Summary Ca 2+ signals play a crucial role in heat stress (HS) resistance. However, how Ca 2+ signals are decoded and regulate the downstream targets of HS signaling remains unclear in plants. Cool‐season grasses are sensitive to high‐temperature conditions and should evolve distinctive heat stress response mechanisms. Here, we identified the calcium‐dependent protein kinase 19 (FaCPK19), which decoded the unique Ca 2+ signal triggered by HS in cool‐season grass tall fescue ( Festuca arundinacea Schreb.). The FaCPK19 was activated upon HS and positively regulated thermotolerance in tall fescue. FaCPK19 interacted with the thermo‐tolerance 1 ( FaTT1 ), which encodes an α2 subunit of the 26S proteasome, and phosphorylated its Thr172 (T172) residue. Under HS, overexpressing FaTT1 T172A ( FaTT1 T172A ‐OE , phospho‐deficient) grasses resulted in significantly lower 20S and 26S proteasome activity and reduced expression of downstream HS‐responsive genes compared to gain‐function transgenic grasses ( FaTT1‐OE ). The Facpk19 mutant was weakened in 20S and 26S proteasome activity under HS. Loss of T172 phosphorylation in FaTT1 T172A ‐OE showed a weakened heat‐tolerance phenotype than FaTT1‐OE. T172 residue at TT1 is evolutionarily conserved from plants to animals, indicating it could be widely used for thermotolerance breeding in grasses and crops. Our findings reveal a Ca 2+ ‐FaCPK19–FaTT1 axis by which an HS‐induced Ca 2+ signal is decoded with enhanced thermotolerance in tall fescue.
Mo et al. (Mon,) studied this question.