Trimethylguanosine Synthase1 (TGS1) Is Essential for Chilling Tolerance.

Chilling stress is a major factor limiting plant development and crop productivity. Because the plant response to chilling is so complex, we are far from understanding the genes important in the response to chilling. To identify new genes important in chilling tolerance, we conducted a novel mutant screen, combining a confirmed SALK T-DNA insertion collection with traditional forward genetics. We screened a pool of more than 3700 confirmed homozygous SALK T-DNA insertion lines for visible defects under prolonged growth at 5°C. Of the chilling-sensitive mutants we observed, mutations at one locus were characterized in detail. This gene, At1g45231, encodes an Arabidopsis () trimethylguanosine synthase (TGS1), previously uncharacterized in the plant kingdom. We confirmed that Arabidopsis TGS1 is a functional ortholog of other trimethylguanosine synthases based both on its in vitro methyltransferase activity and on its ability to rescue the cold-growth inhibition of a mutant in vivo. While mutant plants grew normally at 22°C, their vegetative and reproductive growth was severely compromised under chilling conditions. When we transgenically expressed in the mutant plants, the chilling-sensitive phenotype was relieved, demonstrating that TGS1 is required for chilling tolerance.