ALBA proteins confer thermotolerance through stabilizing HSF messenger RNAs in cytoplasmic granules.

High temperature is one of the major environmental stresses affecting plant growth and fitness. Heat stress transcription factors (HSFs) play critical roles in regulating the expression of heat-responsive genes. However, how HSFs are regulated remains obscure. Here, we show that ALBA4, ALBA5 and ALBA6, which phase separate into stress granules (SGs) and processing bodies (PBs) under heat stress, directly bind selected messenger RNAs, including HSF mRNAs, and recruit them into SGs and PBs to protect them from degradation under heat stress in Arabidopsis. The alba456 triple mutants, but not single and double mutants, display pleiotropic developmental defects and hypersensitivity to heat stress. Mutations in XRN4, a cytoplasmic 5' to 3' exoribonuclease, can rescue the observed developmental and heat-sensitive phenotypes of alba456 seedlings. Our study reveals a new layer of regulation for HSFs whereby HSF mRNAs are stabilized by redundant action of ALBA proteins in SGs and PBs for plant thermotolerance.