Tomatinases

Although tomatinases from different fungal sources share the same substrate, they cleave different sugar moieties from a-tomatine. For example, tomatinases of S. lycopersici [55] and V. albo-atrum [62] remove the terminal P-l,2-linked glucose molecule, while that from B. cinerea removes the terminal P-l,3-linked xylose [67], On the other hand, F. oxysporum sp. lycopersici [63, 64], F. solani [66], and... 

Hostettmann K, Marston A (1995) Saponins. Cambridge University Press, Cambridge Pareja-Jaime Y (2008) Tomatinase from Fusarium oxysporum f sp lycopersici is required for full virulence on tomato plants. Mol Plant Microbe Interact 21 728... 

Cira LA et al (2008) Heterologous expression of Fusarium oxysporum tomatinase in Saccharomyces cerevisiae increases its resistance to saponins and improves ethanol production during the fermentation of Agave tequilana Weber var. azul and Agave salmiana must. Antonie Van Leeuwenhoek. Int J Gen Mol Microbiol 93 259... 

Lairini K et al (1996) Purification and characterization of tomatinase from Fusarium oxysporum f. sp. lycopersici. Appl Environ Microbiol 62 1604... 

The existence of tomatinases in fungal-tomato pathogens supports the idea that tomatine may play a role in resistance to fungal attack because these enzymes seem to act specifically on tomatine. Such a role is also supported by the finding that at least in F. oxysporum f. sp. lycopersici, tomatinase is produced during infection both in roots and stems... 

A second major mechanism of resistance to tomatine involves enzymatic detoxification by tomatinases. Although saponins are very numerous and widely distributed in the plant kingdom, detailed studies on saponin detoxification by fungi have been restricted to pathogens of a few plant species, principally to oat and the Solanaceous tomato and potato. This is because structures and antifungal properties of oat, tomato and potato saponins are well established, and that saponin profiles of these plants are relatively simple, in contrast to other plants like alfalfa, where over 20 predominant different saponins have been identified. 

The mechanisms of action of tomatinases differ, Fig. (2). Whereas some remove a single sugar, most of them release the intact p-lycotetraose group, yielding tomatidine (Table 2). 

Avenacinase, an enzyme from G. graminis var. avenae, is related to tomatinase from S. lycopersici because is able to deglucosilate tomatine by identical mode of action. However, the activity is very low and corresponds to approximately 2% of its activity towards avenacin A-1 [32]. Tomatinase form S. lycopersici, also can cleave avenacin A-l but has less than 0.01% of activity towards it in comparison to its activity towards tomatine [32]. Therefore, the two enzymes are highly specific for their respective host plant saponins. Purification and characterization of S. lycopersici tomatinase revealed that this enzyme shares many properties (including immunological cross-reactivity) with avenacinase... 

Table 2.- Fungal pathogens with tomatinase activity.

Tomatinase from S. lycopersici has been introduced in the hemibiotrophic tomato pathogen C.fulvum, that is very sensitive to tomatine. Tomatinase-producing transformants reveal increased... 

The tomatinase from the vascular wilt pathogen F. oxysporum f. sp. lycopersici has a different mechanism of action than the S. lycopersici tomatinase, releasing the intact p-lycotetraose group to give the aglycone tomatidine, Fig. (2) [35, 83]. The enzyme is extracellular and inducible by tomatine whereas breakdown products (tomatidine and sugars) fail to increase the level of tomatinase activity [35]. 

Although several authors claimed that the glycoalkaloid tomatine is not present in sufficient concentrations in roots or stems to play a major role in resistance to Fusarium oxysporum f. sp. lycopersici [7, 57, 88], the fact that tomatinase is induced in tomato plants during infection, strongly suggests that the enzyme may have a specific role in pathogenicity of tomato plant by F. oxysporum f. sp lycopersici. 

The function of these tomatinase in formae speciales that do not pathogenise tomato is unknown. One possible explanation could be the presence of tomatine or similar saponins in their host plant species. However, (i) tomatine has not yet been reported in these plants [2, 7, 9] and (ii) although some of these species contain small amount of tomatine and other saponins structurally related to tomatine (e.g. potato contains a- solanine and a-chaconine [2, 4, 9, 90]), these are inactive as inducers of tomatinase and, moreover, tomatinase cannot use any of these glycoalkaloids as substrate [89]. In addition, it is clear that tomatinase is not required for pathogenicity in these isolates, at least in the case of F. oxysporum f. sp. melonis, where some strains that are fully pathogens on muskmelon lack tomatinase activity [89]. 

The tomatinase gene from F. oxysporum f. sp. lycopersici has been cloned recently [34]. This gene encodes a protein that has no sequence homology to any previously described saponinase but which is highly similar to xylanases (family 10 of glycosyl hydrolases) [84, 85, 93-95]. Although F. oxysporum tomatinase does not have any detectable xylanase activity, it remains to be determined whether any of the xylanases listed in this family possesses activity against a-tomatine. In any case, it is... 

One of the isolates used by Quidde et al., was unable to metabolize tomatine and was more sensitive to the glycoalkaloid. In addition, this isolate was not able to induce disease symptoms on detached tomato leaves, but could pathogenize bean leaves, indicating that the ability to degrade tomatine may be required for pathogenicity of B. cinerea on tomato [36]. It would be very interesting to transformed this strain with one tomatinase gene to confirm this hypothesis. 

The structural relationship of tomatinase from B. cinerea to other tomatinases is unknown, but its molecular mass (70 kDa) is different from the other two enzymes mentioned before, 50 kDa from F. oxysporum [35, 38] and 110 kDa from S. lycopersici [33]. Moreover, when Quidde et al., attempted cloning of the tomatinase gene from B. cinerea using the tomatinase from S. lycopersici as a probe, they isolated a gene with high sequence homology, whose product had not tomatinase activity but was able to detoxify avenacin A-l [97], the saponin from oats related to some extent to tomatine. 

The occurrence of a tomatine-detoxifying enzyme in F. solani, was studied in liquid medium with or without tomatine. No tomatinase activity was detected in the absence of tomatine, whereas a remarkable increase of tomatinase activity was observed after 12 h of growth in the presence of the glycoalkaloid, the maximal tomatinase activity being observed after 72 h of incubation [37]. Detection of tomatinase activity in F. solani contrasts with the results reported by Defago and Kern [70],... 

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