Programmed Cell Death during Plant Senescence - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 108

PCD was also studied in the petals of Antirrhinum majus , Argyranthemum frutescens ,
and Petunia hybrida , using DNA degradation and changes in nuclear morphology as pa-
rameters by Yamada et al. (2006). The petals exhibit loss of turgor (wilting) as a visible
symptom of PCD. DNA degradation, as shown on agarose gels, occurred in all species
studied, prior to visible wilting. The number of DNA masses in all the petals of a flower,
determined by flow cytometry, markedly increased in Argyranthemum and Petunia ,but
decreased in Antirrhinum . Many small DNA masses were observed in Argyranthemum
and Petunia . The surface of each small DNA mass stained with the lipophilic fluorochrome
3,3 -dihexyloxacarbocyanine iodide (DiOC6), indicating that these masses were surrounded
by a membrane. In Antirrhinum , in contrast, the chromatin fragmented into several small
spherical clumps that remained inside a large membranous structure. Nuclear fragmenta-
tion, therefore, did not occur in Antirrhinum , whereas nuclear fragmentation possibly was
a cause of the small DNA masses in Argyranthemum and Petunia . It is concluded that at
least two contrasting nuclear morphologies exist during PCD. In the first, the chromatin
fragments inside the nucleus, not accompanied or followed by nuclear fragmentation. In the
second, a large number of DNA masses were observed each enveloped by a membrane. The
second type was probably due, at least partially, to nuclear fragmentation (Yamada et al.,
2006).
5.15 PCD functional categorization
A functional categorization of PCD can be made on the basis of the fate of the cell con-
tents. Remobilization is central to leaf, sepal, and petal senescence (Thomas et al., 2003)
and in a different way also to tapetal PCD. But endothecium, synergid, antipodal cell, or
pollen-tube PCD is the selective death of unwanted cells. In green tissues, the chloroplast
is seen by some (Thomas et al., 2003) as the key participant in the senescence process, and
an early sign of senescence in green tissues is conversion of chloroplasts to gerontoplasts.
Sepals, the floral organ that most closely resembles leaves, senesce in a similar way: in
broccoli, sepal chlorophyll degradation is the first visual sign of senescence (Page et al.,
2001). Petals are, however, not usually green, and an early step in their development is a
conversion of chloroplasts to chromoplasts. This conversion has been compared with the
chromoplast/gerontoplast transition (Thomas et al., 2003) with the inference that petals
are most similar to senescent leaves. This agrees with the very early cell death seen in
flowers (Wagstaff et al., 2003) presumably associated with nutrient remobilization. How-
ever, in-silico comparison of transcriptome changes in senescent Arabidopsis leaves and
petals indicates that 25-30% of genes share similar patterns of expression. At a subcellu-
lar level, morphological changes to subcellular compartments during PCD are shared by
many different cell types and tissues (Rogers, 2005). VPEs are found in leaves, roots, and
flowers; ricinosomes are seen in seed and petal tissues; and caspase activity is detected in
pollen tubes undergoing SI, and in many nonfloral tissues during natural senescence and
also during pathogen responses (Sanmartın et al., 2005).
Progress in our understanding of PCD in plants has been rapid in the last 10 years, but
the key regulators of some types of floral organ senescence, such as petal senescence in
ethylene-insensitive species, remain obscure, except gladiolus (Arora and Singh, 2006). It
is also unclear whether regulation of petal senescence and PCD in these species is similar or
divergent. The latter is an important question to resolve before a good model for these species