The cycad Cycas revoluta is a palm-like plant that grows on rocky coastal cliffs in the sub-tropics and tropics. It has a symbiotic relationship with the Nostoc species of bacteria that can convert nitrogen from the atmosphere into ammonia, which the host plant can then use for its growth. Scientists knew that cycad roots produce a compound that can induce Nostoc species within the soil to transform into their motile form, hormogonia, and attracting them to the roots. However, nobody has determined what exactly the compound is.
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Source: Phys.org