THO5 subunit of the <i>Arabidopsis thaliana</i> TREX complex: role in mRNA export and heat tolerance

During eukaryotic gene expression, synthesis, processing and nuclear export of mRNAs are tightly linked and the transcription and export (TREX) complex plays a central role thereby. TREX consists of a hexameric core termed THO that can associate with additional proteins including various export factors. In Arabidopsis thaliana, there are two alternative versions of the THO5 subunit, THO5A and THO5B, whose molecular function so far has not been studied in plants. Proteomics approaches revealed that both THO5 proteins are incorporated into the THO complex that copurified with various mRNA export factors including ALY and UIEF proteins underscoring the involvement in nucleocytosolic mRNA transport. In phenotypic analyses, tho5a/b single-mutants proved almost indistinguishable from wildtype plants, indicating (partial) functional redundancy. Under standard growth conditions, tho5ab double-mutants are mildly affected regarding vegetative and reproductive development. However, tho5ab plants turned out to be very sensitive towards heat stress and mRNA export defects are enhanced at elevated temperatures. As demonstrated by transcript profiling, a subset of mRNAs, characterised by low intron-content and long exons, accumulates in nuclei of tho5ab plants including heat-induced transcripts. Therefore, THO5A/B and proper mRNA export are required for efficient plant tolerance to elevated temperatures.