Functions of Arabidopsis REI1-LIKE cytosolic ribosome biogenesis factor homologs

Functions of Arabidopsis REI1-LIKE (REIL) proteins, two homologs of a yeast ribosome biogenesis protein (RBP) that takes part in the late cytoplasmic steps of 60S ribosomal subunit maturation, were characterized by systems analyses of the 10°C cold acclimating reil1-1 reil2-1 double mutant compared to Col-0 wildtype. The mutant lacked both REIL proteins, was strongly growth inhibited in the cold and complemented by constitutive expression of N-terminal FLUORESCENT PROTEIN (FP)-REIL1 and FP-REIL2 fusion proteins under control of the UBIQUITIN 10 promoter. Wildtype acclimation to 10°C causes relative accumulation of cytosolic ribosome subunits and rRNA. Expression of cytosolic ribosomal genes, known cytosolic RBPs, translation initiation- and elongation-factors was activated. Conserved function of Arabidopsis REIL proteins was indicated by delay of these processes in reil1-1 reil2-1, cytosolic localization of FP-REIL proteins and native REIL protein interactions with 60S containing ribosome fractions. Non-acclimated reil1-1 reil2-1 triggered plant specific metabolic and transcriptomic cold acclimation responses that included activation of the DREB/CBF-regulon with a preference for the cold acclimation factors, CBF1/DREB1B, CBF2/DREB1C, and CBF3/DREB1A. Cold-acclimating reil1-1 reil2-1 maintained cellular integrity and acquired freezing tolerance but did not activate FLOWERING LOCUS T expression in mature leaves. This block was independent of FLOWERING LOCUS C and AGAMOUS-LIKE 19 mediated vernalization. We conclude that Arabidopsis REIL proteins enhance accumulation of cytosolic ribosome subunits after cold shift and either directly or indirectly feedback on temperature perception by suppression of premature cold acclimation at optimized temperature and by triggering growth and the vegetative to generative phase transition in the cold. Transcriptomic profiling demonstrated a hidden acclimation phenotype of the morphologically inconspicuous reil1-1reil2-1 mutant under optimized temperature conditions. Premature triggering of cold acclimation, a severe growth defect at 10°C and compensation responses indicate that REIL function may extend beyond cytosolic ribosome biogenesis towards translation initiation. Cold temperature induced gene expression in Arabidopsis reil1-1reil2-1 mutant in comparison to the wild type Col-0 was measured at 20°C, 24 hours, 1 week and 3 weeks after shift from 20°C to the 10°C temperatures. 3 to 6 replicates were harvested for each line and at each time point before or after cold treatment