HsfAld, a Protein Identified via FOX Hunting Using Thellungiella salsuginea cDNAs Improves Heat Tolerance by Regulating Heat-Stress-Responsive Gene Expression

Thellungiella salsuginea （formerly T. halophila）, a species closely related to Arabidopsis （Arabidopsis thali-ana）, is tolerant not only to high salt levels, but also to chilling, freezing, and ozone. Here, we report that T. salsuginea also shows greater heat tolerance than Arabidopsis. We identified T. salsuginea HsfAld （TsHsfAld） as a gene that can confer marked heat tolerance on Arabidopsis. TsHsfAld was identified via Full-length cDNA Over-eXpressing gene （FOX） hunt-ing from among a collection of heat-stress-related T. salsuginea cDNAs. Transgenic Arabidopsis overexpressing TsHsfAld showed constitutive up-regulation of many genes in the Arabidopsis AtHsfA1 regulon under normal growth tempera-ture. in Arabidopsis mesophyll protoplasts, TsHsfAld was localized in both the nucleus and the cytoplasm. TsHsfAld also interacted with AtHSP90, which negatively regulates AtHsfAls by forming HsfA1-HSP90 complexes in the cytoplasm. It is likely that the partial nuclear localization of TsHsfAld induced the expression of the AtHsfAld regulon in the transgenic plants at normal temperature. We also discovered that transgenic Arabidopsis plants overexpressing AtHsfAldwere more heat-tolerant than wild-type plants and up-regulated the expression of the HsfAld regulon, as was observed in TsHsfAld-overexpressing plants. We propose that the products of both TsHsfAld and AtHsfAld function as positive regulators of Arabidopsis heat-stress response and would be useful for the improvement of heat-stress tolerance in other plants.