The man "appointed himself as the emir" of the group, which controls swathes of Iraq and Syria, The National daily cited a witness as telling a court during his trial.
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He was married to Alaa Bader al-Hashemi, an Emirati woman executed in July for the militant-inspired murder of US school teacher Ibolya Ryan, 47, in the washroom of an Abu Dhabi shopping mall in December 2014.
The couple had performed a "symbolic ceremony to pledge allegiance" to Abu Bakr al-Baghdadi, the IS leader, the witness said.
International media are not allowed access to the trial of the man, 34, at the Federal Supreme court in the UAE capital, which has upped security measures since the wave of Arab Spring protests that swept the region in 2011.
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He is charged with joining Islamic State and plotting attacks on Abu Dhabi's Formula 1 circuit. He is reportedly also accused of plotting to attack the Abu Dhabi branch of furniture chain Ikea, as well as preparing to assassinate an unspecified Emirati leader.
The United Arab Emirates is a member of the US-led coalition that has been bombing Islamic State militants since September 2014.
UAE authorities have enacted tougher anti-terror legislation, including harsher jail terms and even introducing the death penalty for crimes linked to religious hatred and "takfiri groups".
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