Combined Application of Wheat Germ Agglutinin and Bacillus subtilis Mitigates Dehydration Stress in Two Wheat Cultivars Differing in Drought Tolerance by Regulating Water Balance

Drought stress significantly limits wheat productivity worldwide, requiring the development of sustainable strategies to improve crop tolerance. One promising approach involves the use of plant growth-promoting bacteria (PGPB) and biomolecules that can stimulate plant’s intrinsic adaptive mechanisms. Among these molecules, lectins are of particular interest as natural components of plant metabolism involved in stress tolerance. However, the combined effect of lectins and PGPB on protective mechanisms under water deficit remains insufficiently studied. This work was devoted to the comparative analysis of the influence of seed pretreatment with a combination of Bacillus subtilis (B.s.) and wheat germ agglutinin (WGA) on 8-day-old wheat (Triticum aestivum L.) seedlings of two cultivars contrasting in drought resistance: the tolerant Ekada 70 (cv. E70) and the sensitive Zauralskaya Zhemchuzhina (cv. ZZh) under PEG-induced dehydration. Changes in root cell viability, biomass accumulation, water regime (transpiration intensity, relative water content, osmotic potential), and root lignin and suberin deposition were evaluated. Three-day exposure to 15% polyethylene glycol 6000 (PEG) increased root cell death by 14% in E70 and by 57% in ZZh relative to the control, confirming that cv. E70 is more tolerant than cv. ZZh. The pre-sowing (B.s. + WGA) treatment significantly mitigated stress-induced root cell death in both cultivars, with a more pronounced effect in the sensitive ZZh. Furthermore, the pretreatment promoted biomass accumulation in both cultivars. In the drought-tolerant cv. E70, dry weight increased under stress alone and was further enhanced by (B.s. + WGA) application. In the sensitive cv. ZZh, the pretreatment alleviated stress-induced growth inhibition, increasing both fresh and dry weight, indicating its ability to compensate for the lack of intrinsic tolerance mechanisms and support growth recovery under water deficit. (B.s. + WGA)-pretreated seedlings, especially E70, were also characterized by improved water balance under stress, as evidenced by the maintenance of transpiration intensity and relative water content, as well as by a lowered osmotic potential indicating osmotic adjustment. Histochemical analysis revealed that the treatment enhanced lignification in the vascular cylinder of E70 roots, while in ZZh it promoted suberin accumulation, especially under stress conditions. These contrasting responses in lignin and suberin deposition illustrate cultivar-specific strategies of wheat protection. Thus, combined (B.s. + WGA) seed pretreatment may serve as an effective tool for improving drought tolerance in wheat by preserving root cell integrity, mitigating stress-induced growth inhibition, normalizing water relations, and strengthening root barrier properties. Its effectiveness and the underlying adaptive mechanisms are cultivar-dependent, pointing to distinct protective pathways in contrasting genotypes.