Glycoalkaloid are natural toxin and are N-containing plant secondary metabolites. The three well known glycoalkaloids are α-solanine and α-chaconine from potato and α-tomatine from tomato. The saponin α-tomatine is a steroidal glycoalkaloid comprising of an aglycone moiety as tomatidine and also a tetrasaccharide moiety, composed of two molecules of glucose one molecule of galactose and one molecule of xylose. The four carbohydrate residues are attached to the 3-OH group of the aglycone tomatidine (Friedman 2002). The steroidal glycoalkaloid α-solanine, α-chaconine and α-tomatine are able to complex with the sterols cholesterol, sitosterol, stigmasterol, campesterol and ergosterol in vitro. The sterol-complexing ability of tomatine was greater than that of potato alkaloids (Roddick 1979).
α-Tomatine an antifungal glycoalkaloid provides basal defense to tomato plant (Okmen et al., 2013). The tomato pathogens overcome this basal defense barrier by the secretion of tomatinases that degrades α-tomatine to less toxic form. The toxicity of α-tomatine is attributed to its ability to form complex with membrane sterol forming pore and leakage of cell content (Ruiz-Rubio et al., 2001). Tomatine inhibited Colletotrichum truncatum and Fusarium subglutinans growth by 63% and 50% respectively and tomatidine inhibited the growth of these pathogens by 50% and 15% respectively (Hoagland 2009). The biotrophic fungus Cladosporium fulvum is unable to detoxify α-tomatine (Okmen et al., 2013).
The antifungal activity of α-tomatine towards yeast is associated with membrane permeabilization. Removal of a single sugar from the tetrasaccharide chain of α-tomatine results in a substantial reduction in antimicrobial activity but the complete loss of sugars leads to enhanced antifungal activity (Simons et al., 2006). α-Tomatine may induce leakage and/or secretion of proteins from Fusarium oxysporum cells. Ito et al.(2007) demonstrated that α-tomatine induced cell death of Fusarium oxysporum (TICDF)occurs through a programmed cell death accompanied by a rapid generation of reactive oxygen species (ROS) in cells, a process in which mitochondria plays a key role. The fungicidal action of α-tomatine was blocked when ROS production was inhibited suggesting ROS is essential for TICDF.
The structure of tomatine consists of a hydrophilic part (the tetrasaccharide side chain), a hydrophobic part (the steroidal moiety) and polar –NH group which participate in acid-base equilibria. These features govern its biological activities (Friedman 2002). Tomatine in tomatine-rich green tomatoes did not inhibit the growth of the fungal species Botrytis cinerea, Gleosporium fructigenum and Monilia fructigena. Evidently the pH on the cell surface of the tomato acts as a barrier. The two possible reason for tomatine concentration and resistance are (Friedman 2002):
- The fungus lowers the pH on the surface of the leaves reducing tomatine’s effectiveness against the cell membranes.
- α-tomatine induces fungal tomatinases that transforms the former to less active molecules.
α-Tomatine is more toxic at high pH than at low pH to the test fungus Helminthosporium turcicum (Arneson and Durbin 1968). The fungitoxicity of tomatine is correlated with its ability to complex with sterols. Hydrolysis of any of the sugars from α-tomatine eliminates its ability to form complex with cholesterol and thus reduces its fungitoxic activity. The unprotonated α-tomatine will complex with cholesterol while the protonated form will not. As α-tomatine is more toxic at high pH this explains that the unprotonated alkaloid is the active form and that it acts by complexing with the fungal sterols (Arneson and Durbin 1968). Defago and Kern (1983) suggest that tomatine is liberated in the early stages of infection inhibiting the further growth of the wild-type strain of Fusarium solani.
Fusarium oxysporum f. sp. radicis-lycopersici (FORL) produces tomatinase which breaks down α-tomatine and protects the pathogens from the toxic effect of α-tomatine. In contrast the plant has a defense system. The enzymes chitinase and β-1, 3-glucanase were induced in infected tomato plants (Szczechura et al., 2013). The earlier studies reported that chitinase accumulate in the areas where host wall was in close contact with the fungal cells. Chitinase accumulation is mediated by fungal elicitors. Simultaneously β-1, 3-glucanase was found in large amount in uncolonized tissue of resistant plants. The enzyme distribution suggest that chitinases and β-1, 3-glucanases play different role in plant defense against fungal attack. β-1, 3-glucanase is likely to be an early event associated with the protection of plant against fungal invasion, whereas, chitinase accumulation reflect a biochemical response to fungal elicitor released by β-1, 3-glucanase (Benhamou et al., 1990). The plant enzyme chitinase and β-1, 3-glucanase acts synergistically in the degradation of fungal cell walls (Mauch et al., 1988).
Entopathogenic fungus Beauveria bassiana is found to be susceptible to tomatine. Tomatine inhibited colony formation and growth of this fungus more than solanine (Costa and Gaugler 1989). The conidia and hyphal growth of the pathogen may be inhibited if the insect consumed sufficient amount of saponin containing plant tissue. α-tomatine from tomato disrupts liposome membranes containing a 3β-hydroxy sterol. Liposome membranes containing sterol lacking 3β-hydroxy sterol were resistant to α-tomatine. Electrolyte leakage from Phytophthora megasperma was dependent upon incorporation of sterol into mycelium. Sterol substitution may explain why tomato tissue is able to withstand high concentration of α-tomatine (Steel and Drysdale 1988). Fungi evolve two basic mechanisms to avoid the toxic effect of tomatine, it is either changing the composition of cell membrane or producing tomatinase (Ruiz-Rubio et al., 2001). Upto now the understanding of α-tomatine metabolism by phytopathogenic fungi is limited to the removal of sugar residues from the molecule but recent studies in Gibberella pulicaris demonstrate that further hydroxylation of the aglycone moiety, tomatidine occurs after the action of tomatinase (Ruiz-Rubio et al., 2001). Both tomatine and tomatidine increased electrolyte leakage of corn, palm leaf morning glory and wild senna. Tomatidine caused greater electrolyte leakage than tomatine (Hoagland 2009). Nakayasu et al.(2021) observation highlights the role of tomatine in shaping the bacterial communities of the rhizosphere and suggest additional function of tomatine in belowground biological communication.
The interest to reshape the rhizosphere microbiome is to protect the plant from phytopathogens. α-Tomatine the steroidal glycoalkaloid influence plant defense response.
References:
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BIOLOGICAL PROCESSES- A NATURE'S GIFT 