Comprehensive analysis of BpHSP genes and their expression under heat stresses in Betula platyphylla

HSPs are a large family of proteins, including HSP100, HSP90, HSP70, DnaJ and various sHSPs, which are important for plants to resist abiotic stress. However, the precise functions of HSPs involved in heat stress have not been thoroughly elucidated in birch (Betula platyphylla). In this study, 21 HSP genes in birch (named BpHSP1-BpHSP21) were cloned and characterized. The expression patterns of BpHSPs in response to high temperature stress were monitored by quantitative real time-polymerase chain reaction (qRT-PCR). The results showed that most of the BpHSPs were significantly upregulated or downregulated in response to high temperature, indicating that these BpHSPs may be involved in birch heat stress. Among the BpHSPs, the transcription of BpHSP9 was upregulated in stems and leaves under heat stress throughout all study periods, suggesting that this protein may play an important role under heat stress. Furthermore, BpHSP9-overexpressing, RNAi-silencing and empty pROKII vector (as the control) birch plants were generated by transient transformation. Under heat stress, overexpression BpHSP9 transgentic birch plants increased reactive oxygen species (ROS) scavenging, the activities of some antioxidant enzymes and non-enzymatic antioxidants. These effects led to diminished O2− and H2O2 content and reduced electrolyte leakage rates. However, the results were reversed when BpHSP9 was transiently silenced by RNAi. Taken together, these results strongly suggest that BpHSP9 may plays an important physiological role in the heat stress tolerance of birch plants.