SlWRKY81 reduces drought tolerance by attenuating proline biosynthesis in tomato

In plants, WRKY transcription factors (TFs) regulate a number of biological processes and responses to biotic and abiotic stresses. In this study, we used a reverse-genetic approach (virus-induced gene silencing) to unveil the role of tomato WRKY81 in drought tolerance. When non-silenced control (TRV:TRV) and SlWRKY81-silenced plants were grown in 30 % and 15 % substrate water content (SWC) for 7 days, TRV:TRV plants, but not SlWRKY81-silenced plants, showed severe wilting phenotype, which was closely associated with increased transcript abundance of SlWRKY81 in TRV:TRV plants compared with that in SlWRKY81-silenced plants. Although the concentrations of proline drastically increased in response to water deficit in both genotypes, the leaf proline content in SlWRKY81-silenced plants under 15 % SWC was significantly higher than that in TRV:TRV plants. The transcript levels of proline biosynthetic gene SlP5CS1 encoding delta 1-pyrroline-5-carboxylate synthetase (P5CS) were consistent with the changes in the proline contents under different SWC in TRV:TRV and TRV:SlWRKY81 plants. Inspection of 2-kb sequences upstream of the predicted translational start sites of the SlP5CS1 identified two copies of the core W-box (TTGACC/T) sequence in the promoter of SlP5CS1, which correlates well with the expression of SlP5CS1 in response to water deficit, indicating a possible target of SlWRKY81 transcription factor. Taken together, our results suggest that SlWRKY81 acts as a repressor of SlP5CS1 transcription and thus reduces proline biosynthesis to abate drought tolerance in tomato plants.