Unlocking rice growth potential: Synergistic promotion by the fungal-bacterial duo Metarhizium robertsii and Streptomyces spp.

AIMS: This study takes rice as the model system, aiming to dissect the cross-kingdom interaction and synergistic growth-promoting mechanisms between the entomopathogenic fungus Metarhizium robertsii and six Streptomyces strains, and to systematically screen for the optimal synergistic combination. Furthermore, this study seeks to situate the above interaction analysis within the framework of rational design of synthetic microbial communities, thereby providing a transferable mechanistic basis and candidate strain resources for constructing functional SynComs based on fungal-bacterial cross-kingdom synergy. METHODS AND RESULTS: A systematic screening pipeline was established, integrating plate confrontation assays, hydroponic bioassays, and highperformance liquid chromatography (HPLC)based metabolic profiling. Among the tested combinations, Streptomyces pactum L6 exhibited no detectable antagonism toward MAA in vitro. In hydroponic culture, the MAA-L6 consortium significantly and consistently increased rice plant height, root length, root activity, and endogenous indole3acetic acid (IAA) content, and its growth-promoting effect was significantly superior to that of either strain applied alone, demonstrating the strongest synergistic effect among all tested combinations. HPLC analysis revealed novel metabolite peaks in the L6-MAA coculture that were absent in either monoculture, indicating that interspecific interactions induced de novo metabolite synthesis and unique metabolic reprogramming. CONCLUSIONS: Crosskingdom synergy between M. robertsii and Streptomyces can promote rice growth by modulating root physiological activities and metabolic profiles. These findings provide both a theoretical foundation and practical microbial resources for developing fertilizer-reducing synthetic microbial SynComs suited to rice cultivation.