• Arabidopsis USP1 mutants display normal development but are critical under heat stress, as CRISPR–Cas9 usp1 mutants develop normally yet display excessive apoplastic ROS accumulation during heat stress conditions. • USP1 is specifically required for thermopriming, as usp1 mutants show impaired acquired thermotolerance while retaining normal responses to acute heat stress. • Transcriptome reprogramming is altered in usp1 mutants, with strong induction of genes associated with protein folding, electron transport, and oxidative phosphorylation. • USP1 acts as a molecular chaperone for HSFA2, preventing heat-induced denaturation of this key transcription factor during repeated stress exposure. • Heat stress memory is compromised in the absence of USP1, reflected by reduced expression of memory-associated genes and decreased H3K4me3 enrichment at their loci. • USP1 links protein homeostasis to epigenetic regulation, providing a mechanistic basis for sustained transcriptional memory during recurrent heat stress in plants. Plants employ diverse strategies to cope with different types of heat stress. The response to short-term acute heat stress differs significantly from that to moderate heat stress followed by severe stress events. After experiencing moderate heat stress, plants exhibit a more robust response to subsequent severe stress, a phenomenon known as thermopriming or acquired thermotolerance. Thermopriming creates a memory by maintaining the heat stress (HS) memory-related genes in an alert state. In this work, we investigated the role of Arabidopsis Universal Stress Protein 1 (USP1) in plant heat stress responses. CRISPR-Cas9 generated knockout usp1 mutant lines showed no morphological changes during development and normal growth conditions. However, usp1 mutant plants showed enhanced levels of apoplast hydrogen peroxide and superoxide reactive oxygen species accumulation upon heat stress. Transcriptome analyses revealed that genes related to protein folding, electron transport, and oxidative phosphorylation are strongly upregulated in usp1 mutant plants. USP1 is essential for acquired thermotolerance, as usp1 mutants are compromised in heat stress memory but show normal responses to acute heat stress similar to hsfa2 mutants. Biochemical assays showed that USP1 functions as a molecular chaperone, protecting the transcription factor HSFA2 from heat-induced denaturation. Moreover, usp1 mutant plants show decreased transcript levels of heat stress response genes and reduced H3K4me3 enrichment at memory gene loci. These data show that USP1 plays an important role as a chaperone of HSFA2 in mediating plant heat stress memory.
Manickam et al. (Wed,) studied this question.