Balancing mutualism: choice and sanctions in root–microbe symbioses

Madhavan et al., 2026

Plant roots form symbioses with beneficial microorganisms to enhance nutrient acquisition. Most terrestrial plants form arbuscular mycorrhizal symbiosis (AMS) with obligate biotrophic Glomeromycotina fungi, which supply hosts with mineral nutrients in exchange for carbon through specialized symbiotic hyphal structures (arbuscules) that develop within root cortex cells. Legumes form root nodule symbiosis (RNS) with nitrogen-fixing rhizobia, which are housed as differentiated bacteroids within specialized symbiotic organs (nodules) and provide plants with ammonia in return for carbon. RNS exhibits high partner specificity, occurring only between compatible hosts and microbes. Conversely, AMS is less specific, although symbiosis outcomes are context-dependent and influenced by host and fungal genotype, environmental conditions, and microbial competition. In both cases, plants favor high-performing microsymbionts by recognizing them during symbiosis initiation or by punishing low-performing symbionts through postcolonization sanctions. Microbes, in turn, employ strategies to manipulate plants for their own benefit. Here, we review the molecular mechanisms underlying partner preference in beneficial plant–microbe interactions and discuss how host partner selection strategies maintain mutualistic stability in AMS and RNS, alongside microbial strategies to evade host control. Understanding the dynamic interplay of functionally diverse plant–microbe symbioses provides a basis for improving mutualisms in both natural and agricultural systems.

https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.71107