Emerging roles of PGPR in fruit production systems: from growth promotion to system-level regulation, a review.

With the growing importance of sustainable fruit production and the increasing demand for reduced reliance on chemical fertilizers and pesticides, plant growth-promoting rhizobacteria (PGPR) have attracted considerable attention as promising biological resources in orchard systems. However, current research is mainly constrained by an overreliance on single-trait interpretations of PGPR function, which cannot fully explain their performance under the complex, heterogeneous, and long-term conditions of perennial fruit production. This review provides a systematic overview of the emerging roles of PGPR in fruit production systems from a system-level perspective. Drawing on representative recent studies, this review first summarizes the shift from classical growth-promotion functions to integrated system-level regulation. It then discusses current research progress from three interrelated dimensions: rhizosphere engineering and synthetic microbiome assembly, physiological regulation of plant stress tolerance, and molecular signaling associated with induced systemic resistance. Particular emphasis is placed on key mechanisms including extracellular polymeric substance secretion, ACC deaminase activity, antioxidant regulation, hormone crosstalk, and defense priming, as well as on the role of multi-strain consortia and multi-omics approaches in linking microbial traits with host responses and rhizosphere ecological processes. This review shows that PGPR in fruit production systems function not only as direct growth promoters but also as regulators of interconnected plant-soil-microbiome processes involved in nutrient acquisition, stress adaptation, disease resistance, and fruit quality formation. Nevertheless, important limitations remain, including inconsistent field performance, weak colonization persistence, insufficient fruit tree-specific evidence, formulation difficulties, and biosafety concerns. Future research should focus on host- and environment-specific inoculant design, mechanistic validation under orchard conditions, integration of multi-omics with long-term field evaluation, and the development of persistence-oriented and safety-assessed application strategies, thereby providing a stronger theoretical and practical foundation for sustainable orchard management.