Plant-mediated carbon dynamics drive rhizosphere microbiome responses in successive wheat systems

Soil microbial communities regulate key ecosystem functions by driving carbon and nutrient cycling essential for plant productivity. Root exudates—particularly glucose, an easily available carbon source—strongly influence microbial dynamics and rhizosphere biochemical processes. Yet how successive wheat cultivation modifies these exudation patterns and their consequences for microbial activity remains unclear. We hypothesized that continuous wheat cultivation induces plastic responses in plant exudation and gene regulation that constrain microbial growth and function. To test this, we used an optimized in situ glucose-imaging method in root windows installed in field-grown first (W1) and third (W3) wheat crops following break crops. This integrative approach combined spatial glucose mapping with measurements of microbial respiration, enzyme kinetics, and SWEET sugar transporter gene expression in root and leaf tissues. Glucose release hotspots declined by 17.7% in W3 compared to W1, with only 1.35% of the total soil surface showing detectable glucose release in W3. Concurrently, SWEET1a ortholog expression was significantly upregulated in W3, while microbial biomass in the W1 rhizosphere was nearly five times higher than in W3. Enzyme assays revealed significantly higher β-glucosidase V max values in W1, indicating stronger carbon acquisition potential. These findings suggest that reduced glucose exudation and altered SWEET gene expression reflect plant-mediated acclimation responses under continuous cultivation. In addition, reduced aboveground carbon assimilation (source limitation), may further constrain carbon allocation to the rhizosphere and contribute to the observed microbial responses. This study provides novel insights into how continuous wheat cultivation reshapes rhizosphere interactions by altering root exudation, microbial dynamics, gene regulation, and enzyme activity. Moreover, it highlights the utility of in situ glucose imaging for advancing our understanding of plant–soil–microbe interactions in agroecosystems.