"I will present algebro-geometric proofs of a theorem by T. Shiota, and of a theorem by I. Krichever. These results characterize Jacobians of algebraic curves among all irreducible principally polarized abelian varieties. Shiota's characterization is in terms of the KP equation. Krichever's characterization is in terms of trisecant lines to the Kummer variety; I will discuss only the degenerate case of his result. The proofs rely on a new theorem asserting that the base locus of a complete linear system on an abelian variety is reduced. The talk is based on a joint work with E. Arbarello and G. Pareschi. "