Saponins are secondary metabolites produced in healthy plants and derive their name from their soap-like properties. They are pre-formed antimicrobial agents in contrast to phytoalexins which are synthesized in response to pathogen attack (Osbourn et al., 1998). Saponins a glycosylated natural product present in plant exhibits antifungal activity as defense response against phytopathogenic fungi (Osbourn et al., 1998), they also play a role as defensive compound against insect (Nielson et al., 2010) and has molluscicidal effect (Huang et al., 2003). Saponins can be divided into three groups such as triterpenoid (aglycone), a steroid or a steroidal glycoalkaloid (Osbourn 1996 a). There are 11 main classes of saponin: dammaranes, tirucallanes, lupanes, hopanes, oleananes, taraxasteranes, ursanes, cycloartanes, lanostanes, cucurbitanes and steroids (Kregiel et al., 2017). Triterpenoid is primarily found in dicotyledonous plants but may also be present in monocot. Steroid saponins mainly occurs in monocots example Liliaceae, Dioscoraceae and Agavaceae but is also present in certain dicots such as Foxglove (Osbourn 1996 a). Oats (Avena sativa) contain both triterpenoid and steroid saponins (Osbourn 1996 a). Oat kernel saponin is mainly present in endosperm (Onning et al., 1993).
Saponins are associated with defensive roles due to their cytotoxicity and are active against microorganisms (Colson et al., 2020). Saponins may have membranolytic, toxic and fungitoxic effects (Price et al., 1987). Antifungal saponins from oats and solanaceous plants are oat root saponin avenacin A-I (a triterpenoid) and α-tomatine as well as α-chaconine are both steroidal glycoalkaloids and are found in tomato and potato respectively (Morrissey and Osbourn 1999). The mechanisms of antifungal activity of saponins is due to their ability to complex with sterols in fungal membranes and induce membrane disruption (Keukens et al., 1995). Glycoalkaloid is toxic to Cladospoprium fulvum (Dow and Callow 1978). Tomatine exerted both fungistatic and fungicidal effects and caused an irreversible leakage of electrolytes from hyphae (Dow and Callow 1978). Antifungal compounds are commonly sequestered in vacuoles or organelles of healthy plants and so necrotrophic pathogens may encounter fungitoxic levels of these compounds because they cause extensive tissue damage (Morrissey and Osbourn 1999). The antifungal steroidal, glycoalkaloid saponin, alpha-tomatine is present in uninfected tomato plants in substantial concentrations and may contribute to the protection of tomato plants against the attack by phytopathogenic fungi (Melton et al., 1998). Many tomato pathogens has the ability to detoxify alpha-tomatine through the action of enzymes known as tomatinases. In contrast the biotrophic tomato pathogen Cladosporium fulvum is sensitive to alpha-tomatine and is unable to detoxify this saponin (Melton et al., 1998). Number of phytopathogenic fungi degrade the saponins of their respective host by hydrolysis of sugar molecules from sugar chain attached to C-3 of the saponin backbone. Fungi that succeed in breaching antimicrobial plant defences produce saponin detoxifying enzymes (Osbourn 1996 b). Root-infecting pathogen Gaeumannomyces graminis requires saponin detoxifying enzyme avenacinase for infection of oats, the increased disease susceptibility of oat variants lacking avenacins supports the significance of saponin resistance for fungal pathogenesis and to the notion that saponins play a role as antifungal phytoprotectants (Morrissey and Osbourn 1999).
Aescin a saponin has a strong antifungal activity and can also activate plant immunity and provides SA-dependent resistance against both fungi and bacterial pathogens (Trda et al., 2019). The fungistatic activity of saponins was demonstrated both in vivo and in vitro (Gruiz 1996). Antifungal activity of saponins were evaluated in vitro against six pathogenic fungi and it was observed that saponins inhibited the growth of Fusarium oxysporum f. sp. callistephi, Botrytis cinerea, Botrytis tulipae, Phoma narcissi, Fusarium oxysporum narcissi (Saniewska et al., 2006). Trda et al.(2019) showed that aescin triggers plant defense by activating the SA pathway and oxidative burst ultimately leading to resistance of Brassica napus against the fungus Leptosphaeria maculans. The antimycotic activity of saponin A from Styrax and of its debenzoylated derivative saponin B was studied on number of plant pathogens and on Trichoderma viride. Saponin A demonstrated activity against Trichoderma viride, Fusarium oxysporum, Aspergillus niger and Rhizopus mucco. Saponin B had low activity on Trichoderma viride (Zehavi et al., 2008). Zehavi et al.(1993) studied the structure and antifungal activity of medicagenic acid saponins as well as included synthetic glycosides of mannose, galactose, cellobiose and lactose and furthermore 23α-hydroxymethyl analog of medicagenic acid on plant pathogens. The native glucose-containing saponin was more effective antifungal agent. A carboxyl substituent at the 23α position of the sapogenin showed higher fungistatic activity than a methyl carboxylate which in turn was more effective than hydroxymethyl group at the same position (Zehavi et al., 1993). Saponin rich extracts can control the phytopathogenic fungi especially under organic management (Chapagain et al, 2007). Saponins are effective against a wide range of plant pathogens and can be used as natural fungicide.
See Part A for further information ………
References:
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BIOLOGICAL PROCESSES- A NATURE'S GIFT 