| Arbuscular mycorrhizal (AM) fungi are obligately biotrophic organisms, about 80% of plants from all phyla of land plants are hosts of AM fungi. AM fungi improve plant growth through their ability to increase plant uptake of P and other nutrients. AM fungal spore germination and length of germ tube were affected by minerals, pH, temperature, humidity, photoperiod and secretory products of host roots. The establishment of functional symbiosis between AM fungi and host plants involves a sequence of recognition events leading to the morphological integration of the symbionts. The developmental switches in the fungi are triggered by host signals which induce changes in differential hyphal branching, appressorium formation, root colonization and arbuscular development. In this study, (i) three AM fungi were tested for the ability of spore germination in different treatments, the results showed that "the differential method of steriliation" was most effective for spore surface disinfection. After the treatment ,95.0%-97.0% of AMF spores were kept uncontaminated and spore germination rate was up to 81.0%-84.0%; germination rate was improved when AMF spores were germinated in the treatments of soil, peat and secretory products of host root; up to 96% of spores were found to germinate when "the improved sandwich method" was used, (ii) The spore nuclei of Glomus constrictum were stained by using DAPI (4'6-diamidino-2-phenylindol), the result showed that the total number of nuclei in the spore was constant and only part of nuclei flowed from the spore to germ tubes during the spore germination, (iii) when flavonoids were applied to the cultural soils of Astragalus sinicus inoculated with AM fungi, the biomass of Astragalus sinicus, AM fungal infected rate and hyphal enzyme activity of ALP and SDH were significant affected by different concentrations of flavonoids (in the 9th weeks, the biomass of Astragalus sinicus was 11.7g-14.4g, and the control was 8.1g-8,2g; G.intraradices infected rate was 71.5%-94.8%, and the control was 36.6%; G.intraradices hyphal enzyme activity of SDH and ALP were 53.6%-70.7% and 32.2%-42.4%, the controls were 27.5% and 16.5%.). (IV) The dualinoculation with both AM fungi and Rhizobium significantly increased the biomass of host plant, number of nodule produced, AM fungal infection, the hyphal growth and enzyme activity of SDH and ALP, and this promotive effects were more significant when an appropriate amount of flavonoids was added in the Rhizobium +AMF treatment (in the 10th weeks, the biomass of Astragalus sinicus was 14.7g-18.7g, and the control was 9.3g-11.0g; the nodule-generating number of Rhizobium was 353-392, and the control was 212; G.intraradices infected rate was 87.8%-92.6%, and the control was 54.8%; G.intraradices hyphal enzyme activity of SDH and ALP were 65.5%-69.5% and 39.3%-41.7%, the controls were 41.1% and 24.6%) . (V) Leaf mustard is a non-mycorrhizal plant, it does not contain flavonoids and other signal molecules, AM fungi could not infect the roots of leaf mustard and form a symbiont in nature, when leaf mustard inoculated with AM fungi was treated with flavonoids, the results of trypan blue staining showed that two kinds of AM fungi successfully infected the roots of non-mycorrhizal plant leaf mustard, and produced young spores; AM fungi that existed in the roots of leaf mustard were probed by nested PCR and special molecular probes.
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