Assessment of ZnO-NPs toxicity in maize: An integrative microRNAomic approach

Rapid expansion of nanotechnology and indiscriminate discharge of metal oxide nanoparticles (NPs) into the environment pose a serious hazard to the ecological receptors including plants. To better understand the role of miRNAs in ZnO-NPs stress adaptation, two small RNA libraries were prepared from control and ZnO-NPs (800 ppm, <50 nm particle size) stressed maize leaves. Meager performance of ZnO-NPs treated seedlings was associated with elevated tissue zinc accumulation, enhanced ROS generation, loss of root cell viability, increased foliar MDA content, decrease in chlorophyll and carotenoids contents. Deep sequencing identified 3 (2 known and 1 novel) up- and 77 (73 known and 4 novel) down-regulated miRNAs from ZnO-NPs challenged leaves. GO analysis reveals that potential targets of ZnO-NPs responsive miRNAs regulate diverse biological processes viz. plant growth and development (miR159f-3p, zma_18), ROS homeostasis (miR156b, miR166l), heavy metal transport and detoxification (miR444a, miR167c-3p), photosynthesis (miR171b) etc. Up-regulation of SCARECROW 6 in ZnO-NPs treated leaves might be responsible for suppression of chlorophyll biosynthesis leading to yellowing of leaves. miR156b.1 mediated up-regulation of CALLOSE SYNTHASE also does not give much protection against ZnO-NPs treatment. Taken together, the findings shed light on the miRNA-guided stress regulatory networks involved in plant adaptive responses to ZnO-NPs stress.