References

312 E.Davies,B.Stankovic
21.2
Evidence for Our Hypothesis
Sincethispaperisahypothesisbasedonlimitedevidence,wewillfocus
on those aspects of research conducted in our laboratory, supplemented
where necessary by work from others.
21.2.1
Electrical Signals and Translation
We began to suspect a role for electrical signals and their involvement in
wound-modulated gene expression in plants (aged etiolated pea epicotyls)
when we found that polysome formation occurred very rapidly both above
and below the site of wounding, implying that a rapidly generated bidirectionally
transmitted signal, presumably an electrical signal, was involved
(Davies and Schuster 1981). Since there was only a slight increase in the
poly(A)RNA content of the polysome fraction, but a huge increase in the capacity
to make protein both in vivo and in vitro, we assumed that wounding
was increasing translation, presumably by increasing ribosome initiation
(Davies and Schuster 1981).
This interpretation received support from subsequent experiments,
which showed that the reinitiating ability in vitro of ribosomes isolated
from wounded peas was markedly increased (Ramaiah and Davies 1985).
However, it was subsequently disproved when the experiments were redone
so that protein synthesis in vivo was conducted using totally intact (unwounded)
tissue (Davies et al. 1986), rather than in tissue slices (wounded)
as had been done earlier (Davies and Schuster 1981; Schuster and Davies
1983). Using the totally intact systemwe showed that, even though polysome
formation was occurring, protein synthesis in vivo declined markedly
(Fig. 21.2). This effect of wounding (formation of polysomes, inhibition
of protein synthesis) could be mimicked in unwounded tissue by cycloheximide
(Davies at al. 1986), a known inhibitor of elongation/termination.
It became apparent that wounding was blocking ribosome movement
along messenger RNA (mRNA), and we now suspect that this results from
phosphorylation of the translation factor EF2, presumably via a calciumdependent
protein kinase.
21.2.2
Calcium, the Cytoskeleton, and Translation
We reasoned that if influxes of calcium occurred as part of the wound
signal (AP), then cytoplasmic streaming should be inhibited, but to check