AHL-Mediated Remodeling of Microbial Communities by Quorum Sensing and Quenching Bacteria Inhibits Casuarina equisetifolia Growth

Abstract

Casuarina equisetifolia, a key species in subtropical coastal shelterbelts, faces serious challenges in sustainable plantation management due to continuous planting obstacles. This study explores how AHL-mediated quorum sensing (QS) influences these obstacles by regulating microbial dynamics in the rhizosphere of continuously planted C. equisetifolia. Results show that with increasing generations of continuous planting, the abundance of QS bacteria—especially pathogenic strains—significantly increased, with notable shifts in community composition. The number of QS isolates rose from 32 (FCP) to 68 (SCP) and 81 (TCP), with Enterobacteriaceae being the most abundant. Among 10 identified QS species, Pantoea ananatis showed the highest AHL activity, while Burkholderia lata had the lowest. Short-chain AHL (C4-HSL and C6-HSL) were most common, with P. ananatis producing the highest diversity and concentration. Pathogenicity tests indicated that seven QS bacteria damaged roots and inhibited water uptake, leading to wilting. In contrast, three Burkholderia species were non-pathogenic. Meanwhile, the abundance of quorum quenching (QQ) bacteria decreased significantly across planting generations (94, 70, to 63), and key QQ strains like Bacillus spp. showed no pathogenicity. These findings suggest that continuous planting enriches pathogenic AHL-mediated QS bacteria while reducing beneficial QQ populations, altering microbial community structure and exacerbating replant issues. This study offers important theoretical insights into the mechanisms behind continuous planting obstacles in C. equisetifolia.