Plant Molecular Biology 41: 195–206, 1999.
© 1999 Kluwer Academic Publishers. Printed in the Netherlands.
Regulation of developmental senescence is conserved between Arabidopsis
and Brassica napus
Yoo-Sun Noh and Richard M. Amasino
Departmentof Biochemistry, University ofWisconsin, 433Babcock Drive, Madison, WI 53706-1544,USA(
Received 28 April 1999; accepted in revised form 17 August 1999
Key words: BnSAG12-1, BnSAG12-2, development, SAG12, senescence
SAG12 is a developmentally controlled, senescence-speciﬁc gene from Arabidopsis which encodes a cysteine pro-
tease. Using SAG12 as a probe, we isolated two SAG12 homologues (BnSAG12-1 and BnSAG12-2) from Brassica
napus. Structural comparisons and expression studies indicate that these two genes are orthologues of SAG12.
The expression patterns of BnSAG12-1 and BnSAG12-2 in Arabidopsis demonstrate that the senescence-speciﬁc
regulation of this class of cysteine proteases is conserved across these species. Gel-shift assays using the essential
promoter regions of SAG12, BnSAG12-1,andBnSAG12-2show that the extent of binding of a senescence-speciﬁc,
DNA-binding protein from Arabidopsis is proportional to the expression levels of these genes in Arabidopsis.
Therefore, the expression levels of these genes may reﬂect the afﬁnities of the senescence-speciﬁc DNA-binding
protein for the promoter element.
During leaf senescence nutrients are mobilized to de-
veloping parts of the plant, such as growing leaves,
developing ﬂowers, seeds or storage organs. The pro-
gression of natural leaf senescence is predictable and
conserved in most plant species. In senescing leaves,
photosynthesis declines and the recycling of nutri-
ents associated with breakdown of cellular structures
becomes predominant. Natural selection is likely to
have favored the evolution of pathways to reallocate
leaf nutrientswhen the photosyntheticoutput ofleaves
declines or when leaf nutrients are needed to support
the development of other parts of the plant (Bleecker,
1998). Most of the leaf nutrients are contained in
the chloroplasts and the ﬁrst signs of leaf senescence
are visible loss of chlorophyll and the breakdown of
chloroplast membranes (Gepstein, 1988). Mitochon-
drial, nuclear, and plasma membranes maintain their
integrity until the late stages of senescence. With
The nucleotide sequence data reported will appear in the
EMBL and GenBank Nucleotide Sequence Databases under the
accession numbers AF089848 and AF089849.
regard to the similarity of leaf senescence to pro-
grammed cell death, it has been proposed that the
associated cell death is a pleiotropic consequence of
a senescence program developed primarily for the
nutrient salvage (Noodén, 1988; Bleecker, 1998).
Senescence is a highly organized process under
genetic control. The breakdown of chloroplasts can
be inhibited by enucleation and by treatment with
RNA and protein synthesis inhibitors (Yoshida, 1961;
Yu and Kao, 1981). Consistent with the observa-
tion that gene expression is required for the progres-
sion of senescence, many senescence-associatedgenes
(SAGs) which increase at the mRNA level during
senescence have been isolated from several species
(reviewed in Buchanan-Wollaston, 1997; Gan and
Amasino, 1997; Nam, 1997). A few SAGs are speciﬁ-
cally expressed only during senescence whereas other
SAGs are expressed in young leaves and their rel-
ative abundance increases during senescence. Many
SAGs are rapidly induced by exposure to various
senescence-promoting factors including dehydration,
darkness, detachment, abscisic acid (ABA), and ethyl-
ene (Park et al., 1998; Weaver et al., 1998).