Glyphosate Interactions with Actinobacteria Under Phosphate Starvation: Physiological, Ultrastructural and Molecular Insights from Streptomyces sp. Z38

Glyphosate [N-(phosphonomethyl)glycine] is the most widely used herbicide worldwide, and its environmental persistence has prompted increasing interest in microbial processes that may contribute to its dissipation. This study evaluated a collection of 15 soil-derived actinobacterial strains for plant growth-promoting traits, extracellular enzymatic activities, glyphosate tolerance, and glyphosate removal under nutrient-sufficient and phosphate-starved conditions. Herbicide tolerance evaluated on agar plates was widespread across the collection, with all strains sustaining growth at 10 and 50 g L−1 of glyphosate. Under nutrient-sufficient conditions glyphosate removal remained limited, with maximum values of 16.15 ± 2.08% (Streptomyces sp. Con7.16) and 15.34 ± 2.89% (Streptomyces sp. Z38). In contrast, prior phosphate starvation markedly enhanced removal efficiency, reaching 42.21 ± 3.59% in Streptomyces sp. Z38 and 39.46 ± 1.94% in Streptomyces sp. Con7.16. Transmission electron microscopy coupled with X-ray microanalysis in the selected Streptomyces sp. Z38 revealed starvation-associated depletion of intracellular polyphosphate granules, followed by partial replenishment when glyphosate was supplied as the sole phosphorus source, consistent with indirect evidence of glyphosate-derived phosphorus acquisition. Genome mining of Streptomyces sp. Z38 identified candidate genes potentially consistent with a non-canonical, C-P lyase-independent phosphonate utilization route; however, these assignments are based exclusively on bioinformatic evidence and require experimental validation. Collectively, these findings indicate that phosphate limitation enhances glyphosate removal in the selected actinobacteria, and the physiological and genomic data are consistent with a starvation-triggered shift toward alternative phosphorus scavenging strategies. Because this strain is intended for future phytoremediation applications in glyphosate-contaminated agricultural soils, elucidating the underlying phosphorus dynamics is essential for anticipating its functional behavior and environmental relevance.