<PAGE>

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D. C. 20549

                           --------------------------

                                    FORM 8-K

                                 CURRENT REPORT

                       Pursuant to Section 13 or 15(d) of
                       The Securities Exchange Act of 1934

                            -------------------------

       Date of Report (Date of earliest event reported): February 15, 2006

                          MERRILL LYNCH DEPOSITOR, INC.
                 (ON BEHALF OF PREFERREDPLUS TRUST SERIES ALL-1)
             (Exact name of registrant as specified in its charter)

             DELAWARE             001-16835                    13-3891329
         (State or other        (Commission                (I. R. S. Employer
         jurisdiction  of        File Number)              Identification No.)
          incorporation)

      WORLD FINANCIAL CENTER,                                     10080
        NEW YORK, NEW YORK                                     (Zip Code)
       (Address of principal
        executive offices)
                           --------------------------

       Registrant's telephone number, including area code: (212) 449-1000

                      INFORMATION TO BE INCLUDED IN REPORT


Check the appropriate box below if the Form 8-K filing is intended to
simultaneously satisfy the filing obligation of the registrant under any of the
following provisions (see General Instruction A.2.):

[  ]  Written communications pursuant to Rule 425 under the Securities Act (17
      CFR 230.425)

[  ]  Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR
      240.14a-12)

[  ]  Pre-commencement communications pursuant to Rule 14d-2(b) under the
      Exchange Act (17 CFR 240.14d-2(b))

[  ]  Pre-commencement communications pursuant to Rule 13e-4(c) under the
      Exchange Act (17 CFR 240.13e-4(c))

<PAGE>

SECTION 1. REGISTRANT'S BUSINESS AND OPERATIONS

                  Not applicable.

SECTION 2. FINANCIAL INFORMATION

                  Not applicable.

SECTION 3. SECURITIES AND TRADING MARKETS

                  Not applicable.

SECTION 4. MATTERS RELATED TO ACCOUNTANTS AND FINANCIAL STATEMENTS

                  Not applicable.

SECTION 5. CORPORATE GOVERNANCE AND MANAGEMENT

                  Not applicable.

SECTION 6. ASSET-BACKED SECURITIES

                  Not Applicable.

SECTION 7. REGULATION FD

                  Not applicable.

SECTION 8. OTHER EVENTS

         ITEM 8.01 OTHER EVENTS

99.1     Distribution to holders of the PreferredPLUS Trust Certificates Series
         ALL-1 on February 15, 2006.

                           For information with respect to the underlying
                      securities held by PreferredPLUS Trust Series ALL-1,
                      please refer to Allmerica Financial Corporation's
                      (Commission file number 001-13754) periodic reports,
                      including annual reports on Form 10-K, quarterly reports
                      on Form 10-Q and current reports on Form 8-K, and other
                      information on file with the Securities and Exchange
                      Commission (the "SEC"). You can read and copy these
                      reports and other information at the public reference
                      facilities maintained by the SEC at Room 1580, 100 F
                      Street, NE, Washington, D.C. 20549. You may obtain copies
                      of this material for a fee by writing to the SEC's Public
                      Reference Section of the SEC at 100 F Street, NE,
                      Washington, D.C. 20549. You may obtain information about
                      the operation of the Public Reference Room by calling the
                      SEC at 1-800-SEC-0330. You can also access some of this
                      information electronically by means of the SEC's website
                      on the Internet at http://www.sec.gov, which contains
                      reports, proxy and information statements and other
                      information that the underlying securities guarantor and
                      the underlying securities issuer has filed electronically
                      with the SEC.
<PAGE>

                           Although we have no reason to believe the information
                      concerning the underlying securities, the junior
                      subordinated debentures and the underlying guarantee or
                      the underlying securities issuer and the underlying
                      securities guarantor contained in the underlying
                      securities guarantor's Exchange Act reports is not
                      reliable, neither the depositor nor the trustee
                      participated in the preparation of such documents, or made
                      any due diligence inquiry with respect to the information
                      provided therein. No investigation with respect to the
                      underlying securities issuer or the underlying securities
                      guarantor (including, without limitation, no investigation
                      as to their respective financial condition or
                      creditworthiness) or of the underlying securities, the
                      junior subordinated debentures or the underlying guarantee
                      has been made. You should obtain and evaluate the same
                      information concerning the underlying securities issuer
                      and the underlying securities guarantor as you would
                      obtain and evaluate if you were investing directly in the
                      underlying securities or in other securities issued by the
                      underlying securities issuer or the underlying securities
                      guarantor. There can be no assurance that events affecting
                      the underlying securities, the junior subordinated
                      debentures and the underlying guarantee or the underlying
                      securities issuer and the underlying securities guarantor
                      have not occurred or have not yet been publicly disclosed
                      which would affect the accuracy or completeness of the
                      publicly available documents described above.


SECTION 9. FINANCIAL STATEMENTS AND EXHIBITS

         ITEM 9.01 FINANCIAL STATEMENTS AND EXHIBITS

                  (a) Financial statements of business acquired.

                           Not Applicable.

                  (b) Pro forma financial information.

                           Not Applicable.

                  (c) Shell company transactions.

                           Not Applicable.

                  (d) Exhibits.

                           99.1     Trustee's report in respect of the February
                                    15, 2006 distribution to holders of the
                                    PreferredPLUS Trust Certificates Series
                                    ALL-1.

<PAGE>

                                   SIGNATURES

                  Pursuant to the requirements of the Securities Exchange Act of
                  1934, the registrant has duly caused this report to be signed
                  on its behalf by the undersigned, hereunto duly authorized.


                                              MERRILL LYNCH DEPOSITOR, INC.

         Date: February 28, 2006              By:      /s/ Stephan Kuppenheimer
                                                        -----------------------
                                              Name:    Stephan Kuppenheimer
                                              Title:   President
<PAGE>

                                  EXHIBIT INDEX


         99.1     Trustee's report in respect of the February 15, 2006
                  distribution to holders of the PreferredPLUS Trust
                  Certificates Series ALL-1.
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SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D. C. 20549

                           --------------------------

                                    FORM 8-K

                                 CURRENT REPORT

                       Pursuant to Section 13 or 15(d) of
                       The Securities Exchange Act of 1934

                            -------------------------

       Date of Report (Date of earliest event reported): February 15, 2006

                          MERRILL LYNCH DEPOSITOR, INC.
                 (ON BEHALF OF PREFERREDPLUS TRUST SERIES ALL-1)
             (Exact name of registrant as specified in its charter)

             DELAWARE             001-16835                    13-3891329
         (State or other        (Commission                (I. R. S. Employer
         jurisdiction  of        File Number)              Identification No.)
          incorporation)

      WORLD FINANCIAL CENTER,                                     10080
        NEW YORK, NEW YORK                                     (Zip Code)
       (Address of principal
        executive offices)
                           --------------------------

       Registrant's telephone number, including area code: (212) 449-1000

                      INFORMATION TO BE INCLUDED IN REPORT

Check the appropriate box below if the Form 8-K filing is intended to
simultaneously satisfy the filing obligation of the registrant under any of the
following provisions (see General Instruction A.2.):

[  ]  Written communications pursuant to Rule 425 under the Securities Act (17
      CFR 230.425)

[  ]  Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR
      240.14a-12)

[  ]  Pre-commencement communications pursuant to Rule 14d-2(b) under the
      Exchange Act (17 CFR 240.14d-2(b))

[  ]  Pre-commencement communications pursuant to Rule 13e-4(c) under the
      Exchange Act (17 CFR 240.13e-4(c))
SECTION 1. REGISTRANT'S BUSINESS AND OPERATIONS

                  Not applicable.

SECTION 2. FINANCIAL INFORMATION

                  Not applicable.

SECTION 3. SECURITIES AND TRADING MARKETS

                  Not applicable.

SECTION 4. MATTERS RELATED TO ACCOUNTANTS AND FINANCIAL STATEMENTS

                  Not applicable.

SECTION 5. CORPORATE GOVERNANCE AND MANAGEMENT

                  Not applicable.

SECTION 6. ASSET-BACKED SECURITIES

                  Not Applicable.

SECTION 7. REGULATION FD

                  Not applicable.

SECTION 8. OTHER EVENTS

         ITEM 8.01 OTHER EVENTS

99.1     Distribution to holders of the PreferredPLUS Trust Certificates Series
         ALL-1 on February 15, 2006.

                           For information with respect to the underlying
                      securities held by PreferredPLUS Trust Series ALL-1,
                      please refer to Allmerica Financial Corporation's
                      (Commission file number 001-13754) periodic reports,
                      including annual reports on Form 10-K, quarterly reports
                      on Form 10-Q and current reports on Form 8-K, and other
                      information on file with the Securities and Exchange
                      Commission (the "SEC"). You can read and copy these
                      reports and other information at the public reference
                      facilities maintained by the SEC at Room 1580, 100 F
                      Street, NE, Washington, D.C. 20549. You may obtain copies
                      of this material for a fee by writing to the SEC's Public
                      Reference Section of the SEC at 100 F Street, NE,
                      Washington, D.C. 20549. You may obtain information about
                      the operation of the Public Reference Room by calling the
                      SEC at 1-800-SEC-0330. You can also access some of this
                      information electronically by means of the SEC's website
                      on the Internet at http://www.sec.gov, which contains
                      reports, proxy and information statements and other
                      information that the underlying securities guarantor and
                      the underlying securities issuer has filed electronically
                      with the SEC.
Although we have no reason to believe the information
                      concerning the underlying securities, the junior
                      subordinated debentures and the underlying guarantee or
                      the underlying securities issuer and the underlying
                      securities guarantor contained in the underlying
                      securities guarantor's Exchange Act reports is not
                      reliable, neither the depositor nor the trustee
                      participated in the preparation of such documents, or made
                      any due diligence inquiry with respect to the information
                      provided therein. No investigation with respect to the
                      underlying securities issuer or the underlying securities
                      guarantor (including, without limitation, no investigation
                      as to their respective financial condition or
                      creditworthiness) or of the underlying securities, the
                      junior subordinated debentures or the underlying guarantee
                      has been made. You should obtain and evaluate the same
                      information concerning the underlying securities issuer
                      and the underlying securities guarantor as you would
                      obtain and evaluate if you were investing directly in the
                      underlying securities or in other securities issued by the
                      underlying securities issuer or the underlying securities
                      guarantor. There can be no assurance that events affecting
                      the underlying securities, the junior subordinated
                      debentures and the underlying guarantee or the underlying
                      securities issuer and the underlying securities guarantor
                      have not occurred or have not yet been publicly disclosed
                      which would affect the accuracy or completeness of the
                      publicly available documents described above.

SECTION 9. FINANCIAL STATEMENTS AND EXHIBITS

         ITEM 9.01 FINANCIAL STATEMENTS AND EXHIBITS

                  (a) Financial statements of business acquired.

                           Not Applicable.

                  (b) Pro forma financial information.

                           Not Applicable.

                  (c) Shell company transactions.

                           Not Applicable.

                  (d) Exhibits.

                           99.1     Trustee's report in respect of the February
                                    15, 2006 distribution to holders of the
                                    PreferredPLUS Trust Certificates Series
                                    ALL-1.
SIGNATURES

                  Pursuant to the requirements of the Securities Exchange Act of
                  1934, the registrant has duly caused this report to be signed
                  on its behalf by the undersigned, hereunto duly authorized.

                                              MERRILL LYNCH DEPOSITOR, INC.

         Date: February 28, 2006              By:      /s/ Stephan Kuppenheimer
                                                        -----------------------
                                              Name:    Stephan Kuppenheimer
                                              Title:   President
EXHIBIT INDEX

         99.1     Trustee's report in respect of the February 15, 2006
                  distribution to holders of the PreferredPLUS Trust
                  Certificates Series ALL-1.