LHT1/MAC7 contributes to proper alternative splicing under long-term heat stress and mediates variation in the heat tolerance of Arabidopsis.

Natural genetic variation has facilitated the identification of genes underlying complex traits such as stress tolerances. We here evaluated the long-term (L-) heat tolerance (37°C for 5 days) of 174 Arabidopsis thaliana accessions and short-term (S-) heat tolerance (42°C, 50 min) of 88 accessions and found extensive variation, respectively. Interestingly, L-heat-tolerant accessions are not necessarily S-heat tolerant, suggesting that the tolerance mechanisms are different. To elucidate the mechanisms underlying the variation, we performed a chromosomal mapping using the F2 progeny of a cross between Ms-0 (a hypersensitive accession) and Col-0 (a tolerant accession) and found a single locus responsible for the difference in L-heat tolerance between them, which we named Long-term Heat Tolerance 1 (LHT1). LHT1 is identical to MAC7, which encodes a putative RNA helicase involved in mRNA splicing as a component of the MOS4 complex. We found one amino acid deletion in LHT1 of Ms-0 that causes a loss of function. Arabidopsis mutants of other core components of the MOS4 complex-mos4-2, cdc5-1, mac3a mac3b, and prl1 prl2-also showed hypersensitivity to L-heat stress, suggesting that the MOS4 complex plays an important role in L-heat stress responses. L-heat stress induced mRNA processing-related genes and compromised alternative splicing. Loss of LHT1 function caused genome-wide detrimental splicing events, which are thought to produce nonfunctional mRNAs that include retained introns under L-heat stress. These findings suggest that maintaining proper alternative splicing under L-heat stress is important in the heat tolerance of A. thaliana.