Plants use at least two main strategies to protect themselves against pathogens and herbivores: by a physical barrier making the plant content inaccessible to the invader and/or by the production of appetite spoiling or even toxic components (or the precursors thereof). Apart from shaping the habitus of plants, cell walls, a rigid and tightly interwoven fabric consisting mainly of polysaccharides, constitute a formidable physical barrier. The primary cell wall is a relatively thin layer made up of carbohydrates and structural proteins. The secondary cell wall is a thicker layer rich in cellulose and lignin, polymers that greatly contribute to the physical strength of the plant cell wall. Combined the two cell wall elements constitute a compact protective layer. On top of this physical barrier, plants may produce secondary metabolites to reduce its attractiveness as a food source for pathogens and herbivores. Some plant families are well known by the production of specific categories of metabolites that reduce their attractiveness for herbivores. The plantain family (Plantaginaceae) harbours a wide diversity of genera including Plantago (about 200 species). Some Plantago species are known for their anti-toxic and antimicrobial characteristics. Iridoid glucosides such as aucubin and catalpol are terpenoid secondary metabolites produced by a range of plantain species, and the components make these plants unattractive for a wide spectrum of herbivores. Upon cleaving of the sugar residue by beta-glucosidases either from the plant or from the digestive tract of the invading organism, the resulting aglycon acts un-specifically by cross linking proteins and the inhibitions of enzymes. Iridoids are a class of compounds with as a core a cyclopentane fused to a 6-C oxygen heterocycle. All by all, iridoid glucosides are precursors of defence molecules against a broad range of pathogens and herbivores (Dobler et al., 2011). Members of the buttercup family (Ranunculaceae) produce another category of non-specific anti-feedants. The genus Ranunculus L. (buttercups) comprises about 600 herbaceous species, and fresh buttercup plant parts are poisonous for a wide range of organisms. This can be explained by a secondary metabolite named ranunculin. Upon de-compartimentation (chewing, maceration etc.), the enzyme beta-glucosidase, normally stored in the vacuole, comes into contact with ranunculin, and as a result this glucoside is broken down to form the volatile lactone protoanemonin (C5H4O2). This highly reactive component has anti-microbial properties, and ingestion by herbivorous mammals results in erythema and blistering (Sedivy et al., 2012; Martin et al., 1990). Yet another plant family that produces relatively harmless glycosides that are turned into toxic compounds upon the mechanical damaging of plant tissues is the mustard family (Brassicaceae). Over 100 types of glucosinolates are produced by members of this family. This class of secondary metabolites are the most important flavour compounds in edible representatives such as cabbage, 2 mustard and horseradish. The actual reason why these compounds are produced is (again) the defence against pathogens and herbivores. Just like we’ve seen for the Plantaginaceae and the Ranunculaceae, Brassicaceae produce precursors that are converted into toxic components upon de-compartimentation. The activating enzyme myrosinase (a thioglucoside glucohydrolase) is stored in the vacuole. Upon damaging or maceration of the plant tissue, myrosinase cleaves off the thio-linked glucose from its substrate, and the resulting chemically-unstable aglycone is converted into toxic isothiocyanate or related components (Textor & Gershenzon, 2009). The ban of certain categories of soil fumigantia and the severe restrictions with regard to the application of those fumigants that are still on the market to control soil-borne diseases including plant-parasitic fungi and nematodes have been the driving factors behind a search to find effective environmentally acceptable alternatives. One of the options was the use specific types of green manure. Incorporation in the topsoil of plant material harbouring compounds (or precursors thereof) that are toxic for a range of soil borne pathogens is called biofumigation. As compared to aglycones such as protoamonin from buttercups or iridoids from plantain species, the effective products released upon mechanically damaging Brassicaceous plant parts are relatively stable and volatile. Hence it will spread easily throughout the topsoil. In fact, the incorporation of mulched Brassicaceous material could be considered as a natural replacement of the soil fumigant metam sodium (Matthiessen & Kirkegaard, 2006). Recently we investigated in a field experiment in the western part of Germany the impact of Indian mustard (Brassica juncea) on plant parasitic and free-living nematode taxa (Vervoort et al., 2014). Just before the incorporation of plant material in the topsoil, aliquots of above and below plant parts were sampled and the glucosinolate content was determined both qualitatively and quantitatively. As a positive control 2-propenyl ITC was directed applied to the control plots. Although the impact of the incorporation of mulched Indian mustard material on the nematode community was clear and significant, this impact could not be attributed the effect of isothiocyanates on the nematode community. The severe physical disturbance of the soil in combination with addition of a large quantity of green manure alone sufficed to explain the observed changes. Even more remarkable: the positive control treatments by direct application of 2-propenyl ITC (two concentrations, the highest being two times the concentration predicted for the highest producing Indian mustard cultivar) had no significant effect on the nematode taxa under investigation. We cannot rule out the possibility that local circumstances such as soil type, organic matter type and content, moisture content and/or temperature of the soil at the day of biofumigation can be explanatory factors for the absence of a clear glucosinolate-related response. Our results suggest that a more widely acceptance of bio-fumigation as a tool to manage soil borne pathogens would require an increase in the efficacy and the robustness of this tool. In this paper we pay attention to some other plant families that a producing broad-spectrum toxic components (aglycones) upon maceration and/or physical damage. It could be worthwhile to investigate the impact of the simultaneous release of multiple plant-derived aglycones on plant pathogenic soil biota. The two ‘other’ plant families highlighted in this paper, the Plantaginaceae and the Ranunculaceae, should just be considered as examples (not necessarily the best ones) of families producing distinct broad-spectrum anti-microbial aglycones. We hypothesize that simultaneous exposure of soil pathogens to multiple plant defense-related aglycones could contribute to an improved efficacy and robustness of biofumigation.
|Title of host publication||Proceedings of the 5th IS of Biofumigation|
|Editors||M. Back, J. Clarkson, L Lazzeri, V Michel, J Reade|
|Publication status||Published - 2014|
|Event||5th International Symposium of Biofumigation, Newport, UK - |
Duration: 9 Sep 2014 → 12 Sep 2014
|Conference||5th International Symposium of Biofumigation, Newport, UK|
|Period||9/09/14 → 12/09/14|