A DEAD box RNA helicase is essential for mRNA export and important for development and stress responses in Arabidopsis

An Arabidopsis thaliana mutant, cryophyte, was isolated and found to have an enhanced cold stress-induction of the master regulator of cold tolerance, C-repeat binding factor 2 (CBF2), and its downstream target genes. The mutant is more tolerant to chilling and freezing stresses but is more sensitive to heat stress. Under warm but not cold growth temperatures, the mutant has a reduced stature and flowers earlier. Under long day conditions, flowering of the mutant is insensitive to vernalization. The mutant is also hypersensitive to the phytohormone abscisic acid. The mutation was found in a DEAD box RNA helicase gene that is identical to the previously identified low expression of osmotically responsive genes 4 (LOS4) locus, which was defined by the los4-1 mutation that reduces cold regulation of CBFs and their target genes and renders Arabidopsis plants chilling sensitive. We show evidence suggesting that the CRYOPHYTE/LOS4 protein may be enriched in the nuclear rim. In situ poly(A) hybridization indicates that the export of poly(A)+ RNAs is blocked in the cryophyte/los4-2 mutant at warm or high temperatures but not at low temperatures, whereas the los4-1 mutation weakens mRNA export at both low and warm temperatures. These results demonstrate an important role of the CRYOPHYTE/LOS4 RNA helicase in mRNA export, plant development, and stress responses.     

Systematic deletion of the ER lectin chaperone genes reveals their roles in vegetative growth and male gametophyte development in Arabidopsis

Calnexin (CNX) and calreticulin (CRT) are homologous lectin chaperones in the endoplasmic reticulum (ER) that facilitate glycoprotein folding and retain folding intermediates to prevent their transit via the secretary pathway. The Arabidopsis genome has two CNX (CNX1 and CNX2) and three CRT (CRT1, CRT2 and CRT3) homologs. Despite growing evidence of the biological roles of CNXs and CRTs, little is understood about their function in Arabidopsis growth and development under normal conditions. Here, we report that the deletion of CNX1, but not of CNX2, in the crt1 crt2 crt3 triple mutation background had an adverse effect on pollen viability and pollen tube growth, leading to a significant reduction in fertility. The cnx1 crt1 crt2 crt3 quadruple mutation also conferred severe defects in growth and development, including a shortened primary root, increased root hair length and density, and reduced plant height. Disruption of all five members of the CNX/CRT family was revealed to be lethal. Finally, the abnormal phenotype of the cnx1 crt1 crt2 crt3 quadruple mutants was completely rescued by either the CNX1 or CNX2 cDNA under the control of the CNX1 promoter, suggesting functional redundancy between CNX1 and CNX2. Taken together, these results provide genetic evidence that CNX and CRT play essential and overlapping roles during vegetative growth and male gametophyte development in Arabidopsis.