Herbal defense against herbivores
The plant defense against herbivores (feeding defense) is based, like the plant defense against pathogens, on numerous mechanisms, which in many cases are species-specific. Understanding them is of economic interest because significant parts of the harvest worldwide are destroyed by insect damage (13%) and plant diseases (12%). The evolution selected plants like all living things according to their reproductive success, which in plant ecology through symbiosis with fungi ( mycorrhiza ) and pollinating insects ( pollination ) as well as through successful defense against herbivores (herbivores), and pathogens can be achieved. The relationship between plants and their herbivores or pathogens can be understood as a kind of arms race. This coevolution leads to a situation in most ecosystems in which the plants and their herbivores can survive.
Constitutive defense
Morphological aspects
Like animal immune reactions, the plant's defense mechanisms are partly constitutive and partly induced. The constitutive defense has a preventive effect and is already active before interactions with herbivores. It is morphologically realized through the formation of spines (for example in roses and blackberries), thorns (for example in sloe , wild pear and cacti ) or hardened trichomes (for example in cruciferous vegetables). The nettle forms specialized trichomes as stinging hairs , the tip of which breaks off when touched; the remaining shaft acts like an injection cannula, from which irritating substances such as histamine , acetylcholine and formic acid are released. After being wounded, the density of stinging hairs increases on all newly developing leaves. This is already an example of the induced defense reaction. Other mechanical resistance factors are lignified or silicified cell walls, which can act as a physical barrier against leaf damage.
Secondary metabolites
The chemical basis for the constitutive defense is provided by numerous secondary metabolites , which vary greatly in terms of structure, mode of action and occurrence:
20% of vascular plants form alkaloids , most of which are defense substances. Almost all of these substances can be toxic to humans . However, at lower concentrations, some, including nicotine , cocaine and morphine , are used pharmacologically or as intoxicants . The modes of action have been well researched. For example, nicotine inhibits synaptic transmission in the nerves due to its chemical relationship to the neurotransmitter acetylcholine and, among other things, increases adrenaline secretion . Other alkaloids influence membrane transport , protein synthesis or the activity of enzymes .
Legumes form isoflavones , which consist of an aromatic ring system. The isoflavones in clover species can cause sterility in sheep by binding to the estrogen receptors due to their structural similarity to steroids . In this case, the herbivore population is reduced in the long term. Other isoflavones such as Rotenoids are insecticidal . As phytoalexins , some isoflavones protect the plants from infections by fungi or bacteria.
Tannins are phenolic polymers that are widespread in the plant kingdom and tend to accumulate in unripe fruits. Their toxicity is due to the fact that they bind unspecifically to proteins and thus inactivate digestive enzymes in particular. They cause a bitter taste and thus act as a deterrent. The low dose of tannins in red wines could, however, be responsible for blood vessel dilation and thus counteract heart attacks or strokes.
Many plants store secondary metabolites, from which toxins are formed immediately when injured. These include cyanogenic glycosides , which are stored in the vacuoles of epidermal cells. When the leaves are eaten, the vacuoles break open and the cyanogenic glycosides are released into the cytoplasm , where they are enzymatically converted into ketones and hydrocyanic acid ( hydrogen cyanide ). These are antibodies that are often found in grasses ( e.g. dhurrin in millet ), legumes and roses. Glucosinolates , which mainly accumulate in cruciferous plants , are enzymatically broken down like cyanogenic glycosides only when the plant is damaged. In this case, highly reactive and therefore toxic isothiocyanates and nitriles are formed . Melilot ( Melilotus spp. ) And Woodruff ( Galium odoratum ) store in their vacuoles o -Cumarsäure glucoside. If the cell compartments are wounded and thus destroyed, this compound comes into contact with glucosidases from the cell wall. The enzymes catalyze the conversion of the glucoside into o -umaric acid, which reacts spontaneously to form the toxic coumarin .
Non-proteinogenic amino acids such as canavanine represent a further group of defense substances. Canavanine is similar in structure to the amino acid arginine and can therefore be incorporated into the proteins of the herbivores instead, which can lead to the fatal loss of numerous protein functions. Plants such as Canavalia ensiformis , which produce canavanine in large quantities, have a tRNA that only binds arginine and not canavanine in the course of protein synthesis, so there is no risk of self-poisoning. Other non-proteinogenic amino acids interfere with the synthesis or uptake of proteinogenic amino acids in herbivores.
Induced Defense
Jasmonic acid and induced antibodies
The plants' induced defense mechanisms come into play as a direct response to damage from herbivores. Since they are not constantly developed, relatively little energy and nutrients have to be invested in comparison to the constitutive defense.
In many cases, glutamine from the saliva of attacking insects reacts with fatty acids from the destroyed plasma membranes of the plant cells to form fatty acid amides (fatty acid-amino acid conjugates). These compounds are elicitors that trigger defense reactions. Typically, they induce the production of the plant hormone jasmonic acid from linolenic acid . The latter is part of membrane lipids. The synthesis of jasmonic acid begins in the chloroplasts and is completed in the peroxisomes . The plant hormone in turn activates the synthesis of various defense substances via transcription factors. These include, depending on the species, various alkaloids, proteinase inhibitors (including inhibitors of the digestive enzymes trypsin and chymotrypsin ), digestive lectins and inhibitors of the starch-degrading alpha- amylase . Studies on Macaranga tanarius have shown that an increase in leaf nectar production induced by jasmonic acid attracts predators and parasites of herbivores and thus counteracts further leaf damage. This probably also applies to other leaf nectar-forming plants such as elder , cotton , balsa , cashew nuts or cherry trees .
Systemin and systemic response
In the tomato ( Solanum lycopersicum L. ) and other members of the nightshade family , leaf damage leads to the synthesis of proteinase inhibitors and other defense compounds in the entire plant, i.e. also in undamaged organs. The following model was developed for the systemic defense reaction in tomatoes:
The wounded tomato leaves synthesize prosystemin, a protein made up of 200 amino acids . A proteolytic cleavage at the C end releases the 18 amino acid systemin from Prosystemin. Systemin is released from the injured cells into the cell walls. In the phloem parenchyma , it binds to a receptor in the plasma membrane. The receptor activates a phospholipase, which is also located in the plasma membrane , via phosphorylation . The phospholipase cleaves linolenic acid from the membrane lipids , which is then converted into jasmonic acid . Jasmonic acid is transported through the phloem and in this way reaches all tissues in which it activates the genes for the expression of proteinase inhibitors. The systemin signaling path presented here only exists in Solanaceae. However, systemic reactions can also be determined for other plants (for example lima beans, cereal and cotton plant species). The underlying mechanisms are still unknown.
Volatile organic compounds
All plants react to mechanical damage by releasing volatile aldehydes, alcohols and esters. These volatile organic compounds ( English VOC ) lure in many cases the natural enemies attacking insects. Volatile organic compounds released by butterflies after they lay eggs have a deterrent effect on the female butterflies, preventing further oviposition and herbivory. Some volatile organic compounds remain bound to the leaf surface and, because of their taste, have a deterrent effect on leaf-eating insects. Plants can distinguish their herbivores from one another and react to them differently. For example, the wild tobacco species ( Nicotiana attenuata ) usually produces increased nicotine after herbivore infestation. In the case of caterpillars that have developed resistance to nicotine, however, the tobacco plant releases volatile terpenes , which are also counted among the volatile organic compounds. These compounds in turn attract the caterpillars' predators . The herbivores are recognized by the plant by the type of wound or by specific substances in the saliva. Certain volatile organic compounds can induce genes with defense functions in neighboring plants.
Herbal defense mechanisms - a question of the right investment
The production of defense substances is only worthwhile if there is a certain number of herbivores or pathogens, because it requires energy and nutrients. This means that the defensive measures of a plant compete with other physiological processes such as growth and reproduction rates (fruit and seed formation) for the available resources. This is reflected in the experience that cultivated crops are more productive than their wild ancestors, but more susceptible to disease. In addition, laboratory tests have shown that resistance-increasing mutations in the model plant Arabidopsis thaliana lead to reduced seed production. There is still a need to research to what extent fertilization measures can prevent a decline in harvest caused by increased resistance.
See also
literature
- Bob B. Buchanan, Wilhelm Gruissem , Russell L. Jones (Eds.): Biochemistry and molecular biology of plants. American Society of Plant Physiologists, Rockville MD 2000, ISBN 0-943088-37-2 .
- Peter Schopfer, Axel Brennecke (di: Axel Brennicke ): Plant physiology. 6th edition. Elsevier - Spektrum Akademischer Verlag, Heidelberg 2006, ISBN 3-8274-1561-6 .
- Lincoln Taiz, Eduardo Zeiger: Plant Physiology. Sinauer, Sunderland, MA 2006, ISBN 0-87893-856-7 .
- Borchard, F., Berger, A., Bunzel-Drüke, M. & T. Fartmann (2011): Diversity of plant-animal interactions: possibilities for a new plant defense indicator value? In: Ecological Indicators 11 (5), pp. 1311-1318.
Web links
- High Noon in the bed (DIE ZEIT, February 24, 2005, No. 9)
- BSA Online Image Collection, Plant Defense Mechanisms ( Memento from September 7, 2015 in the Internet Archive )
Individual evidence
- ↑ D. Pimentel: Diversification of biological control strategies in agriculture. In: Crop protection. 10, 4, August 1991, pp. 243-253, doi: 10.1016 / 0261-2194 (91) 90001-8 .
- ^ Andreas Schaller: Defense against predators: Self-defense in the plant kingdom. In: Quarterly journal of the Natural Research Society in Zurich. 147, 4, 2002, pp. 141–150, online (PDF; 373 kB) ( Memento of the original dated December 14, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. .
- ↑ Roger Corder, Julie A. Douthwaite, Delphine M. Lees, Noorafza Q. Khan, Ana Carolina Viseu dos Santos, Elizabeth G. Wood, Martin J. Carrier: Health: Endothelin-1 synthesis reduced by red wine . In: Nature . tape 414 , no. 6866 , December 2001, p. 863-864 , doi : 10.1038 / 414863a .
- ↑ Philippe Matile: The toxic compartment of the plant cell. In: Natural Sciences. 71, 1, 1984, pp. 18-24, doi: 10.1007 / BF00365975 .
- ↑ Martin Heil, Thomas Koch, Andrea Hilpert, Brigitte Fiala, Wilhelm Boland, K. Eduard Linsenmair: Extrafloral nectar production of the ant-associated plant, Macaranga tanarius, is an induced, indirect, defensive response elicited by jasmonic acid. In: Proceedings of the National Academy of Sciences (PNAS). 98, 3, 2001, pp. 1083-1088, doi: 10.1073 / pnas.98.3.1083 .
- ↑ Lei Li, Chuanyou Li, Gyu In Lee, Gregg A. Howe: Distinct roles for jasmonate synthesis and action in the systemic wound response of tomato . In: PNAS . tape 99 , no. 9 , April 30, 2002, pp. 6416-6421 , doi : 10.1073 / pnas.072072599 ( ODF [PDF]).
- ↑ M. Heil: Fitness costs of induced resistance: emerging experimental support for a slippery concept . In: Trends in Plant Science . tape 7 , no. 2 , 2002, p. 61-67 , doi : 10.1016 / S1360-1385 (01) 02186-0 ( PDF ). PDF ( Memento of the original from September 23, 2015 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.
