**7. Conclusions**

they could improve embryo tolerance to unfavorable environmental conditions during

We believe that the absence of discrimination of non-heat shock mRNA translation during heat shock is specific to embryo tissues and could be considered an additional mechanism facili‐ tating seed adaptation to unfavorable environmental conditions and successful germination.

Induction of heat shock proteins synthesis is a universal feature of the response to heat shock. According to our data, all tissues isolated from the embryos of dormant recalcitrant horse chestnut seeds responded to heat shock not only by continuation of non-heat shock protein synthesis but also by induction of similar sets of heat shock proteins. Two observations are of interest. We did not observe any dramatic changes in the set of heat shock proteins synthesized by isolated axes in the response to heat shock in the course of stratification, which evidently facilitate seed deep dormancy release. Moreover, heat shock proteins synthesis was readily detected in axes excised from non-stratified seeds, that is, it did not depend on seed capability of germination. This indicates independence of heat shock proteins synthesis at heat shock in the course of stratification of the embryo physiological state and its capability of germination. At the same time, some of our data indicate that stratification still somehow affected embryo physiological state. Thus, in the course of stratification, sensitivity of isolated axis growth to abscisic acid and indol-3-acethyc acid decreased [53], some characteristics of the proteome changed [50], and sensitivity of isolated axis translation to heat shock changed as well. However, this did not affect embryo tissue capacity to respond to heat shock. It is likely that signals providing for dormancy state, its release, and seed germination do not interact with signals leading to heat shock proteins synthesis induction. Furthermore, heat shock proteins' gene expression in isolated axes of dormant recalcitrant horse chestnut seeds was not depend‐ ent on transcription and was controlled predominantly on the level of translation. This means that all components required for the complex molecular mechanism of heat shock proteins gene expression were present in axis cells of mature seeds and were evidently produced still during seed development, may be under the influence of elevated temperatures. After mature seed falling, this mechanism is retained in the cells in the functionally active state and is capable of a rapid initiation of heat shock proteins synthesis in response to heat shock or another stress. However, the realization of this mechanism of heat shock proteins accumulation under natural conditions of stratification seems not very probable because the rate of protein synthesis under low temperature is low and heat signal is absent. Nevertheless, horse chestnut seeds survive successfully the period of long cooling (18–22 weeks), retaining the high water content in their cells and thus they are well adapted to overcome or correct damages arising under these conditions. However, according to current knowledge, heat shock proteins just fulfill this protective function. Therefore, it might be that, in mature dormant horse chestnut seeds, some amounts of required heat shock proteins are already present. These heat shock proteins could be synthesized and accumulated under the influence of elevated temperatures in the embryo cells during seed development or after their falling, that is, in response to heat shock, and they were preserved in the cells after seed entry into deep dormancy in metabolically active state; they could improve embryo tolerance to unfavorable environmental conditions during stratification and thus increase seed viability. This suggestion is supported by our observation

stratification and thus increase seed viability.

328 Abiotic and Biotic Stress in Plants - Recent Advances and Future Perspectives

Plants cannot avoid the exposure to different abiotic factors but adapt morphologically and physiologically by some other mechanisms. Almost all stresses induce the production of groups of proteins called dehydrins and heat shock proteins (HSPs), which comprise several evolutionarily conserved protein families. Accumulation of dehydrins can be induced not only by drought but also by cold, salinity, and treatment with abscisic acid.

Since HSPs accumulation at heat shock in the cells of plant vegetative organs and seedlings was correlated with the development of plant tolerance to subsequent action of lethal tem‐ peratures, it was suggested that the response to heat shock is a manifestation of molecular mechanisms providing cell heat tolerance.

Heat stress induces the known genes for HSPs and chaperones: *hspA, groES, groEL1, groEL2, dnaJ, htpG, dnaK2, clpB1*, and *htrA* for protease, *sigB* for the σ-factor of RNA polymerase, *hik34* for sensory histidine kinase, *sodB* for superoxide dismutase, and some other genes. Using DNA microarrays, it has been shown that none of the aforementioned genes is induced by heat stress specifically [73, 74]. Expression of these genes is induced by high osmolarity, NaCl, oxidative stress (H2O2), high light, and UV-B. This phenomenon has been observed earlier. However, before the application of DNA microarrays, the information on HSPs that respond to various stresses has been fragmented and limited to studies of individual genes. Now it is clear that the genes whose transcription is specifically induced by high temperatures are classified as unknown. The entire list of genes of the genuine HSPs is limited to following titles: *sll0441, sll0688, sll1106, sll1884, slr0852, slr0095*, and *slr1597* [75]. The remaining genes that are induced by heat shock belong rather to a group of the general-stress-responsive (GSR) genes [76], and HSP may be renamed as general stress protein (GSP) [76, 77].
