The following rules apply in an action on a certificated security against the issuer: Unless specifically denied in the pleadings, each signature on a security certificate or in a necessary indorsement is admitted. If the effectiveness of a signature is put in issue, the burden of establishing effectiveness is on the party claiming under the signature, but the signature is presumed to be genuine or authorized. If signatures on a security certificate are admitted or established, production of the certificate entitles a holder to recover on it unless the defendant establishes a defense or a defect going to the validity of the security. If it is shown that a defense or defect exists, the plaintiff has the burden of establishing that the plaintiff or some person under whom the plaintiff claims is a person against whom the defense or defect cannot be asserted.