant phenolic compounds and their role in plant development and defense toward their application in various fields of agriculture.
Classification of phenolic compounds based on the number of carbons.
C6-Simple phenols and benzoquinones are single benzene ringed structures with certain medicinal benefits. For example, embelin is a plant benzoquinone with antispermatogenic effect isolated from seeds of Embelia ribes [11].
C6–C1-Phenolic acids are those compounds possessing one carboxylic acid functional group. These naturally occurring phenolic acids contain two distinctive carbon frameworks, namely the hydroxycinnamic acid and hydroxybenzoic structures. The basic structure remains the same, but the number and position of hydroxyl groups differ between the two. Phenolic acids with hydroxycinnamic acid include cinnamic acid, coumaric acid, ferulic acid, sinapic acid, and caffeic acid. Structures with hydroxybenzoic acid include benzoic acid, p-hydroxybenzoic acid, vanillic acid, gallic acid, protocatechuic acid, syringic acid, gentisic acid, veratric acid, and salicylic acid [12]. C6–C2-Acetophenone is a naturally occurring phenol compound in apple, cheese, apricot, beef, and cauliflower. It is used in fragrances and chewing gum. Phenylacetic acid is an active auxin, a plant hormone found in fruits [13]. C6–C3-Coumarins are notably in high concentration in Dipteryx odorata [14] and produced by plants as a defense chemical to discourage predation. They are widely spread in the grasses and cloves. C6–C4-Naphthoquinones such as 2-hydroxynapthoquinone and naphthazarin show insecticidal activity against tobacco culture insects extracted from Calceolaria andina [15]. Derivatives of 1,4-naphthoquinone are known to possess antibacterial and antitumor properties. Naphthoquinones also exhibit larvicidal and molluscicidal activities. They are effective against Aedes aegypti and Biomphalaria glabrata [16].
C6–C1–C6-Xanthones are present in Bonnetiaceae and Clusiaceae families. They are generally used as an insecticide and as ovicide for codling moth eggs [17]. C6–C2–C6-Stilbenes on hydroxylation from stilbenoids acts as phytoalexins in the plant. Commonly found plant compounds with stilbene structures are trans-resveratrol, trans-piceid, pinosylvin, piceatannol, pinosylvin, trans-pterostilbene, astringin, and rhapontin [18]. Anthraquinones generally present in plants as glycosides. C6–C3–C6-Flavonoids generally occur in plants as glycosylated derivatives. The basic flavonoid structure contains a flavan nucleus. Many classes of flavonoids are present such as flavones (apigenin, luteolin, chrysin), flavan-3-ols (catechin, epicatechin, epigallocatechin), flavanones (naringenin, naringin, hesperetin, hesperidin), flavonols (quercetin, kaempferol, galangin, fisetin, myricetin), flavanonol (taxifolin), isoflavones (genistein, genistin, daidzein, daidzin, ononin), and anthocyanidins (cyanidin, cyanin, peonidin, delphinidin, pelargonidin, and malvidin) [12].
(C6–C3)2-Lignans are phytonutrients with antioxidant property. Examples include pinoresinol, podophyllotoxin, and steganacin. Flax and sesame seeds contain high levels of lignans. (C6–C3–C6)2-Biflavonoids formed through an oxidative coupling of two chalcone units and subsequent modification of the central C3 units. They are characteristic of gymnosperms, the Psilotales, Selaginallales, and several flowering plants. They are not found in Pinaceae or the Gnetales. These biflavonoids act as fungitoxins and insect feeding deterrents [19]. (C6–C3)n-Lignin and tannins are polymer forms of phenolic compounds. Lignin is the largest source of phenolic material in the plant cell walls. Tannins are distributed in plants as ellagitannins, condensed tannins, and gallotannins. These tannins reduce the digestibility of plants by herbivores.
The most crucial property of phenols is their antioxidant capacity. It protects the organism against reactive oxygen species (ROS). Plant polyphenols have multifunctional properties such as reducing agents, hydrogen-donating antioxidants, and singlet oxygen quenchers and flavonoids.
Phenolic compounds exist throughout the plant kingdom, and their presence varies according to the phylum. Bryophytes are the regular producers of polyphenols, including flavonoids, but it is in the vascular plants that the full range of polyphenols was found. These phenolics survived through natural selection and upgraded through ages in types and functions. The taxonomists often use them to classify and separate the plant species. Plants synthesize these phenolic compounds unlike animals because plants are stationary to escape their predators and therefore have evolved this chemical defense against predators. The primary established roles of plant phenolics are ecological, some having dual or even multiple functions (Figure 1). Several studies have indicated a high degree of compartmentation of phenolic compounds and of the enzymes involved in their biosynthesis that occurs through various pathways [20].
Figure 1.
Overview of functions of plant phenolic compounds.
These phenolics are widely distributed in the plant. They usually accumulate in the central vacuoles of guard cells and epidermal cells and subepidermal cells of leaves and shoots. Some found covalently linked to the plant cell wall, and others occur in waxes or on the external surfaces of plant organs. According to some findings, the deposition of flavonoids in nuclei is seen in certain tree species [21]. And it leads to a flavonoid-DNA complex that provides mutual protection against oxidative damage. In plants, phenolics were generally produced during two scenarios such as: (1) preformed phenolics synthesized during the normal development of plant tissues and (2) induced phenolics synthesized by plants in response to physical injury, infection, and response to elicitors such as heavy metal salts, UV irradiation, temperature, etc. [22].
3.1 Role of phenols in plant growth
Majority of phenols are responsible for the growth of the plant by aiding in cell wall formation. Hydroxycinnamic acids, particularly p-coumaric acid and ferulic acid, were found in the insoluble or cell wall fraction as esters. These pools of wall-bound phenolic acids act as a reservoir of phenylpropanoid units for lignin biosynthesis or even that they represent the beginnings of lignification itself. These esters with a large population of bound molecules are responsible for transduction of light energy leading to changes in plant cell wall structure, water flux, turgor pressure, and growth. Auxin (indole acetic acid), a phytohormone, plays a major role in the growth regulation of the plant [23, 24].
Plants are generally rooted and unable to move from place to another and are known to move in certain ways. The circadian rhythmic leaf movement known as nyctinasty was observed in all leguminous plants. Nyctinastic leaf movement is induced by the swelling and shrinking of motor cells in the pulvini, an organ located in the joint of the leaf and believed to be controlled by Schildknecht’s turgorins, which induce leaf-closing movement of the plants [25]. These turgorins are a new class of phytohormones that regulates the turgor of the plants. Some identified phenolic turgorins are: gallic acid 4-O-(β-D-glucopyranosyl-6′-sulfate) and gentisic acid 5-O-β-D-glucopyranoside that are pulvini-localized in Mimosa pudica L., cis-p-coumaric acid 4-O-β-D-glucopyranoside, found in Cassia mimosoides L., and cis-p-coumaroylagmatine, identified in Albizia julibrissin Durazz. Hence, phenols also aid in the movements of plants [26]. In rapidly germinating seeds, coumaric acid β-glucoside is more prevalent, and in non-germinating seeds, such as Melilotus alba found to possess a large number of free coumarins [27]. Another naturally occurring phenolic compound, which inhibits the germination of seeds, is ferulic acid. These phenolics act as germination inhibitors by inhibiting the transport of amino acids and forming of the proteins in the seeds [28].
3.2 Role of phenols in plant signaling
Allelochemicals are known to interact between two plants. Phenolic compounds influence many organic and inorganic nutrients surrounding them. They affect decomposition rate and play a role in nutrient cycle by inhibiting or stimulating spore germination. Phenols are good allelochemicals present in all parts of the plant. Leaf phenolic allelochemicals include p-hydroxybenzoic acid, p-coumaric acid, and bark, rhizosphere, root exudates including quercetin, juglone, catechin, and sorgoleone compounds [29]. Polyphenols stored in the vacuoles of plant encounter the cytoplasmatic proteins and form polyphenols-protein complex [30]. This complex aids in the senescence of plant tissues and causes the brown color of senescent leaves. Flavonoids such as eriodictyol and apigenin-7-O-glucoside isolated from pea root exudates found to play a role in the induction of nod gene expression [31]. Other natural flavanones such as naringenin, hesperitin, chalcones, and isoflavonoids such as daidzein, genistein released from legume plants inducing the nod gene expression. Phenols, in major flavonoids, are responsible for the pigmentation of flowers and fruits in plants and aid in the pollination and seed dispersal. For example, apigenin, luteolin, kaempferol, quercetin, and myricetin produce white, yellow, or ivory colors at their locations in plants [32].
Among the chemical defensive strategies developed by the plant, phenolics generated due to insect herbivory play a significant role in plant resistance and controlling the herbivore damage. Many studies were performed to determine the qualitative and quantitative changes of phenolic compounds in plants and their influence on insects. Studies were conducted on rice plants infested with pests, and an elevated level of phenolic compounds was observed. The elevation was explained as a mechanism of defense that acts as a barrier to insect feeding [33, 34]. Interesting results were obtained in phenolic acid estimations by HPLC in the pest-infested plants. There was an enhancement in the level of phenolic acids in all the groundnut plants infested with the three pests compared with control plants. Also, a quantitative difference in phenolic acids was noted in the infested groundnut plants irrespective of the type of feeding damage [35]. The accumulation of the phenolic compounds by the phenylpropanoid pathway has been reported earlier [36]. Certain phenolic acids such as cinnamic acid derivatives, cinnamic acid, vanillic acid, syringic acid, and p-coumaric acid were found only in the pest-infested rice plants. They were totally absent in normal healthy plants. Similar results were observed with raise in the concentration of phenolic acids such as vanillic acid, syringic acid, cinnamic acid, and p-coumaric acid in the infested rice plants [37, 38] and cotton [39]. Kelly and Felton [40] and Rehman et al. [41] found that increased concentration in plant phenolic compounds is according to the extent of tissue damaged by feeding insects or due to pathogen infection.
Insect damage often alters plant physiology and chemistry. Larvae of the autumnal moth, Epirrita autumnata, on individual branches of its primary host plant, mountain birch, Betula pubescens did not lead to a systemic change of primary nutrients and phenol compounds. However, they affected the larval growth [42]. Changes in phloem phenols occur when pest infestation is seen on the bark of the trees. Phytophthora ramorum, which caused cankers on the oak trees, is analyzed for the phenolic levels against the uninfested oak tree. Ockels et al. [43], quantified nine phenolic compounds and gallic acid, tyrosol levels and showed dose-dependent inhibitory effects against P. ramorum, P. cinnamomi, Pseudocaecilius citricola, and P. citrophthora that are tested through in vitro bioassays. Seeds infested by wheat midge larvae, Sitodiplosis mosellana, showed induced changes in the dynamics of the phenolic acids. Analysis by HPLC of seed extracts produced by alkaline hydrolysis revealed rapid changes in p-coumaric and ferulic acids levels during early seed development [44]. This notified the role of phenols in seed development. Bi et al. [45] concluded that changes in the plant chemicals would induce resistance in the plant. Leaf phenolics and alkaloids variations were seen when Coffea spp. infested by leaf miner Leucoptera coffeella. The insect-plant interactions are studied by Magalhães et al. [46], to determine the pesticide activity of the plant phenols. So, sometimes the phenolic changes in specific insect-plant interaction will affect the other generalist insects of the plant.
4.2 Microbe-plant interactions
Studies indicated that microbial infection on the plant alters the plant’s chemical composition. First identified phenolics providing disease resistance were seen in onion scales infected by Colletotrichum circinans. In order, to prevent this onion smudge disease, plant accumulated sufficient amounts of catechol and protocatechuic acid [47]. A decrease in the phenolic content of the plant was observed in brown spot infection of rice due to infection of Helminthosporium oryzae [48, 49]. Infection suppressed the phenol metabolism in the rice plant due to the Helminthosporium oryzae toxin and aided in pathogen colonization. Phenolic compounds are also involved in defense response of plants by reducing the incidence of Fusarium wilt of tomatoes caused by the fungus Fusarium oxysporum [50]. Alteration in the ferulic, caffeic, and vanillic acid contents and concentrations are identified from recovered leaves and roots. Elicitors from Fusarium oxysporum f.sp. cubense accumulated soluble and wall-bound phenolics and phenolic polymers in the roots of Musa acuminate. White mold fungus, Sclerotium rolfsii Saccodes infection to Arachis hypogea reduced the total soluble phenolic content [51]. However, generally phenolic compounds induce in the infected fungal plants to confer resistance to specific fungal pathogens.
According to Beckman, phenolic compounds play an important role in reducing wilt diseases and aid in signaling for the host defense responses. 4-hydroxycinnamic acid CoA ligase enzyme is vital in the diversion of phenylpropanoids, was altered by the fungi, responsible for the changes in the phenols of the infected plant. In response to Rhizobium and vesicular arbuscular mycorrhizal (VAM) inoculation, enhancement in the phenolic compounds is seen in the Arachis hypogaea roots [52]. These phenolic compounds released at the roots help in maintaining the Rhizobium community at the rhizosphere. Furthermore, these Rhizobia species can use the phenols as a carbon source. Bacterial pathogens also affect phenol accumulation and production. Pseudomonas syringae enhanced the extracellular phenolic accumulation and changed the composition of phenolic acids in the Nicotiana tabacum [53].
Antimicrobial activity of phenolic compounds was observed in Finnish berries against probiotic bacteria and other intestinal bacteria. Myricetin inhibited the growth of lactic acid bacteria and mostly Gram-negative bacteria [54]. Phenolic acids from Olea europaea leaves, namely caffeic acid, verbascoside, oleuropein, luteolin 7-O-glucoside, rutin, apigenin 7-O-glucoside, and luteolin 4′-O-glucoside showed antibacterial and antifungal action. Many herbs and spice plant extracts contain phenols with antibacterial activity against food-borne pathogens [55]. Flavones and flavanones of fruits and vegetables are known to be active against Aspergillus sp., B. cinerea, and F. oxysporum [56]. Resveratrol from grapes is also known to possess antibacterial activity [57]. In these host-microbe interactions, the phenolic metabolites play a key role in providing signals for the interactions [58]. For example, acetosyringone, a phenolic compound produced at the wound site of plants, triggers vir genes in the pathogen. In legume-rhizobial interactions, flavonoids activate the nod genes in Rhizobium responsible for symbiotic relation [59]. The roles played by these phenolic compounds generally include phytoalexins for disease defense and lignin production for structural strength, along with antioxidant nature to combat the pro-oxidants produced during microbial stress.
4.3 Effect of plant phenols on pests
Some plants respond to herbivore damage by increasing chemical, physical, or biotic defenses and responses that can help protect the remaining tissue against further damage [60, 61, 62]. Plant phenolics are believed to play an important role in chemical defense against herbivores through specific physiological effects on insects. These phenols are often described as antifeedant, digestibility reducers, and as toxins. These phenols would promote ROS in the insect digestive tract, particularly in mid-gut, where pH is alkaline. These ROS would result in direct oxidative damage to mid-gut lipids and proteins. Elevated levels of lipid per-oxidation products, oxidized protein, and free ions due to oxidative stress are generated in mid-guts of insects, leading to death [2]. Phytoalexins will disrupt the pathogen metabolism and cellular structure. Some experimental evidence includes the medicarpin by Medicago sativa, rishitin by Solanaceae, and camalexin by Arabidopsis thaliana [63]. Tannins lead to protein inactivation in insects, by binding to salivary proteins and digestive enzymes, including trypsin and chymotrypsin. Insect herbivores that ingest high amounts of tannins fail to gain weight and eventually die [64]. Lignins, which are polymer in nature, provide strong physical barrier in the form of cell walls toward herbivores. On the other hand, furanocoumarins produced in response to the herbivore attack gets activated by ultraviolet light (UV) and integrates with the DNA of insects, leading to death [65].
Plants encounter many challenges of both biotic and abiotic stress factors in nature. Abiotic stress factors include, namely drought, salinity, heat, ultraviolet, and pesticides. Nowadays, these abiotic stresses toward plants have drastically increased due to the environment’s uneven climatic conditions and man-made pollution. The increase of these abiotic stresses will radically impact the growth and development of the plants and could reduce the overall crop yield [66]. Plants to combat these abiotic stress conditions will produce a plethora of defense metabolites [67, 68]. Among them, the plant phenolic compounds are playing a vital role in coping with the abiotic stresses. Under stressful conditions, these phenolics are drastically accumulated in the plant for survival [20, 69]. Phenolic compounds, namely esters, flavonoids, lignin, and tannins, act as antioxidants and fight against these abiotic stress conditions in the plant cells [70].
In certain leafy vegetables, the salinity conditions caused an increase in the phenolic compounds to counterattack the high salt levels in the soil [71]. This increase of phenols assists in the balancing of the mineral composition in the leaves. Heavy metals in the soil also dramatically impact the physiology and metabolic activities of the plant. During such stress conditions, it has been observed that the plant flavonoids are playing a vital defense role by chelating the heavy metals [72, 73]. Climate change is one of the significant factors affecting plant growth and development. Due to adverse climatic conditions, water stress in plants has become a serious concern. Due to the lack of rainfall, drought stress is common environmental stress in many cultivated areas, and crop yield is majorly dependent on it. Under drought stress, it has been observed that plants are producing polyphenols to cope with these stress conditions by controlling the water ions flux [74, 75]. Phenolic compounds also respond to nutrition stress, cold stress, and radiation, thus providing resistance to the plant [76, 77, 78]. Salinity is a significant stress factor that limits crop yield in many areas. Under these extreme conditions, plants adapt to stress through altered metabolic pathways (Figure 2). For example, in a medicinal plant, Salvia mirzayanii, total phenolic content was increased with higher salt levels [79]. Chlorogenic acid, caffeic acid, ellagic acid, ferulic acid, gallic acid, syringic acid, vanillic acid, and p-coumaric acid were enhanced in Aegilops spp. due to salinity conditions [80]. This trend in increase of phenolic contents was reported in many plants under salt stress [81, 82]. This increase of phenols is a tolerance mechanism to maintain redox homeostasis and improve plant health. Similarly, during drought stress, to avoid oxidative damage, plants produce various phenolic compounds. These flavonoids or phenols inhibit further loss of water in the plants through the closure of stomata [83]. Many reports support the production mechanism of phenols during drought stress in plants [84, 85]. The plant phenols are the main accumulator compounds during heavy metal stress. This tendency is for the chelation of toxic metals by phenolic compounds through their carboxyl and hydroxyl groups, which participate in the chelation of the metals [78]. Plant hormones play a vital role during the stress conditions. A wide cross talk of hormones, namely salicylic acid and jasmonic acid, takes place as defense response to the stresses [86]. Proteins associated with these hormones upregulate during defense and provide immunity to the plants through expression of pathogenesis-related genes [87]. Abiotic stresses significantly alter the crop quality and productivity, and to combat it, expression of resistance genes takes place and elevates the levels of defense compounds such as phenolic compounds [88]. These enhanced phenols ensure their endurance and survival of plants during these abiotic stress conditions.
Figure 2.
Mechanistic approach of plant responses to stress conditions.
Phenolic substances are the most resistant metabolites produced by plants. Better understanding of plant phenolics is essential, due to its wide array of functions in the plant development, and its practical applications in many streams such as agriculture, medicine, nutrition, pesticide management, and industry. These phenolic changes in the plant with respect to the herbivory correlated negatively with the larval growth, development, and survival of progeny. The enhanced phenols in plants behave as toxins toward insect feeding, microbial growth, and a mode of induced defense generated by the plant to defend against natural pests. We can summarize that the plant-insect interactions altered the phenolic levels to the pest attacking. So, a change in the phenol level is a defense strategy developed to combat the pest. This change of phytochemical composition from nature will oppose the pest from invading the plant. During abiotic stress also, the plants can produce phenols as tolerance mechanism to cope with the unfavorable conditions. Increased biosynthesis of plant phenols was observed in the plant during abiotic stress factors such as drought, heavy metal stress, salinity, and radiation. In conclusion, this review on the plant phenolic compounds and their role in the plant’s growth, development, and defense will provide the information to understand the plant mechanisms and aid us in further effective application of them in agricultural pest management strategies.
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Written By
Sambangi Pratyusha
Submitted: 19 November 2021Reviewed: 26 January 2022Published: 15 March 2022
Open access peer-reviewed chapter
