Light–Root Microbiome Interactions in Vegetable Crops: From Photoreceptor Signaling to Exudate-Mediated Recruitment

In protected cultivation, light intensity, spectral quality, red/far-red ratio, photoperiod and diel fluctuation can alter the belowground biological environment by modifying carbon allocation, root architecture, root exudation, nutrient acquisition, immune tone and rhizosphere physicochemistry. Direct community-level evidence in vegetables remains sparse, but targeted experiments on bacterial colonization, arbuscular mycorrhizal symbiosis, beneficial fungi and root pathogens show that light can condition specific plant–microbe interactions. This review develops a molecular framework for light–root–microbiome interactions in protected vegetable crops and distinguishes direct community evidence, targeted colonization or symbiosis evidence, crop-specific indirect evidence and mechanistic analogues. We synthesize how photoreceptors and PIF-, HY5-, hormone- and immunity-related pathways regulate root niche construction, while also considering direct microbial photoreception. Experimental examples include tomato rhizosphere responses to shading, R:FR-dependent colonization by Serratia plymuthica, phyB–HY5–strigolactone control of tomato mycorrhization, light-intensity effects on lettuce–AMF interactions, spectrum-dependent Trichoderma harzianum colonization and light sensing by Ralstonia pseudosolanacearum. The evidence supports the view that light acts as a conditional regulator whose effects depend on crop genotype, microbial partner, substrate, nutrient status and developmental stage. Progress will require factorial lighting experiments coupled with exudomics, stable-isotope tracing, absolute microbial quantification, isolate genomics, synthetic communities and pathogen-challenge assays.