Salicylic Acid Pretreatment Reprograms the Subsequent Drought Stress Response of Tomato: Integrated Transcriptomic and Biochemical Evidence for Long-Term Stress Priming

Drought is one of the most critical limiting factors in crop production. Our study investigated how salicylic acid (SA) pretreatment modified the subsequent drought stress response of tomato (Kecskeméti 549). Four groups (control, drought-stressed, SA-pretreated, and SA-pretreated and drought-stressed) were compared using genome-wide transcriptomic analyses based on RNA sequencing (n = 3) and antioxidant enzyme activity (peroxidase, catalase, glutathione reductase, glutathione-s-transferase, superoxide dismutase; POX, CAT, GR, GST, SOD). Chlorophyll content and biochemical factors such as proline, MDA, protein content, total phenols, total flavonoids and soluble sugars were also analyzed. Both SA treatment and drought stress caused extensive gene expression rearrangements, primarily in genes involved in redox homeostasis, signal transduction, proteostasis, and amino acid, carbohydrate, and energy metabolism. Under drought conditions, the transcriptional stress response of SA-pretreated plants was more moderate (with fewer chaperone, proteasome, defense, and redox genes being induced), while several SA-dependent WRKY factors (WRKY70, WRKY41, WRKY51, WRKY60) exhibited an induction response—contrary to the pattern observed in untreated plants. Furthermore, this group displayed the highest activity levels for all five antioxidant enzymes (two-way ANOVA, treatment × stress interaction). Transcript levels of the genes encoding these five antioxidant enzymes showed a strong correlation with their respective enzymatic activities (r = 0.85). Drought stress reduced leaf area by 24.6%, increased MDA content by 2.6 times, and reduced protein content by 79.3% in untreated plants. But in plants pretreated with SA, the leaf area did not decrease, the MDA content did not increase, and protein loss was lower (35.3%). These results indicate a persistent SA-induced priming effect.