Priming Salt Resilience in Melon (Cucumis melo) via Pseudomonas oryzihabitans-Mediated Metabolic Reprogramming

Melon (Cucumis melo L.) is a globally significant crop, yet its productivity is increasingly threatened by rising soil salinity in many cultivation areas. In this context, plant growth-promoting rhizobacteria (PGPR) have emerged as a promising strategy to enhance plant growth and improve tolerance to salinity stress. This study investigated the effect of inoculating melon seedlings with a previously characterized strain AXSa06 (identified as Pseudomonas oryzihabitans) under salt stress (250 mM NaCl), focusing on plant growth, photosynthesis, antioxidant responses, metabolomic profiles, nutrient balance, abscisic acid (ABA) levels, and expression of ABA signaling pathway genes. Inoculation improved the growth and photosynthetic performance of melon plants under salt stress, while reducing Na⁺ accumulation and facilitating better nutrient homeostasis. The elevated levels of malondialdehyde (MDA) and catalase (CAT), along with the increased ABA content upon inoculation, pointed to a priming effect that prepared plants to manage oxidative stress, as evidenced by the reduced antioxidant activity upon stress. Expression of CmPYL9 and several CmPP2AC isoforms was upregulated only in inoculated stressed plants, suggesting improved ABA sensitivity and a fine-tuned transcriptional control of signaling. AXSa06-inoculated stressed plants also displayed notable metabolic reprogramming, primarily affecting carbohydrate metabolism, following shifts in TCA cycle metabolites already evident under control conditions. Taken together, these findings suggest that AXSa06 could serve as a promising biostimulant agent improving melon tolerance to salt stress by modulating physiological and biochemical responses. This study offers valuable insights into PGPR-driven stress priming and its role in developing strategies for sustainable crop production under saline conditions.