MOS4-associated complex contributes to proper splicing and suppression of ER stress under long-term heat stress in Arabidopsis

Plants are often exposed not only to short-term (S-) heat stress but also to long-term (L-) heat stress over several consecutive days. A few Arabidopsis mutants defective in L-heat tolerance have been identified, but the molecular mechanisms involved in L-heat are less well understood than those involved in S-heat one. To elucidate the mechanism of L-heat stress tolerance, we here used a forward genetic screen for sensitive to long-term heat (sloh) mutants and isolated sloh3 and sloh63. sloh3 and sloh63 were hypersensitive to L-heat stress but not in S-heat stress, while only sloh63 was also hypersensitive to salt stress. We identified the causal genes, SLOH3 and SLOH63, which were both splicing-related factors identified as components of the MOS4 associated complex (MAC), which is widely conserved in eukaryotes and has been suggested to interact with spliceosomes. SLOH3 and SLOH63 were induced by L-heat stress in a time-dependent manner. Some abnormal splicing events were observed in both sloh3 and sloh63 under L-heat stress. In addition, ER stress and subsequent UPR were occurred in both sloh3 and sloh63 under L-heat stress, especially prominent in sloh63, suggesting that enhanced ER stress is due to the salt hypersensitivity of sloh63. We found that the splicing inhibitor pladienolide B led to concentration-dependent disturbance of splicing, decreased L-heat tolerance, and enhanced ER stress. These findings suggested that maintaining precise mRNA splicing under L-heat stress are important for L-heat tolerance and suppressing ER stress of Arabidopsis.