Tomato B cell receptor-associated protein 31 homologs enhance thermotolerance by promoting chloroplast PSBW accumulation and stabilizing the PSII–LHCII complex

B-cell receptor-associated protein 31 (BAP31) is an endoplasmic reticulum (ER) transmembrane protein in humans that regulates protein trafficking, apoptosis, and inflammatory responses to maintain cellular homeostasis. However, its role in plants remains largely unknown. Here, we identified two tomato (Solanum lycopersicum) homologs, SlBAP31 and SlBAP31-LIKE, which are strongly induced by heat stress and function redundantly to stabilize photosystem II-light-harvesting complex II (PSII-LHCII) supercomplexes. Although localization to the ER or Golgi apparatus has been detected, both proteins are also present in chloroplasts, where they interact with SlTOC75, a core component of the chloroplast TOC-TIC translocon complex. Loss of either protein reduces the abundance of PSBW under heat stress. Among the two tomato PSBW isoforms, SlPSBW2 plays a predominant role in PSII-LHCII stability, and its accumulation depends in part on glycine 121 at the C terminus and interaction with SlBAP31/SlBAP31-LIKE. Collectively, our results reveal that ER/Golgi- and chloroplast periphery-localized SlBAP31 and SlBAP31-LIKE positively regulate tomato thermotolerance by mediating SlPSBW2 accumulation and stabilizing the PSII-LHCII supercomplex.