Agriculture Reference
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5.12 PCD during petal senescence
Recently, there has been much controversy over the use of the terms “senescence” and
“programmed cell death” (PCD), especially with regard to leaves (Thomas et al., 2003; van
Doorn and Woltering, 2004). In flowers it seems that the distinction is largely unnecessary.
The deterioration of a flower is certainly programmed, and is not a reversible process and
inevitably leads to flower senescence. Thus, the terms used essentially interchangeably,
using PCD more often when discussing the death of individual cell types, and senescence
for whole organs.
Selective removal of reproductive structures is not unique to plants. However, unlike in
mammals, both male and female reproductive structures in plants are only retained while
they are needed, and are developed de novo in perennial species during the following
reproductive cycle. The longevity of the flower is species specific and carefully tailored to
its ecological requirements. This is important because the flower can be a substantial sink on
the plant's resources, and as such, is energetically expensive to maintain beyond its useful
life (Ashman and Schoen, 1994). Another important reason for floral death after pollination
is to remove it from the population so that it does not compete for pollinators with the
remaining blooms. One of the key triggers for petal death is pollination, which initiates a
series of physiological events, orchestrated by plant growth regulators (PGRs). Ethylene is a
clear regulator of petal senescence in some species (Stead and van Doorn, 1994); however, in
other species including lilies such as
Hemerocallis
(daylilies),
Gladiolus
, and
Alstroemeria
,
it appears to play little or no part (Woltering and van Doorn, 1988; Wagstaff et al., 2005;
Arora et al., 2006). How petal senescence in these species is triggered and orchestrated
remains unknown. Given the failure to find a common regulator for these species and their
taxonomic diversity, it seems likely that several interrelated mechanisms may be at play.
Resource allocation has been one trigger proposed, and indeed removal of lower flowers in a
cyme can lead to increased longevity of the first flower (Chanasut et al., 2003). However, this
is clearly not a full explanation for all ethylene-insensitive species. An important feature of
floral death is that the different floral organs play very different roles. Hence, their lifespan
needs to be appropriately coordinated. Likewise, the purpose and fate of the dying cells
depends on the organ and tissues involved. At a whole organ level, petals, anthers, and stigma
are no longer required following pollination, whereas the ovary will mature to contain the
developing seeds. In many species, there is also a mechanism for rescuing resources from
the degenerating organs such as petals, and diverting them to other parts of the plant such as
the developing ovary (Stead and van Doorn, 1994). At a tissue and cell level, the situation
is even more complex as there is a requirement for some reproductive tissues and cells to
die to ensure correct development. For example, the tapetum must degenerate for pollen to
develop properly, and synergid cells must die to allow fertilization. However, the fate of the
dead cells is very different. In the case of the tapetum, cell contents are used to form the coat
of the pollen grains, whereas removal of synergid cells is required for fertilization to occur
(Christensen et al., 2002). Some types of cell death in floral organs also depend on specific
genetic interactions. PCD occurs as a result of incompatible pollination events (Thomas
and Franklin-Tong, 2004) and also as a result of defects in pollen development displayed
in cytoplasmic male sterile lines (Balk and Leaver, 2001). Thus, important questions with
regard to cell death in reproductive organs are the following: How do the cells perceive
and respond to death signals, or how do they know when to die? Are the primary signals
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