References

21 Wounding, electrical signals, the cytoskeleton, and gene expression 315
polysomes” exist in plants. They do. They are present in pea stems, pea
roots, corn endosperm, and every other tissue we have examined (Davies
et al. 1996), and they seem to be more translationally active than free
polysomes (Davies et al. 1998). Indeed, the cytoskeleton seems to play
a major role in the targeting, tethering, transport, and translation of mRNA
(Davies et al. 2001).
21.2.3
Calcium Channels, the Cytoskeleton, and Transcription
Ourworkinitiallyhadbeenontranslation(proteinsynthesis);researchin
this area seems to be less interesting (i.e., less funding is available) than
research on transcription. This need for funding motivated us to change
tracks and work on the model system for wound-evoked changes in transcription
– the tomato plant. With this system we showed that both electrical
stimulation and heat-wounding evoked the accumulation of proteinase inhibitor
(PIN) transcripts, but only heat stimulus could evoke accumulation
of calmodulin mRNA (Stankovic and Davies 1996, 1997a, 1997b).
While the bulk of workers looking at transcriptional responses to electrical
signals wait several hours to make their first measurements, we conjectured
that if plants generate ultrarapid signals, then they presumably
exist in order to evoke ultrarapid responses. In our earlier experiments we
isolated RNA within 15 min of wounding – and found some transcripts
showed maximum accumulation at that time, before declining to basal levels
and then rising again (Fig. 21.4). Interestingly, when we analyzed mRNA
levels in polysomes (the currently translated mRNA), we found that most
of the newly synthesized transcripts were not recruited into polysomes
(i.e., never translated), but were degraded. These same transcripts usually
increased again at a later time and these later-synthesized transcripts were
recruited into polysomes for translation. Indeed, many transcripts showed
this four-phase pattern: (1) a period of rapid accumulation, followed by (2)
aperiodofequallyrapiddecline,followedby(3)aperiodofslowaccumulation
along with (4) their recruitment into polysomes (Fig. 21.4). When we
shortened the wounding period to 2 min and used Northern blots (Davies
et al. 1997), quantitative PCR (Coker et al. 2005) and DNA microarrays (unpublished
data), we found over 20 transcripts reached a peak at that time
point – some increasing as much as tenfold in 2 min. How could transcript
accumulation occur so rapidly?
The simplest explanation for ultrarapid accumulation of transcripts is
through enhancing transcription by activating the major enzyme responsible,
RNA polymerase II (pol2). Evidence from the animal literature shows
that pol2 normally adds 100 or more nucleotides to the nascent transcript,