Root Microbiome Response to Treated Wastewater Irrigation

Irrigation with treated wastewater (TWW) is becoming essential component of agricultural practice in increasing parts of the world. Despite the enhancement in quality, TWW irrigation may still result in undesirable effects on crops, and no clear evidence was found to explain those responses. In this study, we aim to further characterize the root microbiome structure and function response to TWW irrigation. Our hypothesis is that understanding the stress imposed on the root microbiome by the TWW irrigation will teach us about the actual stress the plant roots are experiencing.

Rhizosphere (soil) and rhizoplane (root surface) microbiome structures were examined in lysimeter experiment in which tomato and lettuce plants were grown in two soil types and irrigated with either freshwater (FW) or TWW. Results indicated that rhizosphere bacterial communities compositions were largely determined by soil type and irrigation treatment. However, rhizoplane communities were mainly affected by irrigation water type and the host plant.

Rhizoplane microbial functional profile correlated with soil pH, electrical conductivity and total organic matter. 11.5% of all bacterial gene functional categories varied significantly between TWW and FW irrigated roots, whereas many membrane transport genes were associated with TWW irrigation. Expression of genes involved in inorganic ion transporters suggests that sodium fluxes serves as key factors in the bioenergetics of TWW irrigated roots. Increase in abundance of genes involved in anaerobic pathways, such as denitrification and nitrogen fixation, imply that TWW irrigated roots are more anoxic niches compared to those of FW irrigated roots. Overall, gene abundance patterns indicate that root microbiome under TWW irrigation likely experiences anoxia, salinity, increase in pH and iron limitation.

As rhizoplane bacteria are exposed to the same environmental conditions as the plant roots themselves, we suggest the root microbiome as an indicator to elucidate on the factors that influence the plant root itself.