Acts as component of the THO subcomplex of the TREX complex which is thought to couple mRNA transcription, processing and nuclear export. Contributes to the integrity of the endogenous trans-acting small interfering RNA (ta-siRNA) pathway. May process or transport a long RNA molecule so that it can be a template for secondary siRNA production. May participate in the trafficking of siRNA precursors to the ARGONAUTE catalytic center. Required for the generation of functional messenger ribonucleoproteins (mRNPs). Plays an important roles in plant innate immunity. Component of the THO complex, which is composed of THO1, THO2, THO3, THO5, THO6 and THO7. A number of isoforms are produced. According to EST sequences. Developmental defects as well as reduced levels of endogenous trans-acting small interfering RNA (ta-siRNA).