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Fig. 21.2.
Polysome formation and protein synthesis. Intact tissue - 2-cm-long aged epicotyls
which had been excised 2 days earlier (to provide a cut surface for amino acid uptake and
to permit recovery from the wound) were left undisturbed (unwounded), while others were
nicked at the apex (wounded) for various time periods and then placed with their bases
in labeled methionine for 30 min and then the basal 2 mm was extracted and assayed for
protein synthesis. Similar samples were frozen in liquid nitrogen immediately after the end
of the wound treatment and used for polysome isolation. Excised tissue - similar epicotyls
were used, but the 2-mm basal piece was excised before the period of incubation and
assay for polysomes. Note that similar polysome formation occurred in both tissues after
wounding, but intact tissue exhibited a massive initial decrease in protein synthesis, while
excised tissue exhibited an ongoing increase (Reproduced from Davies et al. 1986, with
permission)
this we needed a system that would allow continuous monitoring of cyto-
plasmic streaming as well as ready uptake of exogenous compounds. For
these studies we chose the aquatic plants,
Elodea
and
Va l l i s n e r i a
(Davies
1990). We showed that wounding did, indeed, inhibit both protein synthe-
sis (Fig. 21.3a) and cytoplasmic streaming (Fig. 21.3b) in these plants, as
did treatment of unwounded plants with calcium plus ionophore, while
removal of calcium relieved the inhibition.
Taken together these data suggest that wounding causes an increase in
cytosolic calcium, which, in turn, inhibits both cytoplasmic streaming and
protein synthesis. The question then became “Does cytoplasmic streaming
dependonproteinsynthesis,doesproteinsynthesisdependoncytoplasmic
streaming, or are both processes co-dependent on some other event?” We
found no evidence for either interdependence, but did find evidence for
a possible co-dependence - both processes might be governed by protein
phosphorylation.
It has been known for some time that phosphorylation of myosin inhibits
cytoplasmic streaming in plants, while phosphorylation of EF2 inhibits
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