How can orchids and fungi live together?
It is well known that our native orchids live in a close association (symbiosis) with certain fungi. This association is termed mycorrhiza. The mycorrhizal fungi colonise the soil and the bark of trees but also penetrate the living tissue of orchids. The fungi are of great importance for young orchid seedlings (protocorms). Lacking leaves and roots the protocorms are dependent on their fungi for the uptake of carbohydrates, minerals and water. However leafy adult orchids still benefit from the association and are often heavily infected. The degree of dependency varies within the genera and species. Epiphytic orchids are in general regarded as less dependent than terrestrial orchids. In protocorms, the fungus stays restricted to the basal part, not penetrating the cell division zone (meristem) and as roots develop they become infected from the soil. In Caladenia and Pterostylis and some other genera native to Australia, where roots are sparse or absent the fungus colonises the thickened stem just below the soil surface ( stem collar). As one examines for the first time an infected root under the microscope, one will be certainly astonished to see the masses of fungus present. The outer region of the root (epidermis) is only sparsely colonised, the inner root region (vascular system) is fungus-free and most of the fungus is crowded in the cortical tissue between. The distribution of the fungus seems to be well organised and it becomes obvious that the plant controls the growth of the fungus within its tissue. The first scientific studies on orchid mycorrhiza were done by the French scientist Noel Bernard. In May 1899 he discovered, under a log in a forest, germinating orchid seeds. Bernard realized that the seedlings contain mycorrhizal fungi and depend on them for nutrition. In the following years until his early death in 1911, Bernard germinated the seeds of many orchid species and conducted numerous experiments. He discovered that the relationship is specific and only certain fungi are able to stimulate the growth of seedlings. For example the neotropical epiphytic orchid Cattleya will form a mycorrhiza with a fungus originating from Cattleya but when combined with a fungus from Phaleonopsis or Odontoglossum, either the fungus infects and kills the orchid or the fungus infects the orchid but becomes killed by reactions of the host plant cells and seedlings do not develop further. The orchids are capable of destroying the fungus. Further, Bernard showed that cuttings from orchid tubers, which do not usually contain mycorrhizal fungi have a strong fungicidal activity. Substances which diffuse out of the tubers can inhibit and stop the growth of an approaching fungus. Bernard concluded from his studies that the mycorrhizal fungi are potential parasites which are controlled by reactions of the host cells. As he writes about the “maladie bien faisante” he sees the association as a disease which turns out to be beneficial to the orchids. Indeed, studies from other researchers in later years revealed that some mycorrhizal fungi of orchids are pathogenic to certain plants (parasitic fungi). Examples are the tree-killing honey fungus Armillaria mellea (a symbiont of Gastrodia elata) and the crop-destroying Rhizoctonia solanii (a symbiont of the European orchid Dactylorhiza purpurella and sometimes isolated from native Prasophyllum and Pterostylis by Jack Warcup). However, most orchid fungi showed no pathogenicity to plants and live on dead, decaying plant material (saprophytic fungi) or live in symbiosis with roots of certain shrubs and trees (ectomycorrhizal fungi). Nevertheless most fungi seem to be capable of producing an array of chemical substances (enzymes) which catalyse the conversion of organic molecules. These enzymes enable the fungi to break down and decompose complex organic material, degrade cell walls and even penetrate living cells of plants and animals, causing diseases. However, like animals, plants have highly effective mechanisms for disease resistance that have contributed to survival under the selection pressure of evolution. Plants are continuously exposed to fungi and other microorganisms but resistance to disease is the rule and susceptibility the exception. The mechanisms of resistance employed by plants are diverse. Plant physiologists differentiate in preinfectional resistance (passive defence) and postinfectional resistance (active defence). The first mechanism refers to compounds which are present in plants before the event of an infection. The latter refers to compounds which accumulate only during or after the course of infection. The molecules that are capable of preventing infection and diseases are not synthesized in the main metabolic pathways of a plant, necessary for growth and differentiation of shoots, leaves, roots and flowers and common to all plants (nucleic acids , proteins, sugars, fatty acids). Antimicrobial compounds are synthesized in particular metabolic pathways and are therefore called secondary metabolites. These chemicals are of great diversity and different plant species produce different compounds. Secondary metabolites can be attractants for pollinators (scents, colours), but others are defence molecules and toxic to feeding predators (e.g. insects) and micro organisms. To give a few examples compounds which may be present in plant cells at the time a potential pathogenic fungus starts to attack and invade a plant tissue are lignin, phenols, terpenes and flavonoids. Many pathogens will successfully overcome these and other defence molecules and colonise the first plant cells. The plant may actively respond by the formation of papillae or cell wall appositions with the aim to encapsulate the invader. Additionally, major disturbance to the metabolism of the infected tissue may result in the immediate death of the infected cells. This localised cell death (hypersensivity reaction) together with the synthesis of new antimicrobial substances (phytoalexins) in significant amounts in the neighbouring healthy tissue will in most cases prevent further invasion and growth of the pathogen. The result of such an encounter between plant and pathogen is then visible as dark pigmented spots on leaves, roots or other plant tissues. However, nutritional stress and environmental constrains often limit the metabolic activities of plants and favour the pathogen. As a result, the disease progresses further and the plant will be lost. The similarity of the orchid mycorrhiza to host-pathogen interactions becomes obvious in symbiotic germination trials. Even a suitable (compatible) fungus which supports the growth of orchid seedlings at the first stage may later become parasitic and kill the protocorms. Break-away parasitism is a major problem in symbiotic in-vitro cultures and limits its application for raising orchid seedlings. However in soil (in nature and in potting mixtures), where the growth of the fungus is sparse compared to in-vitro cultures, break-away parasitism seems rare or absent. Two very different organisms seem to benefit from each other by living together. (At least the fungus does not seem to be harmed in the association). A fungus with the potential to kill and an orchid ready to defend itself against a fungal invader. Little is known what controls the process of infection and balances between orchid and fungus. Several systems seem to be possible:
a) The aggressive nature of the fungus is balanced by defence mechanisms of the orchid. Phytoalexins are thought to be important in these context. In stress conditions, they are continually produced by the orchid and they can be gradually inactivated by the fungus. The oxidases, hydrolases and other enzymes of the fungus are inactivated by defence molecules of the orchid.
b) The fungus alters its metabolism and plant-tissue destroying enzymes are not produced after the establishment of an infection. The orchid does not employ defence reactions as discussed above and other, unknown controlling mechanisms are employed.
c) Some defence reactions operate at a low level (e.g. phytoalexins) additionally to unknown controlling mechanisms.
The possibility a) seems to be supported by the unstable nature of the orchid mycorrhiza. However, the high degree of specificity (only certain fungi are able to form a mycorrhiza with certain orchids) seem to favour possibilities b) and c) as a) should allow for a wider range of fungi. The outcome of an infection is dependent on the orchid, the fungus and the nutrients available to the symbionts. Moderate nutrition will restrict the fungus and allow the establishment of a mycorrhiza. However, high levels of carbohydrates and nitrogenous compounds will result in a profusely growing fungus, parasitic invasion and death of the orchid. To investigate the balance between orchids and their fungi the production of key enzymes, polyphenols and phytoalexins have to be analyzed during the course of an infection. The activities of the enzymes and amounts of defence molecules have to be defined and compared in different interactions, which are analyzed microscopically. To obtain further evidence about the defence molecules involved, inhibitors of pathways of secondary metabolites can be applied and its effect on the symbiosis assessed. Investigations of this kind were carried out at the Waite Institute by Prof. Sally E.Smith (Adelaide, Australia) and myself in cooperation with Prof. R.L.Peterson (Canada) in the years from 1989 to 1993 and the results favoured the possibility b). The research was supported by the Australian Research Council and the Alexander von Humboldt-Foundation, Germany.
© 2000-2010 Heinrich Beyrle