SYDNEY, Oct 24 (Reuters) - U.S. grains and soybeans futures prices rose on Monday, supported by U.S. farmers holding back sales of freshly harvested crops in the hope of higher prices after recent weakness. Commodities and financial markets were, however, cautious as they awaited the outcome of fresh talks about a plan to solve the European debt crisis. Chicago Board of Trade corn for December delivery firmed 0.5 percent to $6.52-3/4 per bushel by 0045 GMT, while wheat for December del