Drosophila heat shock factor (HSF) regulates developmental growth by maintaining the basal expression of HSP83/HSP90

The heat shock transcription factor HSF1 is best known as a master regulator of the proteotoxic stress response, yet its functions remain incompletely understood. In Drosophila melanogaster, heat shock factor (HSF) is essential for viability, but the mechanisms by which it promotes development are unclear. Here, we show that Hsf null larvae arrest at the early second instar stage and exhibit a significant reduction in basal HSP83/HSP90 levels. Tissue-specific knockdown of Hsf revealed widespread, organ-specific requirements, including defects in endoreplication and cell growth in prothoracic and salivary glands, adult wing defects following imaginal disc perturbation, follicle degeneration in the ovary, and melanotic tumors upon simultaneous hemocyte and fat body depletion. In these tissues, HSF loss typically reduced HSP83 levels, and restoring HSP83 expression partially or fully rescued these defects. These findings identify HSP83 as a critical downstream effector of HSF and demonstrate that HSF promotes development largely by maintaining basal chaperone expression. Together, our results establish HSF as a key regulator of developmental progression and highlight a central role for proteostasis in supporting tissue growth under non-stress conditions.