Expression analysis of PtHsp90 genes under heat stress. A. Conditions of heat stress. Seedlings were heated to 37°C for 3 h (pretreatment), returned to 23°C for 2 h, heated to 45°C for 3 h (treatment), and then allowed to recover for 2 h. 1, control; 2, 30 min after pretreatment at 37°C; 3, 2 h after pretreatment at 37°C; 4, 1 h after recovery at 23°C; 5, 30 min after treatment at 45°C; 6, 2 h after treatment at 45°C; 7, 2 h after recovery at 23°C. B-E. The corresponding relative densities of the RT-PCR signals generated from data in Additional file 12B, showing four different types of heat stress responses. Type I is induced by both pretreatment at 37°C and treatment at 45°C (B); Type II is induced by pretreatment at 37°C but is not affected by treatment at 45°C (C); Type III is not affected by pretreatment at 37°C but is negatively regulated by treatment at 45°C (D); and type IV is not affected by either pretreatment at 37°C or treatment at 45°C (E).