The heat shock response is involved in thermotolerance in suspension-cultured apple fruit cells

Cultured apple cells (Malus domestica) were heat-treated at temperatures from 24 to 42 °C and measurements carried out of cell viability and gene expression immediately after heat treatment. The effects on gene expression of temperature, heat treatment duration, and recovery at 25 °C after heat treatment were also studied. An increase in expression of heat shock protein (hsp) mRNA transcripts was detected within 5 °C of the culture growth temperature. Maximum levels of expression, which were maintained for at least 3 h on return to 25 °C after heat treatment, occurred for heat treatments at 38 °C. Cell viability was positively correlated with the expression of hsps. An increase in the expression of hsp transcripts was also associated with a 1 h 38 °C heat pre-treatment that made the apple cells tolerant to a subsequent 1 h 42 °C lethal heat treatment. This acquisition of thermotolerance occurred in the first 24 h after heat treatment and was demonstrated by greater viability of cells that received a pre-treatment prior to a lethal treatment when compared to cells that had received only the lethal heat treatment. The responses to heat stress in cultured apple cells are similar to those in other plant tissues grown at similar ambient temperatures. They suggest that a shift in temperature above growth temperature and its magnitude, rather than the reaching of a threshold temperature, is the stimulus that induces the heat shock response.