Rhizosphere–Canopy Communication in Horticultural Crops: Root Gating, Systemic Signaling, and Links to Yield and Quality

Rhizosphere microbiomes influence nutrient use, stress tolerance, and crop performance, but the processes connecting root microbial activity to canopy function and harvest traits remain incompletely resolved. This review integrates evidence from model plants and horticultural crops through a framework of rhizosphere inputs, root gating, systemic transmission, leaf responses, and belowground feedback. Root gating describes selective entry, perception, and transformation of inputs, rather than a newly discovered signaling pathway. Its value is to distinguish microbial exposure from the host resources and signals that actually reach distal organs. Nutrient fluxes, endogenous hormones and peptides, immune signals, and rapid ionic or physical responses then interact with leaf transcriptional and metabolic networks. Studies in tomato, grapevine, citrus, strawberry, cucurbits, leafy vegetables, and ornamentals connect root interventions to defense, water relations, harvested-organ composition, or ornamental performance. Field evidence includes changes in fruit sugars, pigments, and processing traits, but yield and quality responses can diverge and depend on inoculant, crop, and season. We distinguish functional validation of individual links from systemic intervention effects and observational associations. Resolving the route to fruit quality requires source-specific tracing and functional tests coupled with leaf physiology, fruit-local metabolism, marketable yield, and product composition. This framework supports crop-specific evaluation of microbial interventions without assuming universal benefits or a fully validated root-to-fruit pathway.