The physiological impacts of three AMF species and Bacillus thuringiensis (vegetative and endospore forms) in strawberry under different phosphorus availability

The interactions between arbuscular mycorrhizal fungi (AMFs) and Bacillus thuringiensis can enhance plant growth, yet their combined effects under varying phosphorus (P) conditions remain unclear. This study evaluated three AMF species, Funneliformis mosseae (AMF1), Funneliformis caledonium (AMF2), and Acaulospora langula (AMF3), in combination with vegetative and endospore forms of B. thuringiensis in strawberry grown under no P addition, insoluble P, and P-available conditions. Under insoluble P, vegetative B. thuringiensis produced the greatest growth improvements, particularly with AMF3 (up to 89 % above the uninoculated control) and AMF2 (up to 80 %). Dual inoculation with vegetative cells increased root length to 29.3 cm, biomass to 1.45 g DW, and total protein content to 2.1 mg g⁻¹ FW, though root P sometimes decreased (0.13 %), suggesting a growth-dilution effect. In contrast, In P-available soil, dual inoculation reduced AMF colonization (11 %), root length (16 cm), and biomass (0.45 g DW), but increased root P (0.19 %), reflecting indicating downregulation of symbiosis when external P is sufficient. PAL and POD activities were modulated by both P availability and bacterial form. FTIR analyses showed that vegetative cells enhanced polysaccharide deposition, whereas endospores increased lignin-associated features. Multivariate analyses (PCA, NMDS) identified P availability as the primary driver of plant-microbe interaction outcomes. Overall, the benefits of AMF-B. thuringiensis were most evident under conditions of insoluble P, emphasizing their potential to enhance nutrient uptake in low P soil.