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processed in the same way by the different organs and cells? Is the type of PCD in floral
organs also found in other plant tissues and organs?
5.13 Death signal for cell
In some species, pollination dramatically shortens floral lifespan. For example, orchid flow-
ers will last several months but senesce rapidly once pollinated. In several species, including
Petunia , tobacco, carnation, and orchids, senescence is mediated by the evolution of ethy-
lene following contact between pollen and the stigma surface, which precedes fertilization
(O'Neill, 1997). However, the exact nature of the primary signal resulting in ethylene evolu-
tion has not been established, although other PGRs and low-molecular-weight compounds
have been implicated (O'Neill, 1997). In carnations, ethylene produced from the pollinated
stigma is translocated, via the style and ovary, to the petals. Here, it upregulates ethylene
biosynthetic genes and induces the production of ethylene in the petals (ten Have and
Woltering, 1997). Once initiated, the evolution of ethylene becomes autocatalytic (Wood-
son and Lawton, 1988). This strongly suggests that promoters of the ethylene biosynthetic
genes respond to ethylene and contain ethylene-responsive elements (EREs). To date, this
has not been verified, although an ERE from a senescence and ethylene-regulated gene in
carnation bears similarities to the ERE from an ethylene-responsive fruit-ripening gene, E4,
suggesting commonality of transcription factors in these two processes (Deikman, 1997).
The response to ethylene is regulated by the production of ethylene receptors, but how this
regulation is achieved is not clear. In tomato an ETR1 (“ethylene-resistant”)-type ethylene
receptor was not transcribed in young flowers or senescent flowers, but only in mature flow-
ers (Payton et al., 1996). Furthermore, ethylene receptor expression may itself be regulated
by ethylene production. In pea, transcripts of an ERS (“ethylene response sensor”)-type
ethylene receptor were reduced when unpollinated flowers were treated with an inhibitor
of ethylene biosynthesis (Orzaez et al., 1999). So the balance between receptor produc-
tion and ethylene sensitivity is clearly regulated at several levels. Notably, in species in
which ethylene is a major regulator, ethylene-independent signals are also present. Dis-
ruption of ethylene signaling or biosynthesis in carnation and petunia results in delayed
floral death, but the flowers do eventually die (Michael et al., 1993). Perhaps it is these
endogenous signals that are active in species where ethylene is not a major regulator. Sev-
eral global transcriptome studies (e.g., Alstroemeria , Breeze et al., 2004; Iris , van Doorn
et al., 2003) have attempted to reveal the genes or pathways regulating floral degenera-
tion in these species; however, no clear patterns have yet emerged. A possibility is that
senescence and PCD are regulated posttranscriptionally, as argued by Thomas et al. (2003).
Perhaps a complex network of both transcriptional and posttranscriptional control is in-
volved, as is found in other fundamental cellular processes such as the cell cycle. If the
underlying ethylene-independent lifespan control in ethylene-sensitive species is common
to ethylene-insensitive species, then models such as Arabidopsis and Brassica or tomato
and petunia may serve as better examples to investigate these control networks. Langston
et al. (2005) used this approach to study DNA fragmentation in petunia, showing that the
ethylene induction of a 43-kDa nuclease (PhNUC1) was delayed in 35S:etr1-1 plants but
not eliminated. Likewise, in Arabidopsis , transcriptome and perhaps proteome analysis of
petal senescence in etr1-1 lines may be a fruitful line of enquiry. However, if ethylene-
independent regulation turns out to be species specific, then it is important to continue
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