Figure 10.

Proposed model for regulation of thermotolerance at different stages of pollen development. Thermotolerance of pollen at early stages of development is highly dependent on a priming program that leads to the accumulation of Hsps and other HS-induced genes under nonstress conditions. The induction of these genes is controlled by the master regulator HsfA1a, which further activates HsfA2 and probably other class A Hsfs (HsfAx) to induce downstream genes. HsfA2 is required for the higher expression of some HS genes, including Hsp101, Hsp90-3, Hsp70-9, and MBF1c. Following microspore formation and during microgametogenesis, the capacity of pollen to induce HSR is restored. Here, HsfA1a induces and subsequently interacts with HsfA1b, HsfA2, HsfA3, HsfA6b, and HsfA7 to activate HS genes. HsfA2 induction alleviates the up-regulation of HsfA6b and HsfA7, which in turn are strongly induced in case of limited HsfA2 availability caused by suppression.