(Adds charges against senators, comment by agriculture minister)
BRASILIA, Aug 25 (Reuters) - Brazil's Supreme Court justice on Friday opened a corruption investigation into the nation's agriculture minister, Blairo Maggi, who is accused of leading a graft scheme when he served as governor of Mato Grosso state from 2003 to 2010.
In a separate development in a deepening corruption scandal, the country's top prosecutor filed charges against four senators of the ruling Brazilian Democratic Movement Party (PMDB) and its former president José Sarney.
Justice Luiz Fux approved the investigation request made by Prosecutor General Rodrigo Janot, who alleges that Maggi, a billionaire who made his fortune as Brazil's largest soy farmer, arranged for state lawmakers to be paid off to support his political agenda when he was governor.
Janot's allegations are based on plea-bargain testimony made by Silval Barbosa, who served as Maggi's vice governor from 2007 to 2010. Among the accusations, he alleged Maggi instructed his deputy to pay his campaign debts with public funds.
Maggi denied the accusations in a statement and said he was confident that the investigation would prove his innocence.
"I reject the allegation that I led or organized criminal activities in Mato Grosso. I have never used illegal means in my public life or in my companies," Maggi said.
With agricultural exports the bright light in Brazil's slumped economy, Maggi is a key minister in President Michel Temer's government, which has been weakened by accusations that forced out ministers and even reached the president. The lower house of Congress blocked a corruption charge against Temer earlier this month, saving him from trial by the Supreme Court.
Janot charged the government leader in Senate, Romero Jucá, three other PMDB senators, Renan Calheiros, Garibaldi Alves and Valdir Raupp, plus former president Sarney with corruption and money laundering in a graft case involving Transpetro, the logistics unit of state-run oil company Petrobras.
They received "undue advantages" disguised as donations negotiated with private companies through Transpetro, Janot alleged, based on the plea bargain statement of former Transpetro president Sergio Machado.
Lawyers for the senators denied the charges and said the prosecutor was politically motivated. (Reporting by Ricardo Brito and Lisandra Paraguassú; Editing by Leslie Adler and Diane Craft)