Two SnRK2 protein kinases genes play a negative regulatory role in the osmotic stress response in tomato

Sucrose non-fermenting 1-related protein kinase 2 (SnRK2) proteins positively regulate abscisic acid signaling, and play important roles in plant stress responses. Seven genes encoding SlSnRK2s were isolated from tomato (Solanum lycopersicum cv. Micro Tom), which showed homology to Arabidopsis SnRK2.2/SnRK2.3/SnRK2.6. Transcriptional levels were analyzed in tomato under salinity stress, all seven genes were repressed in the first 24 h under stress treatment. But the transcriptional levels of four SlSnRK2 genes dramatically increased at 72 h, which was not detected in transcriptional level of SlSnRK2.1, SlSnRK2.2 and SlSnRK2.3, indicating SlSnRK2.1, SlSnRK2.2 and SlSnRK2.3 might act differently as other four SlSnRK2 in tomato. SlSnRK2.1 and SlSnRK2.2 were selected for further analysis. The expression patterns of SlSnRK2.1 and SlSnRK2.2 differed among various tissues of tomato plants. A subcellular localization analysis showed that these SlSnRK2s were localized on the cytoplasm and nucleus. Transgenic plants overexpressing SlSnRK2s exhibited a lower germination rate than wild type (WT) under saline conditions. In contrast, RNA-interference lines which down regulate both SlSnRK2.1 and SlSnRk2.2 showed a higher germination rate than that of WT under saline conditions. Furthermore, the activities of peroxidase, catalase and proline content were lower and the malondialdehyde content was higher in overexpressing lines than in WT under salinity and drought conditions. Moreover, overexpression of SlSnRK2s altered the transcriptional levels of the stress-related genes PR5, LHA1, HVA22 and RBOHC under osmotic stress conditions. These results demonstrated that overexpression of SlSnRK2.1 and SlSnRK2.2 decreased osmotic stress tolerance. Therefore, these SlSnRK2s may be involved in osmotic stress signal transduction and may play negative roles in the stress response.