The role of SENSITIVE TO FREEZING6 in plant tolerance to stress

Abstract

The current rate of climate change predicts that plants will become subject to increasing extremes of enviro nmental stress. Rapid population increases in developing countries also demand higher yield from crop production, often from sub-optimal agricultural areas. Genetic engineering can help meet these needs through the development of crops with greater stress tolerance. Identification of Arabidopsis mutants unable to tolerate stress is a powerful approach
to investigate the molecular basis of stress tolerance in plants and so identify targets for modification. The Arabido
psis mutant sfr6 (sensitive to freezing6) is unable to tolerate freezing stress, due to failure of CBF transcription factors to activate downstream cold responsive (COR) gene expression.

In this study, I assessed the suitability of using SFR6 as a molecular tool to improve low temperature tolerance of crop plants. These results show that complementation of sfr6 with the wild type SFR6 gene restores COR gene expression and freezing tolerance; however over-expression of the gene in wild type has no effect. This suggests that SFR6 is essential but not sufficient to initiate COR gene expression controlled by CBF transcription factors. At4g04920, the gene encoding SFR6, has been identified as a component of the multi-subunit transcriptional co-activator, known as mediator, which controls eukaryotic inducible gene expression, which explains these findings.

A homologue of SFR6 was cloned from rice (OsSFR6) and over-
expression of OsSFR6 in the sfr6-1 mutant led to complementation, demonstrating orthology. Rice is a freezing-intolerant species, and therefore the presence of a functional SFR6 orthologue in rice capable of restoring Arabidopsis CBF function suggests that OsSFR6 may play a role in chilling tolerance in rice analogous to that of CBF/DREB1 in other chilling sensitive species.

The responses of sfr6 mutants to a range of biotic and abiotic stress stimuli revealed a hypersensitivity to both UV radiation and infection by both virulent and avirulent Psuedomonas syringae DC3000. This reduced tolerance correlates in sfr6 with reduced mRNA levels of UV irradiance and pathogen infection response genes, including META CASPASE8 (MC8), PATHOGENESIS RELATED1 (PR1) and ENHANCED DISEASE SUSCEPTIBILITY5 (EDS5).