Plant Molecular Biology 44: 387–397, 2000.
E. Lam, H. Fukuda and J. Greenberg (Eds.), Programmed Cell Death in Higher Plants.
© 2000 Kluwer Academic Publishers. Printed in the Netherlands.
, Shigemi Aoyagi
and Hiroo Fukuda
Botanical Gardens, Graduate School of Science,The University of Tokyo, Hakusan 3-7-1, Bunkyo-ku, Tokyo 112-
0001, Japan (
author for correspondence; e-mail: email@example.com);
Department of Biological
Sciences, Graduate School of Science, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan;
Biological Institute, Graduate School of Science, Tohoku University, Aramaki Aza Aoba, Aoba-ku, Sendai 980-
present address: Pharmacology Research, R&D, Kissei Pharmaceutical Co., Ltd., Kashiwabara
4365-1, Hotaka, Minamiazumi, Nagano 399-8304, Japan.
Key words: Ca
-dependent endonuclease, DNA hydrolysis, programmed cell death, Zn
Programmed cell death (PCD) involves hydrolysis of genomic DNA, which must be catalyzed by endonucle-
ase(s) capable of digesting dsDNA. Plants have two major classes of endonucleases active towards dsDNA,
-dependent endonuclease and Ca
-dependent endonuclease. Both classes are found among endonucleases
nominatedfor machineries of PCD in plants. Surveyof plant endonucleasesin relation to PCD leads to a possibility
that a different class of endonuclease reﬂects a different phase of PCD-associated DNA hydrolysis.
At the ﬁnal stage of programmed cell death (PCD),
genomic DNA suffers hydrolysis to be lost, which
strikes the last blow to a cell. During this hydroly-
sis process, DNA becomes fragmented and the ends
of DNA increase. The increased ends can be usually
detected by the TUNEL method, in which labeled
dUMP is added to each of 3
-hydroxyl termini of
DNA by using terminal deoxynucleotidyl transferase.
Thus, PCD-associated DNA hydrolysis must involve
endonucleolytic enzyme(s) that can cleave dsDNA
to produce 3
-hydroxyl termini. This article reviews
such plant endonucleases likely responsible for DNA
hydrolysis during PCD and proposes a possible re-
lationship between the type of endonuclease and the
phase of DNA hydrolysis.
Classiﬁcation of plant endonucleases
Many enzymes capable of cleaving native dsDNA in
an endonucleolytic manner have been puriﬁed from
plant sources and characterized. Biochemical prop-
erties of these enzymes are summarized in Table 1.
(Note that this table does not include unpuriﬁed en-
donucleases or endonucleases implicated in DNA re-
pair.) With the requirement for divalent cations and a
pH optimum as major criteria, they can be categorized
into the following two classes.
The ﬁrst class is Zn
is obviously equivalent to a group of plant nucle-
ase I deﬁned by Wilson (1975, 1982). Enzymes of
this class are characterized primarily by requirement
and by a pH optimum in the acidic range.
It should be noted, however, that Zn
varies apparently under different experimental condi-
tions, as pointed out by Wilson (1975). The Zn
is required for stabilization of the enzymes and for re-
activation of EDTA-inactivated enzymes but does not
always stimulate their activities when simply added to
the standard reaction mixture.
-dependent endonucleases prefer RNA and
ssDNA to dsDNA as a substrate. Although some
-dependent endonucleases were described to be
active speciﬁcally towards single-stranded nucleic