Different Molecular Mechanisms of Plant Thermotolerance at High and Extremely High Temperatures

Many heat shock factors (Hsfs) and heat-induced TAS1 TARGETS (HTTs) have been identified to play important roles in the heat tolerance of Arabidopsis. However, it remain unknown how these genes function in major crops such as Chinese cabbage. To examine the contributions of Hsfs and HTTs genes to thermotolerace, we transferred HsfA1 and HTT1 genes into Chinese cabbage. Overexpression of HsfA1 and HTT1 genes resulted in different levels of thermotolerance at two levels of high temeperatures. Under non-stressed heat (heat), the transgenic plants with HsfA1 or HTT1 showed the enhanced thermotolerance, whereas under stressed heat (extreme heat) these plants failed to show the enhanced thermotolerance. Both HsfA1a and HTT1 activated some Hsps such as Hsp70-14 under non-stressed heat, but did not upregulate the Hsps under extreme heat. Analysis of chloroplasts and reactive oxygen species (ROS) revealed that the molecular mechanisms that govern the thermotoleraces of Hsfs and HTTs genes are different under heat and extreme heat conditions.