Rice-straw extract directly stimulates pre-symbiotic arbuscular mycorrhizal fungal growth and is associated with greater phosphorus acquisition in Sorghum.

Background: Improving arbuscular mycorrhizal fungal establishment may strengthen crop phosphorus acquisition and reduce dependence on phosphorus fertilizers. However, efficient symbiosis depends on fungal activation before root colonization, and practical approaches for stimulating this early stage remain limited. Methods: We tested whether aqueous rice-straw extract (RSE) directly activates AMF and improves subsequent symbiosis with sorghum. Spore germination, hyphal growth, root colonization, recoverable extraradical spore density, plant biomass, phosphorus acquisition, and symbiosis-related gene expression were quantified. Phenolic-depleted RSE was included to assess the contribution of phenolic compounds. Results: RSE showed a pronounced dose-dependent biphasic effect: low-to-moderate concentrations promoted AMF development, peaking at 0.3% with 74.1% spore germination, 18.48 mm hyphal length, and 27.5 branch points per spore, whereas 1.0% RSE markedly suppressed these responses. After 42 days, RSE increased root colonization from 22.5% to 51.3%, spore abundance from 58.4 to 106.4 per 50 g substrate, and extraradical hyphal length from 1.05 to 2.54 m per g. Shoot phosphorus uptake rose from about 2.3 to 5.9 mg per plant. Removing most phenolics markedly weakened these responses. Conclusion: RSE directly stimulated pre-symbiotic AMF development and was associated with greater subsequent colonization and phosphorus acquisition. Reduced activity after partial phenolic depletion suggests that phenolics contributed to the response, although other water-soluble components may also be involved.