Nuclear RNA export and its importance in abiotic stress responses of plants.

Transduction of developmental and environmental cues into the nucleus to induce transcription and the export of RNAs to the cytoplasm through the nuclear pore complex (NPC) play pivotal roles in regulation of gene expression. The process of bulk export of mRNAs from nucleus to cytoplasm is highly conserved across eukaryotes. Assembly of export-competent mRNA ribonucleoprotein (mRNP) is coupled with both transcription and mRNA processing. The export-competent mRNP consists of mRNAs and a dozen nucleocytoplasmic shuttling nuclear proteins, including RNA export factors (Mex67-Mtr2 heterodimer, Npl3), poly(A)-binding proteins, DEAD-box protein 5 (Dbp5), and nucleoporins (NUPs) in yeast. Mobile NUPs help docking of mRNP to the NPC nuclear basket. A partially unfolded mRNP complex appears to be pulled through the NPC by using energy from Dbp5-catalyzed ATP hydrolysis. Dbp5 probably catalyzes the release of mRNA from mRNP in the cytoplasm. In contrast to bulk export of mRNAs by a Mex67-Mtr2/Npl3-dependent pathway, a specific subset of mRNA export under stress and export of microRNAs are mediated through the karyopherin (importin beta) family of proteins in a Ran-GTPase-dependent pathway. Our knowledge of mRNA export mechanisms in flowering plants is in its infancy. Some proteins of the NUP107-160 complex, NUPs and DEAD-box proteins (DBPs), have been studied in flowering plants. Arabidopsis NUP160/SAR1 plays a critical role in mRNA export, regulation of flowering, and hormone and abiotic stress responses, whereas NUP96/ SAR3/MOS3 is required for mRNA export to modulate hormonal and biotic stress responses. DEAD-box proteins have been implicated in mRNA export and abiotic stress response of yeast and higher plants. Arabidopsis DBP CRYOPHYTE/LOS4 plays an important role in mRNA export, abiotic stress response, germination, and plant development. Further studies on various components of nuclear mRNA export in plants during nonstress and stress conditions will be necessary to understand the link between mRNA export and stress-responsive gene expression.