In the context of the input sentence, determine the relationship between 'Syriza' and 'Alexis Tsipras'.
Options: subsidiary, distributed by, legal form, chairperson

Greek Prime Minister Alexis Tsipras is also under pressure from many in his radical left Syriza party who say the new accord will pile further austerity on a weakened economy and goes against the party's campaign pledges.
chairperson