Multi-omics insights reveal a divergent type III secretion system-associated secretome in plant growth-promoting rhizobacterial Pseudomonas fluorescens.
TL;DR
The RspL-T3SS axis is established as an integrated regulatory and secretory module that coordinates transcriptional reprogramming with selective protein secretion during root interaction and yields mechanistic insights into bacterial rhizosphere fitness.
Abstract
The type III secretion system (T3SS) is a well-characterized virulence apparatus in pathogenic bacteria; however, its role in beneficial rhizobacteria remains poorly defined. In the current study, we integrated multi-omics analyses with molecular genetics to systematically characterize the regulatory network and secretory output of the T3SS in the plant growth-promoting rhizobacterium (PGPR) Pseudomonas fluorescens 2P24. The canonical effector repertoire of 2P24 is markedly streamlined, and the conserved effector homolog HopM12P24 has lost virulence-associated activities. The sigma factor RspL is a root-responsive regulator that reprograms 31.92% of detected bacterial genes, promoting energy generation and metabolic pathways while repressing behavioral and signaling programs. Furthermore, T3SS integrity directly governs extracellular protein allocation by facilitating the secretion of non-canonical metabolic enzymes-including a validated hydrolase and decarboxylase-while restricting the release of flagellar components. Collectively, these findings establish the RspL-T3SS axis as an integrated regulatory and secretory module that coordinates transcriptional reprogramming with selective protein secretion during root interaction. This study expands our understanding of the classic virulence-associated T3SS as an adaptive machinery for rhizosphere colonization and yields mechanistic insights into bacterial rhizosphere fitness.