DUBAI, Nov 22 (Reuters) - Leaders of the world’s 20 biggest economies said on Sunday that a multilateral trading system is now as important as ever due to the coronavirus crisis, according to the G20 final communique, as they pledged to keep markets open and achieve a fair and stable trade and investment environment. The G20 also called on the International Monetary Fund to continue exploring additional tools that could help its members as the crisis evolves. (Reporting By Davide Barbuscia and Maha El Dahan; Editing by Hugh Lawson)