Are polyamines involved in the synthesis of heat-shock proteins in cell suspension cultures of tobacco and alfalfa in response to high-temperature stress?

Exposure of plants to sublethal doses of heat stress induces a complex set of responsive mechanisms in order to protect cells and organisms from severe damage and enable them to survive. In the present study, the contents of polyamines (putrescine, spermidine and spermine) and the patterns of heat-shock protein (HSP) production were determined in two differently thermosensitive suspension-cultured cell lines (Nicotiana tabacum L. cv. Bright Yellow 2 (BY2) and Medicago varia cv. RAMBLER) subjected to various elevated temperatures for 3 h after gradual temperature increase. The aim of our investigations was the evaluation of a potential role of polyamines in regulating the synthesis of HSPs in plant cells under heat stress. The more heat-tolerant tobacco BY2 cells exhibited moderate ratios of free putrescine to free spermidine and contained relatively high titres of trichloroacetic acid- (TCA) soluble conjugated polyamines. HSP synthesis in tobacco cells could be detected up to 46 °C. In contrast, excessive accumulation of free putrescine and relatively low levels of TCA-soluble conjugated polyamines compared with those of the corresponding free forms characterised the more heat-susceptible alfalfa cell line. Under high-temperature conditions, the synthesis of HSPs ceased at 40 °C in alfalfa cells. Leakage of soluble polyamines from the cells was low in tobacco over the whole range of temperature investigated, but massive in alfalfa between 40 and 46 °C. The inhibition of putrescine biosynthesis in alfalfa cells by D-arginine decreased markedly the endogenous free putrescine level, reduced polyamine leakage between 40 and 42 °C and improved HSP synthesis. The results indicate that the polyamine metabolic status of cells might influence the extent of HSP synthesis under heat stress by affecting the integrity and properties of cell membranes.