Programmed Cell Death during Plant Senescence - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 112

since maintenance of mitochondrial function may be required during this time. Changes in
levels of ROS and oxidative stress are often observed in association with plant programmed
cell death. However, it remains unclear whether increased ROS are generated actively as a
signal to trigger the PCD pathway, or whether ROS are a byproduct of the stress condition
that induces PCD. If the former possibility is correct, then one would expect to see a
downregulation of antioxidant enzymes such that the ROS signal is not quenched. Such
a downregulation has been observed during developmental PCD in barley aleurone cells
(Fath et al., 2001).
5.16.3 Specific mitochondrial proteins are associated with PCD
It is remarkable that of the eight gene products whose relative abundance increased fol-
lowing the induction of PCD, four are targeted to the mitochondrion. This may either
imply that mitochondrial proteins are particularly important during PCD or that proteins
within the mitochondrion are protected from the proteolytic degradation that is occurring
elsewhere in the cell. If the latter is true, then one would expect to see an increase in
abundance of all mitochondrial proteins relative to total cellular protein. The abundance
of several other mitochondrial proteins by Western blotting was assayed by Swidzinski
et al. (2002). The following proteins, representing different submitochondrial compart-
ments, were analyzed: porin/VDAC (outer membrane), adenine nucleotide translocase (in-
ner membrane), fumarase (matrix), and the E1a subunit of pyruvate dehydrogenase complex
(matrix). The increase in relative abundance of VDAC in both heat-treated and senescent
cells observed on two-dimensional gels was confirmed, but other mitochondrial proteins
did not follow the same pattern. For example, E1a is almost undetectable in heat-treated
cells but appears to be increased during senescence. Conversely, fumarase levels are in-
creased in the former and decreased in the latter. It appears that not all mitochondrial
proteins are maintained during PCD, but specific mitochondrial proteins (including super-
oxide dismutase (MnSOD), a voltage-dependent anion channel (VDAC) Hsr2, aconitase,
and lipoamide dehydrogenase may play important roles in the PCD pathway. The increased
relative abundance of lipoamide dehydrogenase, a subunit that is a part of several mi-
tochondrial multienzyme complexes including pyruvate dehydrogenase complex (PDC)
and 2-oxoglutarate dehydrogenase complex (2-OGDC) (Lutziger and Oliver, 2001), is in-
teresting given that other subunits of these complexes, such as the E1a subunit of PDC,
decrease in abundance during heat-induced PCD. This suggests the increased lipoamide
dehydrogenase content is not related to the function of mitochondrial dehydrogenase com-
plexes but may reflect an alternative function for lipoamide dehydrogenase. One possi-
bility is that changes in the redox state of lipoamide may form part of a redox signaling
mechanism.
5.16.4 Identification of a potential cell-to-cell
PCD signaling mechanism
One of the protein spots that increased in relative abundance during heat- and senescence-
induced PCD was identified as an EP1-like protein. This protein is 51% identical and 67%
similar to an extracellular glycoprotein, EP1, initially characterized in carrot suspension