Tobacco (Nicotiana tabacum) Bright Yellow-2 (TBY-2) cells undergo different fates when exposed for 10 minutes to heat stresses of different severity. A 35 degrees C treatment causes a homeostatic response (HRE) allowing cells to cope with the stress; 55 degrees C triggers processes leading to programmed cell death (PCD), which is complete after 72 h. We have used a proteomic approach to gain insight into the molecular mechanisms defining the fate of TBY-2 cells induced by these two heat stresses. Tandem mass spectrometry (MS/MS) and two-dimensional electrophoresis (2-DE) analysis revealed little overlap of differentially-accumulated proteins: the different severities of heat treatment induced the modulation of specific proteins, some of which are responsible for different cell fates. When the imposed heat shock is beyond a certain threshold, the overall reduced metabolism may be the result of a series of events involving gene expression and oxidative damage that would lead to PCD. Our data suggest that the down-accumulation of several proteins involved in cellular redox homeostasis could provide, until now, an unappreciated contribution to understanding how many partners are involved in promoting the redox impairment leading to PCD. Moreover post-translational modifications seem to play important regulatory roles in the adaptation of TBY-2 cells to different intensities of heat stress.