Question

In forest ecology, the role of mycorrhizal fungi in facilitating inter-plant nutrient transfer was long believed to depend solely on concentration gradients between donor and recipient trees. However, recent investigations into the rhizospheric microbiome have highlighted the modulating role of mycorrhizal helper bacteria (MHB). Rather than merely accelerating hyphal elongation, specific strains of MHB synthesize volatile organic compounds (VOCs) that alter the permeability of fungal cell walls. Specifically, by suppressing the activity of fungal chitin synthase, these bacterial VOCs increase fungal membrane fluidity, thereby elevating the rate of passive glucose transport from host plant roots into the fungal sheath. Crucially, this mechanism operates independently of the host plant's photosynthetic rate, disproving the earlier assumption that fungal carbon uptake is exclusively regulated by host sugar availability. Moreover, while previous models posited that mycorrhizal networks uniformly distribute carbon among interconnected saplings, empirical measurements demonstrate that MHB-mediated transport selectively favors saplings colonized by ectomycorrhizal species over those associated with arbuscular mycorrhizae, due to differential binding affinities of the bacterial VOCs for fungal surface lectins. Consequently, in mixed-species stands, MHB activity creates localized carbon sinks that alter competitive dynamics among understory vegetation.

According to the passage, the volatile organic compounds synthesized by mycorrhizal helper bacteria directly influence fungal carbon uptake by doing which of the following?