Cell-surface redox regulation coordinates chemotropism and hyphal fusion for root colonization in a vascular wilt pathogen

Soilborne fungal pathogens must locate host roots, stably attach to their surface and assemble a persistent mycelial network before invading the plant, yet how host perception is coupled to these cellular outputs remains poorly understood. Here, we characterize HAM-7, a GPI-anchored cell wall protein of Fusarium oxysporum and orthologue of the Neurospora crassa cell wall integrity sensor, and show that it is required for two mechanistically distinct processes essential for root colonization. Targeted deletion of ham-7 strongly impaired stable mycelial retention on tomato roots while leaving germling adhesion to abiotic surfaces and biofilm formation unaffected, indicating a defect in fusion-dependent cohesion rather than in surface adhesion. This defect is coupled with a complete loss of vegetative hyphal fusion (23% WT vs. 0% ham-7Δ), preventing the formation of interconnected mycelial networks that promote stable retention of fungal biomass on the root surface. In parallel, ham-7Δ mutants are severely defective in chemotropism toward plant peroxidases but retain intact responses to synthetic α-pheromone, revealing a specific requirement for HAM-7 upstream of the redox-dependent branch of host cue perception. Consistent with this, ham-7Δ mutants constitutively downregulate nox genes under basal conditions and fail to upregulate noxB and noxR upon exposure to tomato root exudates. Accordingly, exogenous H2O2 fully rescues the chemotropism defect but leaves the fusion deficiency intact, demonstrating that HAM-7 acts upstream of two separable downstream pathways. Combined, these defects significantly attenuate virulence on tomato plants, identifying HAM-7 as an upstream actor of Nox-dependent signalling.