Enrichment is not necessarily recruitment in root microbiome assembly

Area of Science:

• Plant-microbe interactions
• Microbial ecology
• Root biology

Background:

• Rhizosphere enrichment is often mistaken for plant recruitment of soil microbes.
• Enrichment describes microbial composition, while root association is a broader outcome of microbial colonization processes.
• Recruitment implies directional host-driven attraction and attachment, which can occur independently of microbial benefit.

Purpose of the Study:

• To distinguish between rhizosphere enrichment and microbial recruitment.
• To propose a framework for evaluating recruitment claims based on microbial traits and host states.
• To clarify the mechanisms underlying plant-microbe root associations.

Main Methods:

• Conceptual framework development.
• Review of microbial traits (motility, chemotaxis, polysaccharides) and host factors (physiology, immunity).
• Analysis of spatial, temporal, and functional evidence for root association.

Main Results:

• Rhizosphere enrichment is an observable outcome, not direct evidence of recruitment or benefit.
• Microbial motility and host factors influence root association differently for bacteria and fungi.
• Enrichment can be decoupled from plant benefit due to host permissiveness and microbial competition.

Conclusions:

• Precise language is needed to describe plant-microbe interactions, separating enrichment from recruitment.
• Recruitment claims require evidence of specific microbial attraction and attachment mechanisms.
• A framework is proposed to better understand the complex processes of root association and microbial persistence.