Plant cold acclimation and the characterization of hos1, hos2 and los2 mutants that are sensitive to cold stress

The goal of my research was to better understand the mechanism of plant cold acclimation using Arabidopsis thaliana as a model system. Our genetic screening method utilizes a line of transgenic Arabidopsi plants harboring the firefly luciferase reporter gene (LUC) fused to the RD29A promoter (RD29A-LUC), which contains the cold- and osmotic stress-responsive DRE/ CRT element and the ABA-responsive ABRE element (Ishitani et al., 1997). The RD29A-LUC plants were mutagenized with ethylmethane sulfonate and mutants with aberrant luminescence responses were selected from the resulting M₂ population by luminescence imaging using a cooled CCD camera (Ishitani et al., 1997).; hos2-1 (for high expression of osmotically responsive genes) mutant was isolated based on enhanced expression of RD29A and other stress genes under low temperature treatment (Chapter 2). Enhanced gene expression was observed only in response to cold stress, not to osmotic stress or ABA, in hos2 mutants. Compared with the wild-type plants, the hos2-1 mutant plants are less capable of developing freezing tolerance when treated with low non-freezing temperatures, indicating that HOS2 is a negative regulator of low temperature signal transduction important for plant cold acclimation.; Chapter 3 characterized the function of the HOS1 gene that was found to encode a novel protein with a RING (Really Interesting New Gene) finger motif near the amino terminus. The observation that hos1 plants are more sensitive to freezing temperatures even though they have enhanced expression of CBF transcription factors and of their downstream cold responsive genes suggests that controlling the stability of critical proteins in the cell is important in the cellular response to environmental stimuli.; In contrast to hos1 and hos2 mutants, the los2 mutant was identified by its reduced expression of the RD29A-LUC transgene under cold treatment but not ABA, salt or PEG treatments (Chapter 4). LOS2 is critical for chilling as well as freezing resistance. The los2 mutation specifically impairs cold regulated expression of the endogenous COR/KIN/RD/LTI genes. The LOS2 gene encodes an enzyme, enolase, which converts 2-phosphoglycerate to phosphoenolpyruvate in the glycolytic pathway. Several lines of the evidence for possible LOS2 function as a transcriptional regulator are presented.