Isaiah continued to prophesy about the coming of Christ in 2 Nephi 21:1–5. Among the many biblical prophecies that Moroni spoke to Joseph Smith, words in Isaiah 11 (2 Nephi 21) were declared to be "about to be fulfilled." Isaiah 11 speaks of "the stem of Jesse," "a rod [or branch] that will come out of the stem of Jess," and a "root of Jesse." D&C 113:1–6 explains that the stem of Jesse refers to Jesus Christ. Who was Jesse? He was the father of David from the line of Judah. Isaiah was prophesying that the Savior would come through this line. Seeing Christ as the stem of Jesse has had a long and honorable tradition. Many artistic representations show Jesse sleeping with a tree growing from of his body. David is often represented in the tree and Jesus is at the top. A common theme in medieval art was to show the promise given to Jesse—that Christ, the king of Israel, would come through his posterity.
Isaiah also spoke of a rod or branch, as well as of roots, that would come from the stem. The rod and the roots are described in D&C 113:3–6 as being two different people who would both descend from the tribe of Ephraim (and Joseph) and also from the tribe of Judah. Does Joseph Smith fit the bill as a rod or branch of Jesse? Is he a descendant of Jesse? He might be through a collateral ancestral line. We don’t know about that for sure, but we do know that he is of the Tribe of Ephraim.
What should we make of the root? Who is that? I think we are still waiting for further light and knowledge to identify that person. The root prophecy could be looking forward to another time or another person who will eventually bring these two lineages together. I think the prophecy is saying that this branch and this root will bring the House of Judah and the House of Ephraim together. It appears that this prophecy is looking forward to the final millennial stage in Nephi’s world-view because chapter 21 introduces the lamb and the lion lying down together with each other.
Even though the entire prophecy of Isaiah may not be clear, the symbolism of the stem is clear—the stem refers to the coming of Christ.
Book of Mormon Central, "Why Did Moroni Quote Isaiah 11 to Joseph Smith? (2 Nephi 21:10)," KnoWhy 50 (March 9, 2016).