By Malena Castaldi and Felipe Llambias MONTEVIDEO (Reuters) – Uruguay's pioneering move to legalize the planting and sale of marijuana opens the door for a clandestine cottage industry of pot growers to transform into a legitimate business that could even export medical cannabis, a commodity in short supply. More and more countries are setting up medical marijuana programs to help relieve the pain of terminally-ill patients and treat other health conditions, but there are few legal sources of the drug in the world and Uruguay could tap that tight market. Uruguay's domestic marijuana output is expected to expand rapidly under a law that cleared Congress on Tuesday allowing its citizens to grow up to six plants a year in their homes and more in smoking collectives. Smoking marijuana – and indeed the private consumption of all drugs – has not been a crime in Uruguay since 1974, but the small South American nation of 3.3 million people is now the world's first to fully regulate marijuana from cultivation to consumption.