Mycorrhiza helper bacteria as stage-specific modulators of fungal development.

The rhizosphere is a narrow and dynamic soil zone where bacteria and fungi establish symbiotic and synergistic relationships with each other and with plants, collectively influencing nutrient cycling, soil health, and overall ecosystem functioning. Mycorrhiza Helper Bacteria (MHB) are a diverse group of bacterial taxa that promote mycorrhiza establishment, together with those that positively influence already established symbiotic associations. MHB have been largely described from the perspectives of interaction mechanisms, ecological functions, or agricultural applications, treating them as generalized fungal growth-promoters or biofertilizers. However, how their effects change throughout fungal development has received little attention. This review addresses this knowledge gap by reframing MHB not as generic growth promoters, but as stage-specific regulators of the fungal development program. We integrate current evidence on transcriptional responses, metabolite-mediated signaling, and emerging small RNA regulatory networks across five sequential phases of the fungal life cycle: spore germination, presymbiotic hyphal growth, mycelial expansion, root colonization, and nutrient uptake. Elucidating these stage-specific regulatory mechanisms may provide the basis for the rational, stage-targeted design of MHB-based innovative strategies for sustainable agriculture.