horizontal gene transfer from cyanobacterial ancestor. This finding of genetic fossil inside AM fungi raises the hypothesis of the intimate relationship between AM fungi and cyanobacterial ancestors. At the same time, Geosiphon–Nostoc symbiosis was reported previously. Circadian clock evolution in AM fungi? Surprisingly, despite their long time evolution as underground partner of the plant root of which environment is far from light or temperature fluctuation, AMF still have conserved circadian clock with its activation of fungal circadian oscillator (frq) by the blue light, similar to the case of the model circadian fungus Neurospora crassa. The proved conservation of circadian clock and output genes in R. irregulare opens the door to the study of circadian clocks in the fungal partner of AM symbiosis. The characterized AMF frq gene by same research is the first frq gene identified outgroup of Dikarya, which suggest the frq gene evolution in fungal kingdom is much older than previously investigated. Physiology Presymbiosis The development of AM fungi prior to root colonization, known as presymbiosis, consists of three stages: spore germination, hyphal growth, host recognition and appressorium formation. Spore germination Spores of the AM fungi are thick-walled multi-nucleate resting structures. The germination of the spore does not depend on the plant, as spores have been germinated under experimental conditions in the absence of plants both in vitro and in soil. However, the rate of germination can be increased by host root exudates. AM fungal spores germinate given suitable conditions of the soil matrix, temperature, carbon dioxide concentration, pH, and phosphorus concentration. Hyphal growth The growth of AM hyphae through the soil is controlled by host root exudates known as strigolactones, and the soil phosphorus concentration. Low-phosphorus concentrations in the soil increase hyphal growth and branching as well as induce plant exudation of compounds that control hyphal branching intensity. The branching of AM fungal hyphae grown in phosphorus media of 1 mM is significantly reduced, but the length of the germ tube and total hyphal growth were not affected. A concentration of 10 mM phosphorus inhibited both hyphal growth and branching. This phosphorus concentration occurs in natural soil conditions and could thus contribute to reduced mycorrhizal colonization. Host recognition Root exudates from AMF host plants grown in a liquid medium with and without phosphorus have been shown to affect hyphal growth. Spores of Gigaspora margarita were grown in host plant exudates. Hyphae of fungi grown in the exudates