Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

114 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
themselves from the “death signals.” Data from ethylene signaling in the endosperm (Gallie
and Young, 2004) indicate that at least in some systems this may be achieved by a difference
in sensitivity. However, in the TE system another mechanism seems to be operating. Here,
there is evidence for an inhibitor of proteasome-mediated protein degradation which is
released into the apoplastic space (Endo et al., 2001) and which protects living cells from
the hydrolases released during TE PCD. Vacuolation accompanies PCD in more than half
of the examples reviewed, and in generally rupture of the vacuole coincides with release
of hydrolases into the cytoplasm. This suggests a possible model (Fig. 5.5) that would
account for at least some of the systems reviewed. In this model an external signal activates
increases in cytoplasmic calcium, which in turn stimulates the fusion of small vacuoles
derived either from the ER or the golgi to form a large vacuole. This vacuole accumulates
hydrolytic enzymes. Its collapse, resulting either from ROS accumulation, or activation of
a proteolytic cascade, releases hydrolases into the cytoplasm. The hydrolase release results
in organellar breakdown and macromolecule degradation ending in cell death. This model
has clear parallels to animal autophagy although the vacuole rather than the lysosome is the
primary organelle involved.
There are many open questions in the field of PCD in plants. However, we have most of
the experimental tools in place and we should be able to answer these questions in the near
future. Identifying more PCD genes in plants and placing them along the different PCD
PGR signal
Protein P
A, P?
Golgi vesicles
carrying
hydrolases
[Ca 2+ ] increase
A, TE, AE, SI, L, P?
A, AE, L, LS, R, RC, SE,
SU, T, TE, SY
Small vacuoles
derived from
ER
A, R, SU, AE
Large central
vacuole
Vacuolar
collapse
Proteolytic
cascade
AOS
A, P, RC? S
Release of
hydrolases
A, R, P?
Change in
mitochondrial
electron transport
Reduction in
antioxidants?
A, AE, E, R, RC, SU
S, TE, LS, P
Digestion of DNA,
proteins, etc.
Cell death
Organellar
breakdown
E, L, TE,
LS, P
Fig. 5.5 Model showing major signals (black arrows) and cytological/biochemical events (open arrows) during
autophagic-type PCD in plants. Cell types/tissue in which these features have been reported are indicated as
follows: A, aleurone cells; AE, aerenchyma; L, leaf sculpting; LS, leaf senescence; P, petal senescence; R, Ricinus
endosperm; RC, root cap; S, starchy endosperm; SE, supernumerary embryos; SI, pollen tube during SI interaction;
SU, suspensor; SY, synergids; T, tapetum; TE, tracheary elements.