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                                                                    EXHIBIT 10.5

                                 PROMISSORY NOTE

$10,000,000.00                    Dallas, Texas                 January 25, 1999

      FOR VALUE RECEIVED, the undersigned, DNA PLANT TECHNOLOGY CORPORATION, a
Delaware corporation (herein called the "Borrower"), hereby promises to pay to
the order of DNAP Holding Corporation, formerly known as BIONOVA U.S. INC., a
Delaware corporation (herein called the "Lender", which term shall also include
any subsequent transferee of this Note), the principal sum of Ten Million and
No/100 Dollars U.S. ($10,000,000.00), with interest on the unpaid balance
thereof from the date of advancement thereof pursuant to the Loan Agreement (as
hereinafter defined) until maturity at the rate or rates hereinafter provided,
both principal and interest payable as hereinafter provided funds in lawful
money of the United States of America at 6701 San Pablo Avenue, Oakland, CA
94608, or at such other place as from time to time may be designated in writing
by the holder of this Note.

      Borrower promises to pay interest on the principal amount hereof from time
to time outstanding, from the date hereof at the rate of 10.25% per annum (the
"Loan Rate"), compounded annually. Interest shall accrue but not be payable
until the unpaid principal amount hereof or any portion hereof which is prepaid
is due and payable in full to Lender. Such interest shall be payable in like
coin or currency as the principal amount hereof. Anything to the contrary
notwithstanding, if at any time the Loan Rate exceeds the Maximum Rate (as
hereinafter defined), the Loan Rate shall be limited to the Maximum Rate, but
any subsequent reductions in the Loan Rate shall not reduce the rate of interest
which the unpaid principal balance of this Note bears below the Maximum Rate
until such time as the total amount of interest accrued on this Note equals the
amount of interest that would have accrued under this Note if the Loan Rate had
at all times been in effect. All past due principal and/or interest hereunder
shall bear interest from maturity at the Maximum Rate. All interest calculations
hereunder shall be made on the basis of a 365 day year and the actual number of
days elapsed. Any payment made by Borrower to Lender hereunder shall be credited
first to accrued and unpaid expenses hereunder or under the Loan Agreement, next
to accrued and unpaid interest hereunder, and last to unpaid principal
hereunder.

      Unless sooner demanded by Lender as permitted under the Loan Agreement,
the entire unpaid principal amount hereof, together with interest accrued
hereon, shall, without notice, be due and payable in full to Lender in
immediately available funds on January 31, 2001. Upon the occurrence of an
"Event of Default" under the Loan Agreement, Lender may elect to declare and may
at any time declare the entire unpaid principal amount hereof, together with
interest accrued hereon, to be immediately due and payable.

      As provided in the Loan Agreement, this Note is secured by an Assignment
of Patents dated as of January 26, 1996 evidencing a transfer of an interest in
certain patent rights by Borrower to Lender as described therein.


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      This Note may be prepaid in whole or in part at any time without bonus or
penalty, subject to the terms and conditions of the Loan Agreement.

      This Note (a) is issued and delivered under that certain Loan Agreement
dated as of January 26, 1996 between Borrower and Lender (herein, as from time
to time supplemented, amended or restated, called the "Loan Agreement"), and is
the Note as defined therein, and (b) is subject to the terms and provisions of
the Loan Agreement, which contains provisions for payments and prepayments
hereunder and acceleration of the maturity hereof upon the happening of certain
stated events.

      It is the intent of Borrower and Lender in the execution of this Note and
all other instruments now or hereafter executed in connection with this Note to
contract in strict compliance with applicable usury law. In furtherance thereof,
Borrower and Lender stipulate and agree that none of the terms and provisions
contained in this Note, or in any other instrument executed in connection
herewith, shall ever be construed to create a contract to pay for the use,
forbearance or detention of money, interest at a rate in excess of the maximum
interest rate permitted to be charged by applicable law (the "Maximum Rate").
Neither the Borrower nor any guarantors, endorsers or other parties now or
hereafter becoming liable for payment of this Note shall ever be required to pay
interest on this Note at a rate in excess of the Maximum Rate and the provisions
of this paragraph shall control over all other provisions of this Note and any
other instruments now or hereafter executed in connection herewith which may be
in apparent conflict herewith. Lender expressly disavows any intention to charge
or collect excessive unearned interest or finance charges in the event the
maturity of this Note is accelerated. If the maturity of this Note shall be
accelerated for any reason or if the principal of this Note is paid prior to the
end of the term of this Note, and as a result thereof the interest received for
the actual period of existence of the loan evidenced by this Note exceeds the
Maximum Rate, Lender shall refund to Borrower the amount of such excess or shall
credit the amount of such excess against the principal balance of this Note then
outstanding. In the event that Lender shall collect monies which are deemed to
constitute interest which would increase the effective interest rate on this
Note to a rate in excess of the Maximum Rate, all such sums deemed to constitute
interest in excess of the Maximum Rate shall, upon such determination, at the
option of Lender, be either immediately returned to Borrower or credited against
the principal balance of this Note then outstanding, without further penalty to
Lender. By execution of this Note Borrower acknowledges that it believes the
loan evidenced by this Note to be non-usurious and agrees that if, at any time,
Borrower should have reason to believe that such loan is in fact usurious, it
will give Lender notice of such condition and Borrower agrees that Lender shall
have ninety (90) days in which to make appropriate refund or other adjustment in
order to correct such condition if in fact such exists. The term "applicable
law" as used in this Note shall mean the laws of the State of New York or the
laws of the United States, whichever laws allow the greater rate of interest, as
such laws now exist or may be changed or amended or come into effect in the
future.

      Should the indebtedness represented by this Note or any part thereof be
collected at law or in equity or through any bankruptcy, receivership, probate
or other court proceedings or if this Note is placed in the hands of attorneys
for collection after default, Borrower and all endorsers, 


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guarantors and sureties of this Note jointly and severally agree to pay in
addition to the principal and interest due and payable hereon reasonable
attorneys' and collection fees.

      Borrower and all endorsers, guarantors and sureties of this Note and all
other persons liable or to become liable on this Note severally waive
presentment for payment, demand, notice of demand and of dishonor and nonpayment
of this Note, notice of intention to accelerate the maturity of this Note,
protest and notice of protest, diligence in collecting, and the bringing of suit
against any other party, and agree to all renewals, extensions, modifications,
partial payments, releases or substitutions of security, in whole or in part,
with or without notice, before or after maturity.

      This Note and the rights and duties of the parties hereunder shall be
governed for all purposes by the law of the State of New York and the law of the
United States applicable to transactions within such state.

      This Note is given in renewal and extension, but not in novation or
extinguishment, of that certain Promissory Note of Borrower to the order of
Lender in the original principal amount of $10,000,000.00 dated as of January
26, 1996.


                                   DNA PLANT TECHNOLOGY CORPORATION


                                   By:  /s/ ARTHUR H. FINNEL
                                        ------------------------------------
                                        Name: Arthur H. Finnel
                                        Title: Chief Financial Officer


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