Document:

<PAGE>

                                                                    EXHIBIT 10.5

                  ASSIGNMENT, ASSUMPTION AND JOINDER AGREEMENT

          This ASSIGNMENT, ASSUMPTION AND JOINDER AGREEMENT (the "Agreement")
dated as of December 3, 2001 is entered into by and among Senesco Technologies,
Inc., a Delaware corporation (the "Company"), Stanford Venture Capital Holdings,
Inc., a Delaware corporation ("Stanford"), hereafter referred to as the
"Assignor", and those persons listed on Schedule A attached hereto
                                        ----------
(individually, an "Assignee" and collectively, the "Assignees").

                                   RECITALS
                                   --------

          WHEREAS, on November 30, 2001, the Company and Stanford entered into
that certain Securities Purchase Agreement (the "Purchase Agreement"), pursuant
to which Stanford purchased an aggregate of 1,142,858 shares of the Company's
restricted common stock, $0.01 par value per share (the "Common Stock"), and
warrants to purchase 1,000,000 shares of Common Stock (the "Warrants"), for an
aggregate purchase price of $2,000,000; and

          WHEREAS, on November 30, 2001, the Company and Stanford entered into
that certain Registration Rights Agreement (the "Registration Rights Agreement"
and, together with the Purchase Agreement, collectively referred to herein as
the "Financing Transaction Documents"), pursuant to which Stanford is granted
certain rights with respect to the registration of the Common Stock of the
Company issued or issuable upon exercise of the Warrants; and

          WHEREAS, pursuant to Section 5(a) of each Warrant Agreement dated
November 30, 2001, executed by the Company and issued to Stanford, with an
exercise price of $2.00 per share and $3.25 per share, respectively
(collectively, the "Warrant Agreements"), the Assignor desires to transfer,
assign and deliver an aggregate of 500,000 shares of Common Stock underlying the
Warrants (the "Transferred Stock") held in its name to the Assignees, each of
whom is an "accredited investor" as defined in Rule 501(a) of the Securities Act
of 1933, as amended, and as set forth on Schedule A attached hereto; and
                                         ----------

          WHEREAS, pursuant to Article III Section 3.2 of the Registration
Rights Agreement, the parties hereto desire that the terms of the Registration
Rights Agreement shall be binding upon and inure to the benefit of their
respective assigns; and

          WHEREAS, pursuant to Section 5 of the Warrant Agreement governing the
transfer of the Warrants, which sets forth the restrictions and rights on such
transfers, the Registered Holder (as defined therein) may assign to any
accredited investor, all of the rights of the Assignor contained therein,
subject to the Assignees' agreement to accept such obligations; and

          WHEREAS, the Assignor desires to transfer, assign and deliver to the
Assignees, and such Assignees desire to assume all of the Assignor's obligations
under the Financing Transaction Documents, as applicable, with respect to the
Transferred Stock.
<PAGE>

                                   AGREEMENT
                                   ---------

               NOW, THEREFORE, in consideration of the foregoing, the mutual
covenants and agreements herein contained and other good and valuable
consideration, the receipt and adequacy of which is hereby acknowledged, the
parties hereby agree as follows:

          1.   All capitalized terms used in this Agreement that are not defined
herein shall have the meaning set forth in the applicable Financing Transaction
Documents.

          2.   Each Assignee hereby confirms that such Assignee has received a
copy of and is fully familiar with each of the Financing Transaction Documents.

          3.   The Assignor and each Assignee hereby represents and warrants
that such Assignee is an accredited investor.

     4.   The Assignor hereby assigns all of its rights with respect to the
Transferred Stock to each respective Assignee and such Assignee hereby assumes
all of Assignor's liabilities and obligations with respect to the Transferred
Stock, and agrees to perform, comply with and be subject to and bound by,
jointly and severally, each of the Financing Transaction Documents, as
applicable.

     5.   As of and from the date hereof, each Assignee shall be considered, and
deemed to be, for all purposes, a Purchaser under the Purchase Agreement, as
fully as though such Assignee had executed each such document as an initial
Purchaser thereunder and confirms its obligations under each such document, all
in accordance with the terms thereof, as applicable.

     6.   As of and from the date hereof, each Assignee shall be considered, and
deemed to be, for all purposes, a Holder under the Registration Rights Agreement
as fully as though such Assignee had executed the Registration Rights Agreement
as an initial Holder thereunder and confirms its obligations under the
Registration Rights Agreement, all in accordance with the terms thereof.

          7.   Any notice which may or is required to be given to each Assignee
pursuant to the Financing Transaction Documents shall be given in accordance
with the terms of each such document, as applicable, and shall be addressed to:

          Daniel T. Bogar, William R. Fusselmann, Osvaldo Pi and
          Ronald M. Stein:

          c/o Stanford Venture Capital Holdings, Inc.
          201 S. Biscayne Blvd., 12th floor
          Miami, Florida 33131
          Attn.: President
<PAGE>

          With a copy to:

          c/o Stanford Venture Capital Holdings, Inc.
          5050 Westheimer
          Stanford Financial Group Bldg.,
          Houston, Texas 77056
          Facsimile: (713) 964-5245.
          Attention: P. Mauricio Alvarado, Esq., General Counsel

          8.   This Agreement may be executed in counterparts, each of which
shall be deemed an original and all of which taken together, shall constitute
one and the same instrument.

     9.   This Agreement shall be governed by and construed in accordance with
the laws of the State of Delaware without regard to the principles of conflicts
of laws of such state.

     10.  The Assignor and each Assignee shall, at any time and from time to
time after the date hereof, upon the request of the other, execute, acknowledge
and deliver all such further acts, deeds, assignments, transfers, conveyances,
powers of attorney and assurances, and take all such further actions, as shall
be necessary or desirable to give effect to the transactions hereby consummated.

                                     *****
<PAGE>

          IN WITNESS WHEREOF, the undersigned parties have duly executed and
delivered this Assignment, Assumption and Joinder Agreement as of the date first
above written.

                                THE COMPANY:
                                -----------

                                SENESCO TECHNOLOGIES, INC.

                                By:
                                   --------------------------------------------
                                   Name:  Bruce C. Galton
                                   Title:  President and Chief Executive Officer

                                THE ASSIGNOR:
                                ------------

                                STANFORD VENTURE CAPITAL HOLDINGS, INC.

                                By:
                                   --------------------------------------------
                                Name:
                                Title:

     [Signature Page to the Assignment, Assumption and Joinder Agreement]
<PAGE>

          IN WITNESS WHEREOF, the undersigned parties have duly executed and
delivered this Assignment, Assumption and Joinder Agreement as of the date first
above written.

                                THE ASSIGNEES:
                                -------------

                                ------------------------------------
                                Name: Daniel T. Bogar

                                ------------------------------------
                                Name: William R. Fusselmann

                                ------------------------------------
                                Name: Osvaldo Pi

                                ------------------------------------
                                Name: Ronald M. Stein

     [Signature Page to the Assignment, Assumption and Joinder Agreement]
<PAGE>

                                  SCHEDULE A
                TO ASSIGNMENT, ASSUMPTION AND JOINDER AGREEMENT
                         DATED AS OF DECEMBER 3, 2001
<TABLE>
<CAPTION>
--------------------------------------------------------------------------------------------------------------
                                        Number of Warrant       Number of Warrant
                                          Shares to be            Shares to be
                                        Transferred             Transferred
                                        to Assignee Upon        to Assignee Upon
               Assignor           Execution of Agreement          Execution of            Assignee
               --------                  $2.00 Warrants         Agreement                 --------
                                         --------------           $3.25 Warrants
                                                                  --------------
--------------------------------------------------------------------------------------------------------------
<S>                                      <C>                      <C>                     <C>
        Stanford Venture
Capital Holdings, Inc.                   62,500                      62,500              Daniel T. Bogar
--------------------------------------------------------------------------------------------------------------

                                         62,500                      62,500           William R. Fusselmann
--------------------------------------------------------------------------------------------------------------

                                         62,500                      62,500                Osvaldo Pi
--------------------------------------------------------------------------------------------------------------

                                         62,500                      62,500              Ronald M. Stein
--------------------------------------------------------------------------------------------------------------
</TABLE><PAGE>
                                                                   EXHIBIT 4.1
<Table>
<S>                               <C>                                                                             <C>
                                          SPECIMEN OF CLASS A COMMON STOCK OF THE REGISTRANT

         [NUMBER]                                                                                             [SHARES]
         CA                                          INCORPORATED UNDER THE LAWS
                                                       OF THE STATE OF DELAWARE

        CLASS A COMMON STOCK                             UNITEDGLOBALCOM, INC.                            CUSIP 913247 50 8

THIS CERTIFICATE IS TRANSFERABLE IN                                                             SEE REVERSE FOR CERTAIN DEFINITIONS
NEW YORK, NY AND RIDGEFIELD PARK, NJ

        THIS CERTIFIES THAT

        IS THE OWNER OF

                       FULLY PAID AND NON-ASSESSABLE SHARES OF THE CLASS A COMMON STOCK, $.01 PAR VALUE, OF

                                                        UNITEDGLOBALCOM, INC.

transferable on the books of the Corporation by the holder hereof, in person or by duly authorized attorney, upon surrender of this
Certificate properly endorsed.  This certificate is not valid unless countersigned by the Transfer Agent and registered by the
Registrar.

        WITNESS the facsimile seal of the Corporation and the facsimile signatures of its duly authorized officers.

/s/ GENE W. SCHNEIDER                                                           DATED:

CHIEF EXECUTIVE OFFICER                                  [SEAL]                 COUNTERSIGNED AND REGISTERED

/s/ ELLEN P. SPANGLER                                                                   CHASEMELLON SHAREHOLDER SERVICES, L.L.C.

SECRETARY                                                                                          TRANSFER AGENT AND REGISTRAR,

                                                                                  BY
                                                                                                           AUTHORIZED SIGNATURE
</Table>

<PAGE>

                             UNITEDGLOBALCOM, INC.

         The Corporation will furnish without charge to each shareholder who so
requests a full statement of the designations, preferences, limitations and
relative rights of each class of stock or series thereof of the Corporation and
the variations in the relative rights and preferences between the shares of any
series of preferred stock, so far as the same have been fixed and determined,
and the authority of the board of directors to fix and determine the relative
rights and preferences of any series of preferred stock. Such requests may be
made to the Corporation or to the transfer agent.

                                 ABBREVIATIONS

        The following abbreviations, when used in the inscription on the face
of this certificate, shall be construed as though they were written out in full
according to applicable laws or regulations:

<Table>
        <S>                                             <C>
        TEN COM -- as tenants in common                 UNIF GIFT MIN ACT -- _____________ Custodian ___________
        TEN ENT -- as tenants by the entireties                                 (Cust)                  (Minor)
        JT TEN  -- as joint tenants with right of                            under Uniform Gifts to Minors
                   survivorship and not as tenants                           Act __________________
                   in common                                                           (State)
</Table>

   Additional abbreviations may also be used though not in the above list.

FOR VALUE RECEIVED, __________________ hereby sell, assign and transfer unto

PLEASE INSERT SOCIAL SECURITY OR OTHER
   IDENTIFYING NUMBER OF ASSIGNEE
[                                    ]

________________________________________________________________________________
(PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS, INCLUDING ZIP CODE, OF ASSIGNEE)

________________________________________________________________________________

________________________________________________________________________________

________________________________________________________________________ Shares
of the Class A Common Stock represented by the within Certificate, and do hereby
irrevocably constitute and appoint _____________________________________________
Attorney to transfer the said Shares on the books of the within named
Corporation with full power of substitution in the premises.

Dated _____________________________

                                     Signature(s):

                                  X  ______________________________________

                                  X  _______________________________________
                             NOTICE: THE SIGNATURE OF THIS ASSIGNMENT
                                     MUST CORRESPOND WITH THE NAME AS WRITTEN
                                     UPON THE FACE OF THE CERTIFICATE IN
                                     EVERY PARTICULAR, WITHOUT ALTERATION OR
                                     ENLARGEMENT OR ANY CHANGE WHATEVER.

 Signature(s) Guaranteed

By ________________________________________
   THE SIGNATURE(S) MUST BE GUARANTEED BY
   AN ELIGIBLE GUARANTOR INSTITUTION
   (BANKS, STOCKBROKERS, SAVINGS AND LOAN
   ASSOCIATIONS AND CREDIT UNIONS WITH
   MEMBERSHIP IN AN APPROVED SIGNATURE
   GUARANTEE MEDALLION PROGRAM), PURSUANT
   TO S.E.C. RULE 17Ad-15.

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