BERLIN (Reuters) - Germany's largest trade union, who sit on Volkswagen's supervisory board, said on Thursday they will leave no stone unturned in coming up with alternatives to the carmaker's threats of plant closures, with a 4-day week as one option.

The job security agreement put in place in 1994 was designed specifically to get the carmaker through difficult situations and should not be thrown overboard in difficult times, Thorsten Groeger, head of IG Metall for the Lower Saxony region where Volkswagen is based, said.

(Reporting by Victoria Waldersee; editing by Matthias Williams)