ChaWRKY40 Enhances Drought Tolerance of 'Dawei' Hazelnuts by Positively Regulating Proline Synthesis

Hazelnuts are among the most important nuts worldwide. Drought has severely restricted the development of the hazelnut industry in the wake of global warming and lack of water resources. Delta-1-pyrroline-5-carboxylic acid synthase (P5CS) is closely related to drought stress as the rate-limiting enzyme of proline synthesis. WRKY40 had been proven to be an important transcription factor regulating drought tolerance in several plants. In this study, the hybrid hazelnut 'Dawei' exhibiting drought tolerance was used as the test material. Tests for simulated drought stress and ChaWRKY40 overexpression, and the yeast one-hybrid assay were performed. The results showed that the relative water content of leaves gradually decreased, but the proline content, electrolyte leakage, and expression of ChaWRKY40 and ChaP5CS increased with increasing PEG-6000 concentration in the leaves. A transient ChaWRKY40 overexpression trial indicated that overexpression of ChaWRKY40 improved the proline content and the transcription level of ChaP5CS. The Y1H experiment suggested that ChaWRKY40 directly binds to the W-box-acting element (W-box) on the promoter of ChaP5CS. In conclusion, ChaWRKY40 may increase the proline content by positively regulating the expression of the ChaP5CS gene, thereby improving the drought resistance of hazelnuts.