Biochemistry of Flower Senescence - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 67

leaves and flowers ( SAG23 ) or upregulation detected only in senescing leaves ( SAG28 and
SAG24 ). SG27 shows strictly leaf senescence-specific expression (Quirino et al., 1999).
Buchanan-Wollaston group has also identified a number of SR genes from B. na-
pus utilizing both differential library screening and subtractive hybridization (Buchanan-
Wollaston, 1994, 1997; Hanfrey et al., 1996; Buchanan-Wollaston and Ainsworth, 1997).
It is not known whether any of these genes are upregulated during petal senescence, but
similar to the Arabidopsis SAGs, they show differential patterns of expression during the
development of the leaf. The homolog of the Arabidopsis ( SAG12 ) gene has been cloned
in B. napus and shows a similar senescence-specified expression after the onset of leaf
senescence (Noh and Amasino, 1999b).
There is significant evidence that petal senescence in some flowers is a genetically
programmed event that requires de novo protein synthesis and transcription of few genes
(Woodson, 1987, 1994). In vitro translation of carnation petal mRNAs has revealed that
the initiation of petal senescence is associated with increases in certain mRNAs (Woodson,
1994). Differential screening of a cDNA library from senescing carnation petals has iden-
tified nine cDNAs that represent unique senescence-related mRNAs. A cysteine protease
( DCCP1 ) and three ACC-synthase ( DCACS1 , DCACS2 , and DCACS3 ) cDNAs were identi-
fied from carnation petals using reverse transcriptase-polymerase chain reaction (RT-PCR)
(ten Have and Woltering, 1997; Jones and Woodson, 1999). Only SR139 and DCCP1 tran-
scripts are detected in preclimacteric petals. Most of the SR genes are detected in petals at 5
days after harvest, corresponding to the first detectable ethylene production from the petals.
Eight of the eleven SR genes from carnation are flower specific, while low levels of SR139 ,
SR123 , and DCCP1 are detectable in leaves (Woodson et al., 1992, 1993). Identifying the
function of additional flower-specific SR genes will help to identify differences between
the regulation of vegetative and floral senescence.
4.11 Regulation of SR gene expression by stress
Little is known about the transcriptional regulation of the SR genes other than that their
mRNAs accumulate in one or more senescing tissues. While senescence is under develop-
mental regulation, it can be accelerated by certain environmental stresses (Nooden, 1988).
A few of the genes that have recently been identified as senescence related were previously
identified based on increased transcription, following exposure to various stresses including
drought and darkness (Park et al., 1998; Weaver et al., 1998). Studies by Weaver et al. (1998)
have shown that a number of the SAGs from Arabidopsis are induced by stresses. Exposing
excised leaves to darkness is one of the strongest inducers of SAG transcript accumulation.
While detaching leaves in the light did not result in leaf chlorosis, it did result in the en-
hanced expression of most SAGs (Weaver et al., 1998). Only about half of the SAGs tested
showed increased expression following dehydration treatments. In general, these responses
were influenced by the developmental stage of the leaves with strongest induction in older
leaves and no effect or only moderate induction detected in young leaves (Weaver et al.,
1998).
Physiological and biochemical changes such as decline in photosynthetic rate and
chlorophyll content have been used to identify stages of senescence, but artificial meth-
ods such as detachment and darkness that rapidly induce senescence have become
popular models for studying leaf senescence (Thomas and Stoddart, 1980). While the