Title: Coordinated plant and microbial transcriptional responses to oil-sands process-affected water

Constructed wetland treatment systems (CWTSs) are promising options for treating oil-sands process-affected water (OSPW), which contains toxic naphthenic acid fraction compounds (NAFCs). However, the molecular mechanisms underlying NAFCs attenuation by plants and root microbes remain poorly resolved. In our previous mesocosm study, common cattail (Typha latifolia) increased NAFCs removal by 2.5-fold relative to unplanted controls with no significant effect on plant growth. Here, using RNA from the same experimental system, we applied metatranscriptomics to 40 root samples collected over 60 days to examine plant and active-microbial responses to OSPW exposure. The active-root-associated-microbial community was dominated by Pseudomonadota, which showed a slight increase with exposure to OSPW. Burkholderiales were the most active family, though their relative activity decreased in OSPW systems, where Flavobacteriaceae (Bacteroidota) activity increased. Clear microbial-community shifts were driven by time and OSPW exposure. Although 18 previously proposed microbial NAFC-degradation genes were not differentially expressed, 42 other genes with potential roles in NAFC or related organic compound degradation showed differential expression in OSPW-filled mesocosms. This activity was dominated by specific oxidoreductases from Burkholderiales and Rhizobiales. Crucially, host plant actively responded to OSPW, robustly up-regulating genes encoding oxidoreductases, transporters, and glycosyltransferases, some of which are related to xenobiotic stress and detoxification. Taken together, these results show coordinated plant and microbial transcriptional responses in a system where NAFC removal had already been measured chemically. They help explain the response of OSPW-exposed mesocosms, but the observed patterns likely reflect the broader OSPW mixture rather than NAFCs alone.