Electrical Signals, the Cytoskeleton, and Gene Expression: a Hypothesis on the Coherence of the Cellular Responses to Environmental Insult - Communication in Plants: Neuronal Aspects of Plant Life - page 318

1992; Beelman and Parker 1995; Hentzke and Kolozik 1999). Thus, in our
heat-wounded tomato system, these rapidly synthesized transcripts are
not recruited into polysomes; instead they are degraded and never used for
protein synthesis. Thus heat-wounding of a mature leaf leads to a complex
chain of events occurring systemically in the youngest leaf (summarized in
Table 21.1).
21.3
Conclusions and Perspectives:
The “Help! It's a Virus” Hypothesis
Why would a cell make a copious amount of mRNA, but degrade it without
usingitforproteinsynthesis.Onepossibilityisthatithasnofunction-
butinthiscaseonewouldexpectittobesuchaneconomicdisadvantage
that it would have been weeded out by evolutionary pressure. Another
possibility is that the RNA, including viral RNA, is used for intercellular
(interorgan) signaling. This does not seem to be the case, however, since
virtually identical kinetics of transcript accumulation and disappearance
take place in other parts of the plant, so there is nowhere else for preexisting
mRNA to originate.
Our preferred hypothesis is that when the plant's integrity is compro-
mised, the plant does not know what hit it, but mounts a defense against
its most potent nemesis, a virus. Thus, essentially the instant its integrity is
compromised, the wounded leaf informs the entire plant via the hydraulic
surge and calcium channels open in all the cells capable of detecting such
changes in turgor. Subsequent inhibition of cytoplasmic streaming will
slow down virus movement within the cell; plugging of plasmodesmata
will compartmentalize the virus and prevent its spread; slowing of ribo-
some movement along preexisting polysomes will cause ribosome “pile-
up” on preexisting transcripts, thus protecting them from degradation by
RNase;increasingtherateoftranscriptionwillcauseanincreaseinerror
rate; flooding the cytoplasm with mismade RNA will lessen the availability
of ribosomes to translate viral RNA as well as to stimulate breakdown of
unprotected RNA. Within minutes, the plant has repelled its enemy, these
(automatic?) defensive reactions cease, and now, having identified the spe-
cific nature of the recent insult, the plant generates an appropriate response
and concludes “All is well with the world.”
References
Abler ML, Green PJ (1996) Control of mRNA stability in higher plants. Plant Mol Biol
32:63-71