Micro-CT reveals how Fusarium invades tomato roots

Researchers have identified physical mechanisms of root penetration and colonization by Fusarium oxysporum, vascular pathogen associated with wilt in tomato and other crops. The research employed phase-contrast X-ray micro-computed tomography and advanced microscopy. This approach allowed for the visualization, in three dimensions, of stages of root invasion that were previously difficult to access using conventional techniques.

The study analyzed the interaction between Fusarium oxysporum f.sp. lycopersici and tomato roots (Solanum lycopersicum). The authors observed bulbous structures similar to appressoria (appressoria-like structures). These structures generated thin invasive hyphae, capable of penetrating epidermal cells and advancing through internal root spaces.

Interpretation of infection

The discovery alters the interpretation of soil vascular pathogen infection, the scientists explain. The literature describes classic appressoria in foliar pathogens, such as magnaporthe oryzae. The study indicates the presence of similar functional structures in Fusarium oxysporum, even without classic fasteners.

Microtomography revealed clear colonization of host tissues three days after inoculation. By day five, researchers recorded an increase in fungal biomass and the formation of air spaces. These spaces were associated with the presence of hyphae. The article links this process to the loss of tissue integrity and physical interference with water transport.

The fungus showed high morphological plasticity. According to the study, hyphae with a normal diameter of 4 µm to 6 µm were able to traverse passages of up to 220 nm. This reduction exceeds 20 times the initial diameter. The researchers tested this capacity in PDMS microfluidic devices, in nylon membranes with defined pores, and in cellophane barriers without pores.

Invasion mechanism

The results indicate an invasion mechanism dependent on mechanical force. Mutants deficient in cellulolytic activity were able to cross cellophane membranes without pores. Therefore, the authors concluded that enzymatic degradation of cellulose alone does not explain surface rupture. The study attributes a central role to adhesion, the formation of invasive hyphae, and the generation of force.

Colonization occurred predominantly through the apoplast. Images of infected roots showed thin hyphae at intercellular junctions and localized cell wall separation. Confocal microscopy confirmed hyphal networks in intercellular spaces. Controls without inoculation showed no fungal signal under the same acquisition conditions.

Three MAPK waterfalls

The study also described the role of three MAPK cascades. The Fmk1 pathway regulated the formation of appressorium-like structures and force-dependent penetration. fmk1Δ mutants showed almost no internal colonization and remained restricted to the root surface. The fungal biomass of these mutants decreased by 99% compared to the wild type.

The Hog1 pathway played a role in osmotic adaptation during colonization in narrow spaces. Hog1Δ mutants penetrated the surface, but showed reduced colonization efficiency. This limitation increased as pore diameter decreased and osmotic conditions deviated from isotonicity.

The Mpk1 pathway participated in guiding growth towards vascular tissues. mpk1Δ mutants maintained colonization close to that of the wild type in total volume, but induced greater formation of air spaces and exhibited less vascular colonization. The article associates this pattern with loss of orientation by host signals.

Scientists propose a step-by-step model. Fmk1 controls adhesion and penetration. Hog1 allows survival during extreme hyphal remodeling in confined environments. Mpk1 directs growth towards the xylem. This set of responses allows the pathogen to cross physical root barriers and occupy internal tissues.

The research points to potential targets for resistance strategies against vascular wilt. The study highlights signaling pathways and mechanical processes linked to invasion, rather than focusing solely on cell wall-degrading enzymes.

Further information can be found at doi.org/10.1038/s41467-026-72805-x