(1) Single employer. In the case of a plan maintained by a single employer, the employer is the plan administrator. If the employer is a corporation, the corporation is the plan administrator. However, the corporation's board of directors may authorize a person or group of persons to fulfill responsibilities of the corporation as plan administrator. In the absence of such authorization, any corporate officer authorized under law, corporate by-laws, or resolution of the board of directors to act on behalf of the corporation with respect to contracts of a value equivalent to the fair market value of the assets of the plan shall be presumed to have authority to fulfill responsibilities of the corporation as plan administrator. For purposes of this paragraph (b) (1), employer means the employer as defined in section 3 (5) of the Employee Retirement Income Security Act of 1974 (29 U.S.C. 1003 (5)).