A Guard Cell Specific F-box Protein DOR Negatively Regulates Drought Tolerance In Arabidopsis Thaliana

DOR (DrOught Resistance) is AtSFL35, a member of Arabidopsis AtSFL (S-locus F-box-like) superfamily related to the Antirrhinum S-locus F-box gene(SLF). A T- DNA insertional mutant of DOR results in a significant increase in tolerance to drought. Detached leaves, MS plus glycerol and soil-grown plants all exhibit a drought- tolerant phenotype compared to wild-type plants. Functional complement action experiment showed that DOR mutation leads to the drought tolerance, suggesting that DOR encodes a novel drought tolerance repressor. Quantitative PCR showed that DOR expressed ubiquitously at a low level. Interestingly, GUS reporter gene studies indicated DOR is specifically expressed in guard cell, suggesting that the DOR may function in stomatal movement under drought stress. Furthermore, we compared the stomatal aperture under normal grown condition and KAT1 and KAT2 expression under drought stress between wild-type and dor mutant and revealed that the dor could limit its transpirational water loss in response to drought stress. To investigate the molecular mechanisms for the DOR action, we analysed the DREB2A, RD29A, COR15A and RD22 expression by RNA gel blot. The results showed that the dor mutant induced the expression of drought-responsive genes after PEG treatment, indicating that DOR may function in both ABA-dependent and ABA-independent pathways for stress-responsive gene expression. Meanwhile, we found that DOR also function in cold signal transduction. Yeast two-hybrid and pull-down assays showed that DOR protein is capable of interacting with ASK14 and CULLINl, suggesting that it could form an SCF (Skp1/Cullin or CDC53/F-box) complex. Taken together, these results suggest that the ubiqutin/26S proteasome-mediate proteolysis mediated by DOR acts a negative regulator of drought tolerance by inhibiting the stomatal closure under drought stress.