Bioactive Molecules in Plant Defense

There are approximately 450,000 plant species exist on earth, and one third of these plants are under the risk of extinction (Pimm and Joppa 2015). The current estimated total number of plant-produced metabolites within a given plant species are greater than 10,000, however, at present, it has been projected that with currently available metabolome techniques, only less than 20% of these metabolites can be analyzed (Lei et al. 2011; Abdelrahman et al. 2018). During life span, human has frequently used plant derived natural products as traditional medicines for millennia. However, the full potential of these plant derived natural products remains to be exploited, because they are difficult to synthesis in vitro, exist in very low amounts in a given plant species and/or produced by rare plant species and thus cannot be utilized for the large scale production. Generally speaking plants synthesize a diverse array of primary and secondary metabolites, which have different structures and vary greatly in their richness (Arbona et al. 2013; Hong et al. 2016). For instance, primary metabolites are crucial for plant growth and development, whereas secondary metabolites have more explicit functions; and both types of metabolites have major roles for plant responses to a specific stress (Fujii et al. 2015; Abdelrahman et al. 2019).

Saponins are triterpenoid or steroid glycosides that play various biological activities in different plant species. The broad-spread existence of saponin in several plant species, and the potential for saponin pharmaceutical applications have resulted in the extraction and identification of many saponin compounds in various plant species. Although an extensive efforts have been invested in the extraction, isolation and chemical structure identification of saponin compounds, which are important to extend our knowledge of saponin structures, recent progress has been given to the biosynthesis and distribution of saponin compounds and their biological activity in various plants. In this chapter, we summarized and discussed the recent progress on saponin biosynthesis pathway and genes involved in the up and downstream pathway of saponins.

The ‘saponin’ word is originated from Latin name ‘sāpō’ means ‘soap’, as saponins make foams when they are shaken using water. These are a varied class of surface active and nonvolatile secondary metabolites are broadly dispersed in nature, existing in diverse species of plants, including both monocot and dicot. Saponins are 30-carbon skeleton molecules derived from oxidosqualene precursor that consisted of nonpolar aglycones, to which one or more polar monosugar molecules are attached. The polar (sugar moieties) and nonpolar (aglycones) structures mixture in the saponin compounds describe their soap like behavior in water and provide the base for their biological activities. Although saponin is considered major group of plant natural products, their functions in plant biological process are not fully understood and saponins are usually recognized to have significant functions in plant defense mechanisms against pathogens, herbivores and pests. Saponin compounds have a wide array of characters, such as emulsifying and foaming, bitterness and sweetness, antimicrobial, insecticidal, as well as pharmacological and medicinal properties. Although in the early times it may be suitable to categorize saponin compounds according to their biological and/or physicochemical activities, currently with the high throughput in chemistry and mass spectrometry, the structural diversity of saponin compounds became the main classification scheme. In this chapter, we will try to describe the different types of saponin compounds and their distributions in the different plant species. The new isolated saponin compounds from different plants will also be listed as a source information for future biological studies.

Saponins, a group of phytoanticipins are recognized as the first biochemical barriers against wide range of fungal pathogens. Although the detailed mechanisms of saponin antifungal mode of action is not well established, it is believed that saponin aglycone-sugar structure forms complex with the pathogen sterols in the cell membrane, leading to lose of the cell membrane and pore formation and consequently loss of membrane integrity. In this chapter we will discussed different saponin compounds and their mode of action against wide range of phytopathogens.

Pathogenic fungi usually use different tactics to counteract induced and constitutive plant defense mechanisms that include degradation of any chemical compound and inhibition of plant triggered defenses by producing enzymes. Saponins as major bioactive compounds located in several monocot and dicot plant species and have been proposed to be involved in the defense of plants against pathogen outbreak. However, the capability of several pathogenic fungi to produce saponin-neutralizing enzymes would suggest that they play a major role in ascertaining the effect of interaction between plant and pathogen. Most of the saponin-detoxifying enzymes are glycosyl hydrolases, which catalyze hydrolysis of sugars from saponin aglycone that consists of a sugar chain attached to the C3 carbon, resulting in loss of saponin membranolytic properties and consequently loss of toxicity. In this chapter we will discuss and summarize different saponin-detoxifying enzymes and their effects in plant defense, as ultimate objective to increase crop plant productivity.

Saponins are broadly dispersed natural products in the plant kingdom with massive structural and functional diversity, and therefore being regarded as an active components in medicinal plants. Saponin compounds have a significant roles in pharmaceutical industry, however the specific roles of saponins in plant defense as well as other biological process are remain underexplored. Saponins are glycosides of steroids, triterpenes or alkaloids, which are primarily found in roots and shoots of different plant species. Therefore, saponin compounds can be classified into steroidal, triterpenoidal or alkaloidal saponin depending on the nature of their aglycone structure. In this chapter, we will discussed the two major saponin classes, including triterpene saponins and steroidal saponins and their biological activities in pharmaceutical industries and plant-microbe interactions. In addition, saponin biosynthesis pathway and methods of induction of saponin contents will be also covered in this chapter.

Saponins are commonly found in adequate amounts in the root tissue of plant, however recent studies have reported that saponins can be also found in considerable amounts in plant aerial tissues such as leaf and stem. Thus, quantification of total saponin contents in different plant species and organs are very important to understand their biological functions in plant defense. There are several methods have been developed for measuring saponin contents in medicinal as well as crop plant species. The classical colorimetric and biological methods are remain popular methods for saponin quantification. However, biological and colorimetric determinations of saponin contents doesn’t provide accurate information and sometimes might resulted in a misleading information, due the large structural variation of individual saponins not only within different species, but even also among same species. Thus, more sensitive methods have been recently introduced to measure and quantify saponin contents in different plant extracts. High performance (HP)-thin-layer chromatography (TLC) on normal (HPTLC) or reversed-phase (two-dimension, 2D-HPTLC) provides more precise and reliable saponin qualitative information, especially when these HPTLC methods are combined with a computer flying-spot scanner with dual-wavelength. After screening the saponin profile on the TLC, a 2D-analytical software can applied for the quantification of saponin level in plant extracts. However, for reliable measurements a proper saponin standards must be run with the saponin extracts for comparative analysis. Standardization and identification of the peaks by HPLC chromatograms has been also developed for saponin quantification, which relay on the comparisons of the retention times with those observed for authentic standards. On the other hand, there are limited applications of gas chromatography (GC) for quantification and determination of saponin compounds, due to the high molecular weights of the saponin compounds. In this chapter we will discussed some of these methods and the amount of saponin detected in different plant species.

Plant is a main source of natural products with diverse chemical structures and biological activities. Among different plant-derived natural products, saponins secondary metabolites are widely distributed across diverse plant species and have great potentials for the pharmaceutical industry, detergents, pesticides and plant disease management. Triterpene glycosides characterized by a 30 carbon atoms, namely oxidosqualene, a major precursor for triterpene aglycone (sapogenin), to which sugar chains are attached to yield the corresponding saponin compounds. However, saponin applications are often limited due to the low yields or accumulation in planta, inadequate of natural resources and the continuous need of the new compounds with superior biological activities. In addition, the biosynthesis and regulatory pathways of the saponin compounds in different plant species are still very limited. Thus development of new methods to improve and diversify the production of saponin glycosides, with a foresight into metabolic engineering, can be an alternative solution to avoid the problem associated with saponin large scale production. In this chapter, we will summarized and discussed the available information regarding saponin engineering in plant and their potential applications for plant disease resistance.