page_title: Strigolactone
section_title: Plant-fungi interaction
hierarchical_section_title: Strigolactone / Ecology / Plant-fungi interaction
caption_attribution_description: Lotus corniculatus var. japonicus, a model organism of Legume マメ科のモデル植物のひとつ、ミヤコグサの写真。
context_page_description: Strigolactones are a group of chemical compounds produced by a plant's roots. Due to their mechanism of action, these molecules have been classified as plant hormones or phytohormones. So far, strigolactones have been identified to be responsible for three different physiological processes: First, they promote the germination of parasitic organisms that grow in the host plant's roots, such as Striga lutea and other plants of the genus Striga. Second, strigolactones are fundamental for the recognition of the plant by symbiotic fungi, especially arbuscular mycorrhizal fungi, because they establish a mutualistic association with these plants, and provide phosphate and other soil nutrients. Third, strigolactones have been identified as branching inhibition hormones in plants; when present, these compounds prevent excess bud growing in stem terminals, stopping the branching mechanism in plants.
  Strigolactones comprise a diverse group, but they all have core common chemical structure, as shown in the image to the right.
context_section_description: Strigolactones play a fundamental role in plant-fungi interaction. One of the first studies made in Lotus japonicus had already demonstrated that compounds extracted from the root were necessary for the development of arbuscular mycorrhizal fungi that will establish a symbiotic relationship with the plant's root. These same findings were true for different plants such as maize and sorghum. Later on, the compounds responsible for the branching of the arbuscular fungi were isolated, and they include 5-deoxystrigol, strigol and sorgolactone, all of them belonging to the strigolactone family of compounds. The process of branching is crucial to establish the symbiosis. Since this branching only occurs after the germination of the spores and the initial growth of the hypha, the strigolactones required for germination have to be secreted by the plant and reached to fungi, meaning that strigolactones are also part of the recognition process by the fungi.
  Due the fact that arbuscula mychorriza can form symbiotic associations with the majority of the angiosperms, and many gymnosperms, it is expected to found different strigolactones compounds distributed in a whole variety of plants. Unfortunately, while strigolactones are supposedly found in most plants, the studies done with strigolactones and AM fungi so far, have only studied a very limited range of plant species, mostly due the difficulty to extract these compounds and due its ease to disintegrate in solution.
  Strigolactones are not only necessary for the recognition of the plant by the fungi, they are also required by the recognition of the fungi by the plant. The mechanism of fungal recognition occurs in a similar fashion that the recognition of bacterias such as Rhizobia sp. In fact, it has been proposed that the recognition mechanism for bacterias evolved from the mechanism to recognize fungi, because the latter is known to be more primitive and ancient. Just like bacteria use Nod factors, the fungi use a set of molecules denominated Myc factor. These fungal products can be recognized by different plants and are not designed to be plant-specific. When these Myc factors are recognized by the plant's root, they stimulate the expression of different genes involved in the initiation of the symbiotic association. However, the secretion of the Myc factor by the fungi occurs only after being previously stimulated by strigolactones from the plant, demonstrating the necessary role of these compound for both recognition (from fungi and from plant). Strigolactones also have been reported to produces other changes in fungal cells, such as an increase in the concentration of intracellular calcium and an increase in lipochitoolisaccharides (LCOs), the latter has been proved to be one of the Myc factors produced by the fungi for its recognition by the plant.
  One of the main roles of arbuscular fungi contained in symbiotic association with plants, is to provide soil nutrients to the plants, especially phosphate. Thus when the phosphate in the depletion zone gets really low, the plant depend mainly in the AM fungi to fulfill its phosphate demands. Studies with tomato plants have shown that, when plants undergo a deficit in phosphate, they produce higher amount of strigolactones, which in turn will increase the branching of AM fungi. This excess development of the fungi is expected to provide the additional phosphate required for the plant, since the fungi can now spread to more soil areas. However, since strigolactone also stimulate the germination of parasitic plants, these phosphate-deficient plants, also present higher invasion of parasitic species such as Striga sp. Providing adequate phosphate through soil fertilization have been proved to reduce the proliferation of these parasites, because they require strigolactone for its germination.
caption: Lotus Japonicus (Image from Wikipedia Commons)
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