Figure 9.

Model of the HS response in plants. The left panel shows possible activities of Hsfs under non-HS and moderate HS conditions. HsfB1 and HsfB2b repress the expression of HS-inducible Hsfs and HSPs; the HsfA1s are inactive under non-HS conditions. The middle panel shows the activity of Hsfs in the early stage of the HS condition. HsfA1s are activated rapidly in response to HS to overcome the repressive activities of HsfBs. The expression of genes for Hsfs (HsfA2, HsfA7a, HsfB1, and HsfB2b) and HSPs is strongly induced by HsfA1s. The right panel shows the activity of Hsfs in an extended HS response. Under this condition, the plant cells acquire thermotolerance by the activities of HSPs, which have been produced in the early stage of the HS response. Several HSPs suppress the activity of class A Hsfs by interacting with them to interfere with their trimer formation and nuclear import. On the other hand, the HS response is enhanced and maintained by HS-induced HsfAs (e.g. HsfA2). At the same time, the HS-induced HsfBs (HsfB1 and HsfB2b), the expression of which is induced by HsfA1s in the early stage of the HS response, repress the expression of heat-inducible HsfA2 to attenuate the HS response. The font size indicates the quantity of active proteins. Black and blue arrows indicate positive (black) and negative (blue) regulation of the transcription of each gene. Red and light blue arrows indicate activation (red) and inactivation (light blue) of HsfA1 proteins.