While Johnson isn't himself an impeachment witness, his statements attempting to defend the president have involved mention of the quid pro quo. According to the Wall Street Journal, Johnson said he was told by Sondland of a potential quid pro quo between Trump and the Ukrainian president.
In an interview with the Milwaukee Journal Sentinel, Johnson said Trump had conveyed the quid pro quo in question to him via an August 31 phone conversation. He said he tried to get Trump to release Ukrainian military aid but was unsuccessful.
Despite his close involvement with Ukraine, Johnson has emerged as one of the staunchest defenders of Trump during the impeachment hearings.