**2. Defense against pathogens with secondary metabolism**

Knowledge of how these molecules affect the exploitation of natural materials is often followed by an understanding of the role of metabolism in the producing organism. In plants, well-understood secondary metabolism is involved in pathogen protection or perception and signaling. In terms of pathogen protection, fungal diseases pose a major threat to plant health, with estimates below of 13,000 phytopathogenic fungal species in the United States alone. Therefore, it is not surprising that plants have developed comprehensive protection mechanisms against fungal pathogens, with chemical protection being one of the key weapons in the plant arsenal [5]. Although thousands of different molecular companies are believed to play a role in plant protection against bacterial and fungal pathogens, the mechanism of action of relatively few has been the subject of extensive research.

#### *Biological Activity of Defence-Related Plant Secondary Metabolites DOI: http://dx.doi.org/10.5772/intechopen.101379*

Plant defense molecules may be pre-formed in plant tissues (**Figure 1**) or synthesized in response to the pathogenic attack, resulting in variations leading to the terms phyto antiseptics and phytoalexins, respectively [6]. This difference does not provide any specific information about the chemical structure or mechanism of metabolism and in some cases, misleading defense molecules are pre-manufactured but concentrated in high concentrations at the site of infection are reasonably considered to be phyto antiseptics or phytoalexins. In practice, when studying the range of possible biological functions involved in metabolism, the chemical structure of natural products is more relevant than the exact time produced at the plant.

## **3. The role of secondary metabolites in plant-microbial signaling**

In terms of signal, the most comprehensible metabolites are flavonoids involved in symbiotic lentil-rhizobia interactions that lead to the formation of nitrogen-fixing nodules in root tissues [7]. Collectively, plants produce more than 5000 different flavonoids, with only a small subgroup involved in specific interactions with Rhizobia. This interaction begins with the secretion of signal flavonoids at the root exudates, followed by the bacterial understanding of the signal and direct contact with the bacterial nodule transcriptional activator. This triggers a series of events that create convenient rhizobial infection of the plant root and nitrogen-fixing nodules.

The other major beneficial plant-microbe interaction that occurs in nature is the formation of mycorrhizal roots. Once again, there is a facilitated infection of plant roots, this time by arbuscular mycorrhiza fungi, which develop specialized structures called arbuscles within the root for nutrient exchange between the plant and fungus. Although a role for signaling has long been postulated, it is only in recent years that the first experimental evidence demonstrating a role for a plant chemical has been obtained, showing that a particular class of sesquiterpene, the strigolactones, can induce hyphal branching, an important step in the symbiosis [8].

As an added twist, several studies have shown that certain strigolactones actually play a role in regulating plant hormones and spruce branches in the plant, thus regulating processes above and below the ground [9].

### **4. Allelopathic reactions**

Allelopathy is defined as the inhibition of the growth of one species by chemicals produced by another species, and although this is a matter of controversy in the scientific literature, this concept has been generally accepted in recent years [10]. This definition is significantly shorter than the original use of the word, which may involve both positive and negative interactions, but it is also a reflection of the importance of allelopathy between domestic and introduced plant species, especially when introduced species can invade and displace native plants. Engineering mills, especially those with grains, have a considerable interest in controlling weeds in their own surroundings using allelopathy in agriculture.

The basic premise of allelopathy is that plants secrete phytotoxic metabolites in their surroundings (primarily rhizosphere) and inhibit the growth of plants that are susceptible to these metabolites. This process can be reasonably classified as protective or signal and, in fact, molecules such as strigolactones may have dual roles. Allelopathy is believed to have an evolutionary dimension, so long-term coexisting plants have developed co-adaptation and tolerance mechanisms, whereas ecologically separated plants may not have these tolerance or resistance mechanisms. The various molecules present in the root glands are known to have phytotoxic properties at biologically related concentrations (**Figure 1**).

The majority are phenolics, including simple phenolics, flavonoids, and quinones; terbenes, monoterpenoids, sesquiterpene lactones, diterbenes and Benzoxazinoids or glucosinolates. An important feature when considering plant protection against microbial or insect pathogens, signaling and allopathy is that overall classes of molecules are also included in these cases. Our ability to determine whether, specific metabolites may first be lost in evolutionary history as signal molecules, as protection against pathogens, or as phytotoxic agents to enhance competitiveness. With regard to the exploitation of these natural products (lead) as herbicides, plant protection products, or drugs, it is now an important quest to understand their mechanism of action in targeted and non-targeted organisms.

### **5. Process of plant natural products**

#### **5.1 Identifying the targets of plant metabolites**

Despite the vast number of biological reactions in biological structures and cells, a relatively small number are exploited by man. For example, 270 herbicides in commercial use target only 17 different processes and medicinal and agricultural fungicides target only six different processes [11]. As the synthesis of natural substances in plants runs into many thousands of different molecules, many new inhibitors of cellular functions can be identified. This belief drives most of the research on plant natural products and their mode of action. Although many plant metabolites have been described chemically and have played many roles in signaling, defense, and allelopathy, the exact action of some has been determined in no detail. In cases where attempts have been made to determine how chemicals cause their effects, the interpretation of the results is often complicated by several goals, including difficulty in separating primary and secondary effects and difficulties in determining whether data obtained from *in-vivo* studies are related to *in-vivo*. To a large extent, these difficulties are indicative of limitations with the test methods used, and there is certainly a case for conducting studies in genetically controllable systems such as yeast. Nevertheless, it was possible to identify key processes that are normally targeted by plant metabolites and specific enzymes that can be inhibited by specific metabolites.

#### **5.2 Inhibition of specific enzymes**

Plant secondary metabolites can inhibit specific enzymes in plants or other organisms, such as fungi or animals. In some cases, it appears to be the only function of the metabolism, while in others, it forms part of a set of enzyme inhibitory effects. It should be noted, however, that the uniqueness of some of the findings and the biological relevance of *in-vivo* are questionable. An example of this is the inhibition of various enzyme reactions, including the plant hormone biosynthetic enzymes, catalase, maltase, and phosphatase by phenolics and phenolic acids [12].

Sesquiterpenes are one of the largest families of plant natural products and have many common effects associated with this type of molecule. It is believed that some

#### *Biological Activity of Defence-Related Plant Secondary Metabolites DOI: http://dx.doi.org/10.5772/intechopen.101379*

sesquiterpenes inhibit the activity of enzymes containing sulfhydryl-containing enzymes (e.g., phosphor-fructokinase) and that this may be due to the general apoptotic effects of plant sesquiterpenes on animal cells, but more detailed investigations are needed in this area. In contrast to those common effects, quinone sorgoleone (**Figure 1**) inhibits the enzyme 4-hydroxyphenylpyruvate dioxygenase (HPPD) [11]. Plastoquinone and ultimately chloroplast synthesis require HPPD activity and are targeted to sulcotrione and other herbicides [13]. Other quinones, such as juglone made from the walnut tree, can also inhibit HPPD activity.

Another example is the steroidal alkaloid tomatidine, which in particular inhibits the C24 sterol methyltransferase reaction, which is essential for the synthesis of the essential fungal membrane sterol, ergosterol. This anti-fungal metabolism is

**Figure 2.** *Structures of some plant secondary metabolites.*

synthesized in tomatoes in a glycosylated form called α-tomatine and is closed to the steroidal alkaloid tomatidine by fungal enzymes during plant infection (**Figure 2**). Studies with yeast *Saccharomyces cerevisiae* have unique modes of action with tomatidine, which is 50 times more potent than α-tomatine and tomaditin action α-tomatine.

Interestingly, the importance of C24 sterol methyltransferase for ergosterol biosynthesis has already been recognized and commercial fungicides such as fenpropimorph target the same enzyme. The fact that the enzymes in question have already been identified, and used as pharmacological targets in both sorgoleone/HPPT and tomatidine/C24 sterol methyltransferase confirms the technique of identifying new enzyme targets of plant natural products as intervention drugs or chemicals. Some new natural ingredients or enzymes are under investigation in this regard. For example, 1,4-cineole (monoterpene) inhibits the synthesis of asparagine and quassinoids (diterpenes) are believed to inhibit membrane NADH oxidase [14].

### **6. Inhibition of electron transport systems**

#### **6.1 Target of photosynthesis and respiration**

Photosynthesis is centrally important for plant health; It is, therefore, a clear target for natural and synthetic inhibitory molecules. At least 59 different herbicides target Photo System II (PSII), primarily by interfering with electron transport [13]. PS-II quinone was found to be the main target of sorgoleone, the same metabolite that inhibits the enzyme HPPD (above). Sorgoleone is believed to compete with plastoquinone for binding to D1 proteins in PS-II [15] and is secreted in droplets from the root hairs, which accumulate in the soil around the plant roots at 10–100 μM.

The imbalance between the number of herbicides and natural metabolites that inhibit photosynthesis is surprising and suggests that there may be many more natural inhibitors of photosynthesis yet to be identified. Respiration is another important function of the cell based on electron transport chains and also is the target of inhibitory molecules. The clearest example is probably the cyanogenic glycosides that are produced by more than 200 different types of plants. These are synthesized by converting amino acid precursors to oximes, which are then glycosylated. The hydrolysis of cyanogenic glycosides in response to tissue damage produces hydrogen cyanide (HCN), a potent respiratory toxin [6]. Glucosinolates are molecules associated with the evolution that is synthesized only by a subgroup of organisms, mainly within the order capparalase, including the agriculturally important Brassicaceae family [16].

The hydrolysis of glucosinolates yields isothiocyanates, thiocyanates, and nitriles and although the fungal pattern of these metabolites has not been demonstrated, cyanide moiety is said to be the target of some of these metabolites. Other low molecular weight natural products are also believed to target respiration, but in many cases, it is difficult to establish definitively and studies with isolated mitochondria have sometimes produced conflicting results. Therefore, although some phenolic acids inhibit the absorption of iodine by the mitochondria, the concentrations of phenolics appear to be unreliable, while there are suggestions that phenolics may inhibit electron transport in the b/c1 cytochrome complex and those phenolics actually induce respiration in some cases [17].
