Use of Plant Secondary Metabolites to Reduce Crop Biotic and Abiotic Stresses: A Review

*Ziming Yue, Varsha Singh, Josiane Argenta, Worlanyo Segbefia, Alyssa Miller and Te Ming Tseng*

### **Abstract**

Plant secondary metabolites (PSM) are small molecules of organic compounds produced in plant metabolism that have various ecological functions, such as defense against pathogens, herbivores, and neighboring plants. They can also help to reduce abiotic stresses, such as drought, salinity, temperature, and UV. This chapter reviewed the ecological functions of the PSM and how people utilize these metabolites to reduce crop biotic and abiotic stresses in agriculture. Specific topics covered in this review are (1) extraction of PSM from plant parts and its application on crops; (2) screening of crop/cover crop germplasms for high PSM content and with resistance to pathogens, herbivores, and/or neighboring plants; (3) regulation of PSM biosynthesis (including plant hormones and defense activators) to increase plant readiness for defense; (4) transcriptome and genome technology improvements in the last decade leading to valuable tools to characterize differential gene expression and gene composition in a genome, and lineage-specific gene family expansion and contraction. In addition, there is a critical need to understand how the biosynthesis and release of allelochemicals occur. Filling this knowledge gap will help us to improve and encourage sustainable weed control practices in agriculture.

**Keywords:** allelopathy, pathogen defense, herbivore defense, plant defense, cover crops, sustainable pest management, organic farming

### **1. Introduction**

Plant secondary metabolites (PSM) are small organic molecules produced during plant metabolism that can function as a plant defense against herbivores, pathogens, neighboring plants, or environmental stresses [1–3]. Although proven to be incorrect, PSM [4, 5] used to be defined as (1) the part of metabolites not present in nonplant organisms or as (2) the part of plant metabolites not required for simple growth and development. These outdated PSM definitions still reflected some properties of PSM—they are widespread in the plant kingdom and are beyond the highly conserved primary metabolites, which are required in plant growth and development, such as proteins, carbohydrates, lipids, and nucleic acids. Hence, they represent plant diversity. The description of PSM often starts

from the sessile property of terrestrial plants [1, 2, 6], where they cannot flee from the threat or stress from the environment and hence have to develop strategies to defend or reduce the threat or stress. PSM are their strategies.

Environmental factors, such as temperature, salinity, and water, are also called abiotic stresses [7]. The herbivores, pathogens, and neighboring plants are also called biotic stresses. Plant metabolites can be classified into primary metabolites, secondary metabolites, and plant hormones [3]. The defense function of secondary metabolites is often realized by integration with physical structures, such as cell wall, cutin, suberin, wax, and bark. According to Hartman [1], plant secondary metabolites are often lineage-specific and aid plants in interacting with the biotic and abiotic environment. For example, pine trees and mint plants often contain terpenes, peppers often contain capsaicin, and sicklepod contains anthraquinone derivatives for defense. The production of secondary metabolites can be constitutive or induced. Some plant secondary metabolites, such as anthraquinone derivatives, in sicklepod are routinely produced, and they are called constitutive secondary metabolites. The production of secondary metabolites demands a high metabolic cost on the host plant; thus, many of these compounds are not produced in large quantities until after insects have begun to feed. These secondary metabolites are called induced secondary metabolites [7].

The number of secondary metabolites reported is vast, and they have widespread applications. The most prominent application of the plant secondary metabolites is in the pharmaceutical industry, where about 25% of the drugs in use by humans are derived from medicinal plants [8]. The type and concentration(s) of the secondary molecule(s) produced by a plant are determined by the species, genotype, physiology, developmental stage, and environmental factors during its growth [2].

The application of plant secondary metabolites in agriculture is the focus of this chapter. In standard agricultural practices, the species, physiology, and development stages usually follow biological laws, and we cannot do much to change them. The genotype and environmental factors are currently where most work has been focused on in agriculture. According to Hartman [1], the functions of plant secondary metabolites could fall into three categories—(1) defense and competition involving herbivores (arthropods, vertebrates, and invertebrates), pathogens (viruses, bacteria, and fungi), and plants (allelopathy); (2) attraction and stimulation (pollination, seed dispersal, food-plant recognition, oviposition, sequestration, and symbiosis); and, (3) abiotic stresses defense. Compared to other reviews on secondary metabolites, this review chapter focuses on the agricultural applications of plant secondary metabolites, specifically categories (1) and (3).

#### **2. Secondary metabolites as resources to reduce crop biotic stresses**

#### **2.1 Main groups of plant secondary metabolites**

PSM are widely spread in the whole plant kingdom. As they are lineage-specific, the total number of PSM is much more than the number of primary metabolites [5]. PSM derive from primary metabolites using a limited number of key pathways. Their functional diversity is gained by adding diverse combination of reactive functional groups [9]. Terpenoids are the largest group of PSM and occur in all plants, including over 22,000 compounds. The simplest terpenoid is isoprene (C5H8), a volatile gas produced during photosynthesis in leaves. Terpenoids are classified into monoterpenoids consisting of two isoprene units, sesquiterpenoids (three units), diterpenoids

#### *DOI: http://dx.doi.org/10.5772/intechopen.104553 Use of Plant Secondary Metabolites to Reduce Crop Biotic and Abiotic Stresses: A Review*

(four units), and triterpenoids (six units), depending on how many isoprene units are in their structures [7]. Mint plants (Mentha spp.) produce large quantities of the monoterpenoids menthol and menthone stored in glandular trichomes on the epidermis [7]. Pyrethrins are monoterpenoid esters produced by chrysanthemum plants that act as insect neurotoxins (Saxona 1988). Gossypol (*Gossypium hirsutum*) from cotton is a diterpenoid [7]. The fresh scent of lemon and orange peel results from a class of triterpenoids called limonoids. The active ingredient of neem oil, azadirachtin, is a powerful limonoid isolated from neem trees (*Azadirachta indica*) [10]. Phenolics are another large group of PSM, which includes a wide variety of defense-related compounds, such as flavonoids, anthocyanins, phytoalexins, tannins, lignin, and furanocoumarins [7]. Flavonoids are one of the largest classes of phenolics. Soybean contains a large amount of isoflavone [7]. Tannins are water-soluble flavonoid polymers produced by plants and stored in vacuoles. Tannins are toxic to insects because they bind to salivary proteins and digestive enzymes, including trypsin and chymotrypsin, resulting in protein inactivation. Alkaloids are a large class of bitter-tasting nitrogenous compounds found in many vascular plants and include caffeine, cocaine, morphine, and nicotine [7]. Capsaicin and related capsaicinoids produced by members of the genus *Capsicum* are the active components of chili peppers and have their characteristic burning sensation in hot and spicy foods [7]. Anthraquinones are present in different plant families, such as Leguminosae, Polygonaceae, Rubiaceae, Rhamnaceae, Scrophulariaceae, Liliaceae, Verbenaceae, and Valerianaceae [11]. Anthraquinone derivatives from sicklepod (Leguminosae) have been used to repel deer from browsing soybean [12]. Chlorogenic acid (CGA) or caffeoylquinic acid (CQA) exists in all plants [13], suggesting they are among the oldest PSMs.

#### **2.2 PSM as resources to reduce crop biotic and abiotic stresses**

Crop biotic stresses come from microbial pathogens, nematodes, insects, and mammalian herbivores. Crop abiotic stresses come from drought, salinity, temperature, ultraviolet, etc. Plant secondary metabolites can help to reduce these stresses. For example, some secondary metabolites containing benzene rings can absorb ultraviolet (UV) light and release the energy in the visible light range as fluorescence to avoid crop damage from UV light.

#### **3. Use of secondary metabolites to reduce biotic and abiotic stresses**

#### **3.1 Extraction of secondary metabolites**

Secondary metabolites have a defense function in plants [1, 2]. The simplest way to utilize secondary metabolites for crop protection is to extract the secondary metabolites and apply them to crops for protection against pathogens, insects, and mammalian herbivores.

#### *3.1.1 Secondary metabolites used as a deer repellent*

Deer is the primary pest in row crop production in the US. This was first concerned in the 1960s and gradually confirmed by the agricultural community during the following 40 years [14, 15]. The annual loss of row crops in the US was estimated to be up to \$4.53 billion [14]. Deer repellent is one of the primary strategies to solve

crop deer damage. Among them, deer repellent with putrescent egg solids as active ingredients occurred in the 1990s and still dominates the deer repellent market today. Deer acceptance of food is dependent on the concentration of secondary metabolites present [16]. They usually avoid plants containing high concentrations of terpenes, tannins [17], and gossypol (cotton). Sicklepod (*Senna obtusifolia* L.) is one of the southern US's top ten most troublesome weeds [18]. It belongs to the Leguminosae family and is famous for its high concentrations of anthraquinone derivatives [19], another group of secondary metabolites. Anthraquinone was reported as a mammalian animal repellent since the 1940s [20, 21]. To protect soybean damage from deer, deer repellents were developed using sicklepod fruits [12]. After several modifications of the extraction protocol, the sicklepod extract matched the deer repelling efficacy of Liquid Fence® Deer & Rabbit Repellent, a popular commercial deer repellent with putrescent egg solids as active ingredients. Besides the anthraquinone derivatives, some other plant secondary metabolites were used as deer repellents, such as capsaicin in pepper plants, and monoterpenoids menthol and menthone in peppermint (the active ingredients in Deer Out™, a commercial deer repellent).

#### *3.1.2 Secondary metabolites as insecticides*

One of the best examples of secondary metabolites used as an insecticide was the development of the popular insecticide bifenthrin. The pyrethrins from chrysanthemum (*Chrysanthemum cinerariaefolium*) flower extract were used to develop this insecticide*.* The safety of this product is, however, questionable. Sesbania extracts developed using a similar extraction method were applied on soybean leaves and exposed to soybean loopers in a 40 mm rearing cup for 24 hours. The looper mortality reached 60% in cups containing sesbania extract-treated soybean leaves.
