The response of many animal and plant systems to heat shock has been examined almost exclusively at the upper temperature ranges. Maize grows normally over a broad temperature range (exceeding 15 C - 35 C) and offers the opportunity to investigate the response not only to heat shock, but to a range of upward or downward temperature shifts which may still be within the "normal" growing range. We have examined a series of temperature shift regimes to ascertain whether heat shock polypeptides (HSPs) were specifically produced following a "heat shock" (40-45 C for Oh43 seedlings grown at 27 C) or whether they represented a general synthetic response to any upward shift in the culture temperature.
Seedlings of Oh43 were grown at 17 C and at 27 C until plumules were 1 to 2 cm long, at which time they were subjected to a 10 C increase in the incubation temperature for one hour. One- and two-dimensional PAGE separations and fluorographic analysis revealed that, while each temperature regime resulted in synthesis of some unique polypeptides, both 10 C shifts yielded enhanced synthesis of the same six Mr classes of HSPs. However, fewer isoelectric variants of these HSPs were noted in the 17 to 27 C shifts.
We subsequently investigated the effects of prolonged exposures to a shift temperature to determine if these conditions led to continued HSP synthesis, or to the establishment of a stable, temperature-specific polypeptide synthetic pattern (i.e., acclimatization). Seedlings grown as before were subjected to the same 10 C upward shift, but for 3, 12 or 24 hours (which included a two-hour labelling period). In both the 17 to 27 C and in the 27 to 37 C shifts, a 3-hour incubation resulted in new and/or enhanced synthesis of the six Mr classes of polypeptide described previously. By 12 hours at the shift temperature, very little synthesis of these polypeptides was evident, and by 24 hours, the synthetic patterns were similar, but not identical to the control patterns. These observations suggest that these HSPs may represent transition proteins synthesized during the rapid transfer from one incubation temperature to another. In experiments where the seedlings were shifted down from 27 to 17 C for 3, 12 or 24 hours, no changes were detected in one-dimensional electropherograms, suggesting that this enhanced synthesis is specific to upward temperature shifts.
To further investigate the possibility of an "acclimatization" in polypeptide synthetic pattern, seedlings grown at 27 C were shifted down to 21, 18 or 15 C for 6, 12, 24 and 48 hours, and then returned to 27 C, where plumules were labelled for two hours prior to extraction. Results indicate that return to the control temperature following a downward temperature shift leads to a time- and temperature-dependent enhancement of HSP synthesis. For example, 24 hours at 21 C are required before a response is observed, while synthesis of this same group of polypeptides is noted after only 6 hours at 18 C or 15 C. Thus, the greater the interval over which the seedlings are shifted, the shorter the time required at the shift temperature before response is noted when seedlings are returned to the control temperature, suggesting that the seedling is responding to cumulative 'heat units,' as distinct from changes in temperature.
It appears, therefore, that prolonged exposure to an increased or decreased incubation temperature leads to acclimatization as monitored by changes in polypeptide synthetic patterns. The observations in this report and in the previous one, that new and/or enhanced polypeptide synthesis occurs following a variety of temperature shifts and not only in response to high temperature shifts, suggests that it may be more appropriate to refer to these polypeptides as temperature shift polypeptides or proteins (TSPs) of which the HSPs would be a specific subclass induced during high temperature stress.
C. L. Baszczynski, J. G. Boothe, B. G. Atkinson and D. B. Walden
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