Strengthening physiological resilience, osmotic adjustment, and ion homeostasis in water-deficit stressed muskmelon via ACC deaminase-producing rhizobacterial partnership.

Here, ACC deaminase ACC Deaminase producing rhizobacteria, Priestia aryabhattai MD-85 (Accession no. PV155249.1) and Enterobacter cloacae MD-79 (Accession no. PV155250.1), were assessed for their potential to enhance water-deficit stress tolerance in muskmelon. Both strains produced ACC Deaminase and exhibited drought tolerance, with MD-79 showing 78.9 ± 7.6 µM α-ketobutyrate mg⁻¹ protein h⁻¹ at 18%-PEG, and MD-85 showing 68.4 ± 5.4 µM α-ketobutyrate mg⁻¹ protein h⁻¹ at 21%-PEG. Both strains produced multi-functional growth-promoting substances under PEG-induced stress, conferring their significant drought tolerance potential. Increasing water stress negatively impacted growth and physiological characteristics of soil-grown muskmelon plants. However, ACC Deaminase-producing strains, especially when applied in combination (P. aryabhattai MD-85 + E. cloacae MD-79), effectively mitigated adverse effects of drought stress. For instance, under 3%-polyethylene glycol (PEG)-induced stress in muskmelon, co-inoculation (MD-79 + MD-85) enhanced root length (44.3%), shoot length (47.6%), root dry and fresh wight ratio (40.7%), leaf dry and fresh wight ratios (51.7%), total chlorophyll (41.5%), and carotenoids (38.8%). Further, bacterial consortia significantly (p ≤ 0.05) enhanced chlorophyll colour index (56.7%), net photosynthetic rate (64.3%), Fv/Fm (50.8%), stomatal conductance (64.3%) and relative water content (62.3%) in leaf tissues of 3%-PEG-stressed muskmelon. Single/combined bacterial inoculation lowered drought-induced oxidative stress markers in muskmelon. Moreover, bacterial partners strengthened antioxidant enzymes in water-deficit affected muskmelon. The 15%-PEG + MD-79 + MD-85 treatment exhibited greater increase in catalase (79.3%), ascorbate peroxidase (65.3%), peroxidase (55.7%), and superoxide dismutase (72%), activities over their respective untreated controls. Additionally, bacterial strains modulated ion homeostasis in PEG-stressed muskmelon roots, enhancing drought tolerance. Notably, combined inoculation synergistically enhanced drought tolerance compared to single-strain treatments. This study emphasizes the potential of ACC Deaminase-producing PGPR as a sustainable and long-term strategy to improve muskmelon resilience under water-deficit condition by modulating physiological, biochemical, and ionic responses. These findings underscore the use of PGPR in drought management to enhance crop productivity and stress tolerance.