Types of plant secondary metabolites.
Medicinal plants are widely used worldwide to treat various diseases. Its widespread use is due in part to the cultural acceptance of traditional medicine in different regions of the world, as well as its effectiveness in treating various diseases. Many of its active substances or secondary metabolites are formed to a response of various situations that generate stress in their habitat, such as sudden changes in environmental temperature, humidity, rain, drought, and infections by phytopathogens (fungi, bacteria, viruses, nematodes, protozoa). The production of these secondary metabolites is a mechanism of defense of plants. In this context, the objective of this chapter is to study the secondary metabolites of medicinal plants that could have a promising application in the control of different phytopathogens in crops of agricultural and economic interest.
- medicinal plants
- secondary metabolites
- biotic and abiotic elicitors
Phytopathogens generally attack plants during their growth, causing alterations in their cellular metabolism and/or interfering with the absorption of nutrients . The crops of cereals, vegetables, and fruits are affected by these organisms during harvest and postharvest . However, one of the main control measures to eradicate phytopathogens is the use of pesticides. Although they are effective, easy to access, and easy to use, they have several disadvantages, generate resistance, and are considered toxic substances, not only for bacteria, fungi, viruses, protozoa, and nematodes but also for the humans, animals, and the environment [3, 4]. In this context, the pesticides can induce acute and chronic toxicity, to persist in the environment and pollute soil and water. So, they are easily incorporated into the food chain, bioaccumulation, and biomagnification . Regarding their toxicity mechanisms, it has been described that they can act as endocrine disruptors and as reactive species that generate oxidative stress in the cell [6, 7, 8, 9].
On the other hand, the study of medicinal plants as possible natural sources of obtaining active compound (secondary metabolites) against phytopathogens has gained increasing interest in recent years, due to several aspects, mainly that they are obtained from a natural source through the production or synthesis of secondary metabolites considered as nontoxic such as phenols, flavonoids, terpenes, alkaloids, etc. [10, 11, 12, 13]. Another advantage is that phytopathogens still do not develop resistance to the antifungal, antimicrobial, and nematicide effect of the phytochemical compounds produced by some medicinal plants. When carrying out an exhaustive search in the literature, it was found that the potential use of the secondary metabolites obtained from medicinal plant extracts is fungicide [14, 15, 16]. Most of the research in this area focuses on evaluating the effects of these active compounds on fungi such as
2. Pesticides in the control of phytopathogens
In the market, there are a variety of pesticides that are used alone or in combination to eradicate, control, or prevent pests . Pesticides can be classified according to the chemical group to which they belong, to their selectivity toward a certain phytopathogen, its mechanism of action, and its use or application. However, the most widely used for their effectiveness and a broad spectrum of activity against various pests and diseases in plants are insecticides, herbicides, and fungicides [4, 19].
Pesticides used in agriculture mainly contaminate the soil by direct application and water by leaching, and it is very easy for them to be present either in trace quantities or high in food and to enter the food chain, which facilitates its accumulation and biomagnification [5, 20]. In general pesticides are considered dangerous substances for living beings since they can produce acute or chronic toxicity; however the magnitude of the poisoning depends on several aspects to be considered such as the physicochemical characteristics of the pesticide, the concentration, the exposure time, the route of entry to organisms, their toxicodynamics and toxicokinetics (absorption, distribution, half-life, metabolism, and elimination), as well as the use of mixtures of different pesticides, the components of their formulation, and the general state of health of the individual [21, 22]. All these aspects influence that pesticides represent a risk or danger for those who use them in the fields of cultivation, as well as for those who consume foods that contain substances in trace quantities in prolonged consumption.
Regarding its toxicity, it has been described that pesticides act as endocrine disruptors and generators of free radicals and enzymatic inhibitors [8, 9]. Unfortunately, the cellular targets to which most of these pesticides are directed coincide with cellular targets that are also present in man, such as the case of the mechanisms of action of organophosphorus insecticides, which inhibit the activity of acetylcholinesterase enzyme present in different insects; unfortunately man and other mammals also have acetylcholinesterase, so their toxicity is not selective toward the pests that they wish to control, but they also affect man, and depending on the magnitude of the poisoning, they can cause death [19, 20, 21, 22]. However, until today an ideal pesticide does not exist, and the correct use of herbicides, fungicides, insecticides, etc. has many benefits to control plagues and increase the yield of the crops .
3. Secondary metabolites of medicinal plants as biological control of phytopathogens
There are several methods of biological control against phytopathogens. The use of extracts of medicinal plants to eradicate diseases in crops caused mainly by viruses, bacteria, and fungi is one of them . The above makes sense if we analyze the fact that plants have mechanisms to protect themselves from both biotic and abiotic stress agents. That is, if the phytopathogens (biotic agents) are attacking the plants, why not think what the plant does to defend itself?
In this context, it is interesting to analyze the secondary metabolism of plants know which phytochemical substances are produced and what biological activity they present.
3.1 The bioactive potential of secondary metabolites derived from the medicinal plant
Plants are formed by a primary metabolism that is responsible for the physiological processes and development of the plant, such as lipids, carbohydrates, and proteins . The secondary metabolism is not essential in the basic processes of plants. However, these bioactive compounds play an important role in the defense of plants, and these secondary metabolites can be classified as phenolic compounds, carotenoids, terpenes, alkaloids, and sulfur compounds, among others, as shown in Table 1 .
Phenolic compounds are aromatic substances formed during the passage of the shikimic acid pathway or mainly the mevalonic pathway. These can be divided into insoluble compounds such as condensed tannins, lignins, and hydroxamic acids bound to the cell walls, and soluble compounds are phenolic acids, flavonoids, and kinases . Carotenoids are lipophilic molecules and are found in plants giving orange tones. The importance of these compounds is the intervention they have in photosynthesis, and they also protect the photosynthetic apparatus from excess energy . The carotenoid contents in plants are affected by various factors, such as plant development, stress conditions, postharvest conditions, or cooking treatments, but the interest of these compounds has been increasing due to their potential antioxidant activity . Terpenes are lipid-soluble compounds that include one- or more five-carbon isoprene units, which are synthesized by all organisms through two pathways, mevalonate and deoxy-D-xylulose . Terpenoids are classified according to the number of isoprene units they contain; terpenes and terpenoids are basic constituents of many types of plant essential oils . Alkaloids are bioactive compounds that generally contain nitrogen derived from an amino acid of great importance because it has physiological and medicinal properties, for example, caffeine, nicotine, morphine, atropine, and quinine .
Now well, all these compounds mentioned above help the plants to develop complex defense systems against different types of stress for the survival or the systematic forces in their metabolism for resistance against pests and diseases. Stress provoked in the plant involves several signaling response pathways for pathogens and insects, and some of these response pathways are induced by the microorganisms themselves. Also, the plants have specific recognition and signaling systems allowing them to detect the pathogens and initiate an effective defense response [30, 31]. The defence system broadest have the plants against pathogens are the phenolic compounds (phenylpropanoids and flavonoids). These substances have different mechanisms of action they can dissociate the ions of the phenolic hydroxyl and forming phenolates, ionic and hydrogen bonds with peptides and proteins causing a high astringency and protein denaturation. In the other hand, they interfere with the pathogen's cell signalling compounds and affect their physiological activities through enzymatic inhibition, DNA alkylation and altering their reproductive system . The compounds with allelopathic effects affect positively or negatively on the ecosystem’s structure to remove or eliminate microorganisms from the plants. Some phenolic compounds are allelochemicals that have been shown to have an activity as antibiotics, antifungals, and antipredator . Phenolic acids, such as benzoic, hydroxybenzoic, vanillic, and caffeic, have antimicrobial and antifungal properties produced by the inhibition of enzymes. Caffeic, chlorogenic, sinapic, ferulic, and p-coumaric acids have antioxidant activity by the inhibition of oxidation of lipids and the elimination of reactive oxygen species. These effects are important to the plant defense .
3.2 Improving production of plant secondary metabolites through biotic and abiotic stresses
Classification of secondary metabolites related to the defense of plants is commonly used in the form of synthesis and accumulation of phytochemicals with interaction effect of the pathogenic plant against plant insect, virus, fungi, and antibacterial compounds. For example, phytoalexins are produced very quickly after infection of a pathogen producing toxicity to an ambiguous environment of fungi or bacteria [33, 34].
Phenylpropanoids and flavonoids have hydroxyl groups that contain phenolic compounds, which dissociate into phenolate ions, and the phenolic hydroxyl groups form ionic bonds and hydrogen bonds with peptides and protons, producing a high astringency and denaturation that thus show an antifungal effect acting together with cellular signaling compounds and physiological activities or acting on the parts of the pathogen, reproductive system, enzymatic inhibition, etc. . The properties of the proteins change with any change in protein conformation, for example, by changing the three-dimensional structure forming covalent bonds with SH, OH or free amino groups there is inactivation or protein function loss. When polyphenols of the plants bind to some proteins of phytopathogens are less toxic for them but can protect the plant of abiotic elicitors . On the other hand, phytoalexins are induced against the attack of microbes and insects activated by β-glucosidase by the release of biocidal aglycones . In the same way act the benzoxazinoids (BX), these phytochemical compounds are produced and released by tissue damage and hydrolysis by β-glucosidase and act as insect repellents too .
At present, several biotechnological strategies have been used to increase the productivity of secondary metabolites, using different inducers of secondary metabolites such as at the cellular, organic, and plant levels, as well as the most effective methods to improve the synthesis of these secondary metabolites in endemic and medicinal plants . These secondary metabolites accumulate in plants when they are prone to various stress types, inducers, or signal molecules. Thus, there are different modulating factors of secondary metabolites, as well as microbial, physical, or chemical effects such as abiotic or biotic elicitors, inducing the biosynthesis of specific compound that plays an important role in the adaptations of plants to stress conditions, and these phenomena cause a greater synthesis and accumulation of secondary metabolites . In Table 2 the authors focus on the abiotic elicitors that are substances of biological origin such as proteins and carbohydrates that are initiator compounds or coupling responses at the cellular level activating several enzymes or signaling canals. There are also microorganisms and chemical compounds with elicitor effect that stress the plant and produce the expression of a greater amount of metabolites or new metabolites which cause physiological changes in the plant against pathogens. As shown in Table 2, glycoprotein-type proteins produce phytoalexins that have been used to identify ion channels in cell membranes and thus transfer signals by external stimuli, as demonstrated by Alami  where the
On the other hand, Table 3 shows some research that has the influence of different abiotic elicitors that are considered substance and that are not of biological origin such as salt, drought, light or heavy metals, and temperature, among others. Table 3 shows different perspectives of research on medicinal or aromatic plants in hydroponic crops, outdoors, and the application of elicitors in different stages of growth or postharvest. For example, heavy metals such as Al3+, Cr3+, Co2+, Ni2+, Cu2+, Zn2+, and Cd2+, among others, are considered high toxicity compounds depending on the concentrations applied in the sprinkler system or because they are used as biocontrol since they alter the production of metabolites in plants. Similarly, Zobayed  demonstrated that the temperature in high concentrations in
Recent studies focused on evaluating the secondary metabolites of medicinal plants that are active against phytopathogens show that the potential use that these compounds can have in the future is for the control of phytopathogenic fungi, mainly against different species of
Finally, in the realization of a retrospective of the secondary metabolite modulating factors in our workgroup, Garcia-Mier  demonstrated that the use of mixtures of elicitors such as jasmonic acid, hydrogen peroxide, and chitosan in different concentrations applied in various stages of plant development of the sweet bell red pepper and in different stages of ripening of the fruit has a positive effect on the increase of polyphenolic and carotenoid compounds, where the results showed that the maturation stage of 95% produces a greater quantity of bioactive compounds. On the other hand, Vargas-Hernández  demonstrated that the foliar application of hydrogen peroxide in
The phytochemicals that produce medicinal plants derived from their secondary metabolism represent a safe and effective alternative to control various phytopathogens that affect various crops of agricultural products of economic and nutritional interest. There are different challenges in the use of biopesticides obtained from medicinal plants, such as evaluating the costs of obtaining these compounds on a large scale or exploring the possibility of them being obtained through chemical synthesis to increase yield and reduce costs. On the other hand, the various studies that exist on the effectiveness of these compounds are only at the laboratory level, which is why it is still necessary to explore and evaluate their effectiveness at the greenhouse and field levels.
The authors would like to acknowledge the University of Guanajuato for the grant of this publication.
Conflict of interest
The authors declare no conflict of interest.