Diurnal plant and Rhizophagus irregularis transcriptional patterns are linked to shifts in cassava tissue partitioning in the field

Cassava is a globally-important crop whose yields can radically increase with arbuscular mycorrhizal fungi (AMF) inoculation. However, extensive background noise in field environments makes it very challenging to understand how the cassava-AMF symbiosis confers benefits, which is especially important for future applications of AMF treatments. In two field experiments, we combined transcriptomics and allometric analyses to investigate functional variation in cassava-AMF interactions using sterile, single isolates of Rhizophagus irregularis. We developed a novel Index of Symbiotic Transcriptional Activity (ISTA) and accounted for sampling times to reduce transcriptomic noise and improve links to biomass traits. ISTA significantly correlated with cassava shoot biomass in an isolate-dependent manner, and allometric analyses revealed that R. irregularis isolates can either reinforce or uncouple cassava shoot-root relationships to maximize root yields. Differential expression and co-expression network analyses uncovered isolate-specific plant and fungal gene module responses. Including ISTA and sampling time as random effects enhanced detection of gene candidates, including down-regulated genes linked to higher yield. Our study uses novel and translatable transcriptomic tools to readily dissect variably field data, allowing new links to be found between AMF symbiotic functions and cassava yields.