I have always tried to have an owner or officer sign plan documents and amendments.

I have a Plan where the controller is a trustee, but not an owner or Pres., VP., Secretary or treasurer.

Does a trustee have the authority to sign the amendments? In this case, the 415 amendment?

Your plan document might specify who can sign. Also see the US Supreme Court case for some ramblings about authority to make plan amendments. Schoonejongen

As J Simmons points out, look first to the PD.

I recommend the general practice of many of the large insurance companies. They require the signature of the CEO or next in command as per the corporation's by-laws. Alternatively a Board Resolution authorizing a named officer. A Board Resolution form seems to have become standard in document packages.

By the way, I have seen many cases where the owner is not an officer.

Check the plan document. If the document says "the Company" reserves the right to amend the plan then (1) the board of directors may take action, on behalf of the company, to do so or (2) the board may have delegated its authority to do so to an individual or to a committee and they/it may do so. Even if someone, e.g., a corporate officer, signs an amendment and he/she has not been delegated the authority to do, the board may ratify his/her action later (if it wishes to do so). Ratification of the officer's signature effectively constitutes a limited delegation of authority to act on behalf of the company.

Don't get this issue confused with the general contract law principles of who can bind the company with third-parties.