1. Aba Shaul was in charge of making the dough for the house of Rebbi. He would buy Terumah Teme'ah grain for fuel.
2. The Mishnah states that "seedlings of Terumah Teme'ah that were planted become Tahor from their Tum'ah, and they may not be eaten."
3. Rav Ashi explains this Mishnah (#2) differently.
4. There is a dispute about why Terumah that is not guarded from becoming Tamei is considered Tamei.
5. The difference between these reasons (#4) is in the case above (#2-3).
A BIT MORE
1. The Gemara explains that there was no suspicion that he would use this grain for the dough instead of using it for fuel for the oven, since when he bought the grain it was already disgusting.
2. Rabah explains that the Mishnah means that what grows from these seeds is in fact Tahor, and may not be eaten by non-Kohanim who are forbidden from eating Terumah.
3. Rav Ashi understands that the Mishnah is referring to the seeds themselves that are forbidden for the Kohanim to eat, since they were not guarded from becoming Tamei.
4. There is a dispute about whether it is deemed Tamei because it might have actually become Tamei, or because the mere fact that it was not watched makes it intrinsically Tamei.
5. Rav Ashi's explanation (#3) is valid only if it is intrinsically Tamei, as it certainly did not become Tamei when it was in the ground. Moreover, if Eliyahu ha'Navi would tell us that it did not become Tamei, it still would be Tamei because of the intrinsic Tum'ah that an item acquires as a result of not being watched (and not just because it might have become Tamei).