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SLs have been identified as a seed germination stimulant of parasitic plants Striga ssp. and Orobanche spp. These parasitic plants play a major impact on the destruction of crop such as maize, sorghum, millet, rice, and some legumes. Generally, the parasitic seeds are dormant until they receive host-derived stimulants, including SLs. After germination, parasitic seedlings attach host plants with a haustorium which is a special structure for penetration into host root. Parasite plants remain in soil for several weeks or months and then emerge the flower shoots under the ground. The parasite can produce a large number of small seeds which can survive in soil for many years and they will cause the problem for future crop. However, Striga and Orobanche cannot survive without their host plants and cannot germinate without SLs exuded from host roots. The control of SLs in soil is the essential key to protect crop from parasitic plants. Not only parasitic plant but SLs also contribute a seed germination in non-parasitic plant such as Arabidosis thaliana. SLs have been reported on the alleviation of seed germination under high temperature. Base on the thermoinhibition, SLs play a role with other phytohormones such as, gibberellin, cytokinin, ethylene and ABA to control seed germination.
The role of SLs in shoot-branching was elucidated by SL-signaling and SL-deficient mutants. For example, the more axillary growth 2 (max2), max3 and max4 mutants in Arabidopsis exhibit a large number of rosette branching. However, the more bud outgrowth in SL-biosynthesis mutants can be recovered by the application of both natural SLs e.g. 5-deoxystrigol, strigol and synthetic SL analogues e.g. GR24. Crosstalk between SLs and auxins in controlling bud outgrowth has been investigated. In SL biosynthesis mutants including max1, max3 and max4 auxin transport and the accumulation of auxin exporter protein PIN1 clearly increased. Treatment of SL accelerates the depletion of PIN1 locating in the plasma membrane of xylem. Therefore, it is thought that SL inhibits bud outgrowth by impairing auxin transport.
Root is the main organ for absorption water and nutrient into whole plant. Several reports demonstrated that SLs affect root development in many aspects. In the initial stage of root development, SLs have positive effects on root hair formation. SLs are involved in root hair elongation by regulating auxin efflux in root. Moreover, SLs and other plant hormones such as cytokinins and auxins have been suggested to regulate cell division, cell elongation and differentiation in the root apical meristem. SL mutants display shorter primary root length than wild-type plant. The shortened primary root in SL-deficient mutant (max1, max3 and max4) but not in SL signaling mutant, max2, can be rescued by exogenous SL application. In the secondary development of root, SLs repress lateral root and adventitious root formation. Treatment of GR24 in Arabidopsis seedling reduces lateral root density. Auxins promote adventitious root development, while SLs and cytokinins suppress adventitious root formation. However, role of these three hormones on adventitious roots are independent each other. The SL-deficient mutants in Arabidopsis and pea display more formation of adventitious root. Overall, SLs should play important roles in root from primary growth to secondary development.
Arbuscular mycorrhizal (AM) fungi are microorganisms in soil that have the symbiotic associations with about 80% of all plant species. AM fungi cannot be alive without a host root. At the initiation stage of AM fungi symbiosis with their plant host, a plant signal molecule, so called branching factor, exuded from the host root regulates hyphal development of AM fungi. The branching factor exudated from host roots was isolated and identified as SL such as 5-deoxystrigol. To confirm SL function as a branching factor in AM fungi, natural SL, 5-deoxystrigol, sorgolactone and strigol, and a synthetic analogue, GR24, were applied to AM fungi. The result showed that these SLs could induce the hyphal branching in germinating spores of the AM fungus at very low concentrations. SLs are essential for the symbiotic growth in AM fungi and may enhance the mycorrhizal signaling factors leading to increase the symbiotic signals.
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