Saponin avenacin

Fig. 1A-C. Examples of different classes of saponins A the triterpenoid saponin avenacin A-1 from roots of Avena spp B the steroidal saponin gracillin, from Costus speciosus C the steroidal glycoalkaloid a-tomatine from tomato (Lycopersicon spp.)...

When oat roots were used for bioassay, zoospores were first attracted and then caused lysis which raises the possibility that oat crops and their residues might be used to reduce soil populations of zoosporic plant pathogens. The cystospores were insensitive to concentrations of saponins or oat root extract that lysed zoospores. The zoospores lytic principle in oat root extract/exudate was believed to be a saponin, avenacin (61). The effects of avenacin (61) or root extracts on zoospores were identical to those of /0-aescin and consistent with a role of membrane active agents. Toxic effects of saponins on zoospores were also observed in Pseudoperonospora humuli Miyabe Takah. and Phytophthora spp. [139,140]. 

Fig. 2.1 Strucnires of some common saponins. The strucnires of some of the saponins described in the text are shown, including aescin from horse chesnut, avenacin A-1 and avenacoside A from oat, and a-tomatine from tomato. The glucose molecule enclosed in square brackets in the structure of avenacoside A highlights the glucose moiety that is cleaved off by hydrolysis by glycosidases in disrupted oat leaf tissue, leading to the fungitoxic 26-desglucosyl avenacoside A. Redrawn from [94]...

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... 

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. 

Osbourn, A. (1996) Saponins and plant defence - a soap story. Trends Plant Sci. 1, 4-9 Osbourn, A.E., Clarke, B.R., Lunness, P., Scott, PR. and Daniels, M.J. (1994) An oat species lacking avenacin is susceptible to infection by Gaeumannomyces graminis var. tritici. Physiol. Mol. Plant Pathol. 45, 457-467 Papadopoulou, K., Melton, R.E., Leggett, M., Daniels, M.J. and Osbourn, A. (1999) Compromised disease resistance in saponin-deficient plants. Proc. Natl Acad. Scl USA 96, 12923-12928... 

The saponins such as avenacin Ai which is released from oat roots are known to be lytic to a broad range of oomycete zoospores. Our screening assays revealed that crude extract of stem bark bom an Anacardiaceae plant, Lannea coromandelica had similar properties. A solution of 200 ppm of the crude extract was found to inhibit the motility of the zoospores and subsequently cause them to lyse (3i). 

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