Overlapping locality between rhizosphere and mycorrhizosphere regulates glucose exudation pattern in rhizosphere and enzyme distribution

Glucose is one of the major primary metabolites in the plant root exudates to mediate the cross-talk between plants and microbes, but their contribution to drought resistance of plants is largely unknown. To test this, we inoculated arbuscular mycorrhizal fungi (AMF) in soybean, quantified rhizospheric microbial hotspots, visualized glucose exudation pattern, and analyzed microbial activities, such as kinetic properties of β-glucosidase and acid phosphomonoesterase enzymes, and microbial biomass phosphorus.

Drought reduced glucose exudation, mainly allocated to root tips under optimum conditions, and narrowed the rhizosphere enzymatic hotspot by three times. However, AMF inoculation enhanced glucose exudation compared to non-mycorrhizal plants, and enlarged enzymatic hotspot area by 53% under drought condition. Despite the 50% reduction in β-glucosidase and acid phosphomonoesterase activities owing to water deficit, AMF symbiont triggered up to 36% enzyme activities in correlation with the non-mycorrhizal ones. Therefore, the drought resistance of these two enzymes was enhanced by up to 63% in mycorrhizal plants. The biomass of microbial phosphorus increased by 45% under drought conditions in plants inoculated with AMF.

Overall, the greater resistance of enzyme activities to drought in AMF-inoculated than in non-mycorrhizal suggest that microorganisms associated with mycorrhizal root have higher capability to react to altered abiotic environmental conditions than those associated with non-mycorrhizal roots. The mycorrhization induced an interactive regulation of soybean glucose exudation and rhizosphere expansion for enzyme activities. This contributed to the resistance of microbial functions (e.g., enzyme expression) to drought stress in AMF-inoculated than in non-mycorrhizal soybean. AMF-inoculation suppressed adverse drought effects on plant and microbial nutrient mining, which has substantial implications for controlling microbial roles in organic matter decomposition and P cycling.

Keywords: glucose imaging, arbuscular mycorrhiza fungi, soybean, hotspot, drought resistance