Heat Stress Responses in Cultured Plant Cells

Abstract

Using cultured pear (Pyrus communis cv Bartlett) cells, heat tolerance induced by heat shock was compared to that developed during growth at high temperature. After growth at 22°C, cells exposed to 38°C for 20 minutes (heat shock) showed maximum increased tolerance within 6 hours. Cells grown at 30°C developed maximum heat tolerance after 5 to 6 days; this maximum was well below that induced by heat shock. Heat shock-induced tolerance was fully retained at 22°C for 2 days and was only partly lost after 4 days. However, pear cells acclimated at 30°C lost all acquired heat tolerance I to 2 days after transfer to 22°C. In addition, cells which had been heat-acclimated by growth at 30°C showed an additional increase in heat tolerance in response to 39°C heat shock. The most striking difference between heat shock and high growth temperature effects on heat tolerance was revealed when tolerance was determined using viability tests based on different cell functions. Growth at 30°C produced a general hardening, i.e. increased heat tolerance was observed with all three viability tests. In contrast, significantly increased tolerance of heat-shocked cells was observed only with the culture regrowth test. The two types of treatment evoke different mechanisms of heat acclimation.are constants). If so, then heat acclimation induced by brief HS and that induced by prolonged but more moderate heat (e.g. 30C) would have similar mechanisms. However, results presented in this paper indicate that HS and prolonged exposure to 30°C increase the heat tolerance of pear cells in clearly different ways. MATERIALS AND METHODSAll experiments were conducted with suspension-cultured cells of pear (Pyrus communis cv Bartlett) which were used in our previous studies (13,14). The culture medium and most methods (culture maintenance, growth conditions and measurement, heat stress treatment, and viability tests) were as described before (13). All stock cultures and experimental controls were grown at 22C.Heat shock was administered to cells from 7-d-old suspension cultures using 5-ml aliquots in 1 x 10 cm test tubes. Heat treatment was accomplished in a water bath at the HS temperature; temperature equilibration in the 5-ml aliquots occurred within 3 min. The HS treatments lasted 20 min after which six of the 5-ml heat-shocked cell suspensions were transferred to empty, sterile 125-ml Erlenmeyer flasks and maintained at 22°C with shaking. Non-heat-shocked controls were subjected to the same sample handling procedures.Heat shock, i.e. brief exposure to supraoptimal temperature, alters gene expression and leads to increased heat tolerance in a wide range of organisms (10) 2Abbreviations: HS, heat shock; TTC, triphenyl-tetrazolium chloride. RESULTS AND DISCUSSIONHS-Induced Tolerance. Pear cells exposed to 38C for 20 min and then incubated for 24 h at 22C showed greatly increased tolerance of a subsequent heat stress treatment (Fig. 1) (8) and soybean seedlings (7). Schroeder (11) showed that heat tolerance was induced in avocado tissue cultu...

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