Flavonoid-associated microbial defense and Cellvibrio sp. C74 pretreatment collectively enhance straw degradation by Stropharia rugosoannulata.

Stropharia rugosoannulata efficiently utilizes straw in natural habitats, yet how it simultaneously defends against competing environmental microorganisms and degrades lignocellulose remains unclear. Here, transcriptomic, metabolomic, and microbiome analyses were integrated with flavonoid antibacterial assays and bacterial pretreatment experiments to characterize fungus-bacterium interactions during straw degradation. The flavonoid biosynthesis-related genes chi, fns, and fls were significantly upregulated during degradation, and the total flavonoid content reached 101.06 μg/g dry weight at the end of the process. Flavonoid accumulation was also significantly associated with specific bacterial taxa. At concentrations ≥ 25 μg/mL, both exogenous flavone and crude endogenous mycelial flavonoid extracts significantly inhibited Sphingomonas sp. S125, Cellvibrio sp. C74, and Sphingobacterium sp. S85, supporting a role for accumulated flavonoids in microbial defense. Conversely, pretreatment with Cellvibrio sp. C74 disrupted the straw surface and increased lignocellulolytic enzyme activities during subsequent fungal degradation, resulting in a 24.39 %-26.97 % increase in degradation efficiency relative to the sterilized-straw control. These findings extend the mechanistic understanding of straw degradation from fungal enzyme systems to fungus-bacterium interactions, revealing a dual mode of flavonoid-associated defense and Cellvibrio sp. C74-assisted degradation. Moreover, Cellvibrio sp. C74 represents a promising biological pretreatment agent for efficient straw bioconversion.