Stress of plant cellWe conducted our study according to the following basic tenets of general biology. It is evident that if environmental conditions vary within the tolerance range of living system functioning, then they do not disturb the specialized mechanisms controlling catalytic capacities of the cell provided by regulatory enzyme action. The deviations appearing in a plant organism relax through operating homeostatic mechanisms. After removing the disturbance, the system rapidly returns to its initial state without any evident residual consequences.
If the extent of influence exceeds the tolerance range, fast adaptive homeostatic responses become insufficient for maintaining an intact living system. In this case, a "higher level" mechanism of adaptation starts operating that is associated with the repression or activation of various genes. This adaptive act takes much more time compared to homeostatic mechanism performance. The cell takes hours and even days to realize pre-existing genome programs. We attempted to elucidate the cell alterations that maintain cell intactness when external influence exceeds the ability of homeostatic mechanisms to support vital activity, but newly appearing gene-dependent adaptive transformations have not been completed yet because of their slow operation.
One likely proposal is that the cell switches to a new discrete steady state with enhanced resistance during this phase of reaction. We propose to consider only this phase of cell reaction as a stress state. If the cell has no chance to switch its activity over to a new, more stable state, it will die. Additional opportunities appear in this special state for the adaptive process developing: reparation mechanisms start operating, thus providing time for realizing the slow, pre-existing "silent" program of the cell genome.
The cell transition to the new state, as it was elucidated by analysis of dosage relationships, represents a stepwise process since it proceeds within a rather narrow range of gradually increasing external factors. The responsive heat tolerance enhancement in the thylakoid membranes of pea chloroplasts that accompanies photosynthetic activity inhibition measured as the retardation of O2 evolution, thus indicating a drop in the energy supply in the cell. Analysis of other structural and functional changes occurring at this time suggests that the cell resists an unfavourable influence. The rate of photorespiration and the extent of the transthylakoid proton gradient, which both protect the pigment apparatus from light-induced damage, are enhanced. The system removing excess high-reactive and under-reduced O2 molecules is activated. Antioxidants protecting the lipid bilayer of cell membranes from peroxidation and other protectants (proline, polyamines, ets.) are accumulated in cells. Main protein synthesis stops with the simultaneous appearance of stressproteins at this stage of development of the cell response to the external factor. The structural and dynamic changes occurring during the phase of reactive enhancement of cell resistance have previously been described in detail.
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