Lightner says that Joseph was commanded by an angel in 1834 to practice plural marriage.
Mary E. Lightner to A. M. Chase, letter dated 20 April 1904, quoted in J.D. Stead, Doctrines and Dogmas of Brighamism Exposed (Lamoni, Iowa: Reorganized Church of Jesus Christ of Latter Day Saints, 1911), 218-219
Joseph the Seer taught me the principle of plural marriage in Nauvoo. He said God gave him a commandment in 1834, to take other wives besides Emma, and I was the one he was commanded to take, though I was a thousand miles from him. . . .Joseph told me that he was afraid when the angel appeared to him and told him to take other wives. He hesitated, and the angel appeared to him the third time with a drawn sword in his hand and threatened his life if he did not fulfill the commandment.