From invitation to eviction: How plants control arbuscular mycorrhizal symbiosis.

Arbuscular mycorrhizal (AM) symbiosis is often presented as a linear sequence of fungal recognition, root colonization, arbuscule formation, and nutrient exchange. This view underrepresents the repeated regulatory transitions through which plants influence symbiotic establishment, function, and persistence. Here, we propose a checkpoint framework organized around invitation, admission, accommodation, investment, maintenance, and termination and renewal. We examine how nutrient status, carbon availability, hormonal and immune signaling, systemic root-shoot communication, and environmental context regulate fungal recruitment, intracellular entry, interface construction, resource exchange, and arbuscule turnover. We distinguish strong mechanistic evidence from correlative observations and emphasize that gene expression, colonization abundance, or arbuscule degeneration alone do not demonstrate resource flux, performance-sensitive evaluation, partner-level sanction, or selective interface termination. Evidence is strongest for plant control of presymbiotic signaling, cellular accommodation, interface construction and resourcing, nutrient acquisition, and regulated arbuscule turnover. By contrast, direct performance-sensitive evaluation of individual fungal interfaces remains insufficiently demonstrated. AM fungi also providing regulatory inputs: fungal signals, secreted molecules, and small RNAs can modify host processes, while fungal physiology and extraradical networks influence nutrient acquisition and allocation. Plant control is therefore substantial but not absolute, operating through asymmetrical, reciprocal regulation across interface, whole-plant, and fungal-network scales. We conclude by outlining experiments that link fungal nutrient contribution, plant response or allocation, and subsequent fate at the same interface, with fungal fitness additionally required for claims of partner-level sanction. Such experiments provide tests for distinguishing developmental and physiological regulation from performance-sensitive interface maintenance.