Exogenous C4-HSL Improves Casuarina equisetifolia Growth in Continuous Planting by Modulating Rhizosphere Bacterial...

N-acyl-homoserine lactones (AHLs) serve as key microbial quorum sensing (QS) signaling molecules in plant-microbe interactions. In this study, Casuarina equisetifolia Forst. (C. equisetifolia) in continuous planting was used to explore the effects of different concentrations of N-butyryl-L-homoserine lactone (C4-HSL) on its root growth, rhizosphere microenvironment, and microbial community. The results showed that C4-HSL significantly promoted root development of C. equisetifolia in continuous planting, increasing total root length (89.1%), surface area (156.3%), volume (247.1%), tip number (106.0%), and fork number (174.5%). Meanwhile, C4-HSL elevated rhizosphere soil enzyme activities (urease: 78.3%; protease: 32.6%; acid phosphatase: 33.3%) and nutrient availability (available nitrogen: 26.8%; available phosphorus: 21.7%; available potassium: 13.3%). Microbial community analysis showed that C4-HSL altered bacterial community structure, enriching characteristic bacteria such as Pedosphaera, Rhodoplanes, and Gaiella, while enhancing photosynthetic and nitrogen metabolic functions. Structural equation modeling showed that C4-HSL positively regulated characteristic bacterial communities (0.99***), subsequently improving soil enzyme activity (0.90***) and nutrient availability (0.98***), ultimately promoting root growth of C. equisetifolia in continuous planting (0.96***). These findings elucidate the mechanism by which C4-HSL fosters C. equisetifolia growth via a "microbial community-soil function-plant growth" cascade, providing a theoretical foundation for leveraging QS signals to enhance rhizosphere microenvironments and plant productivity.