microRNAomic profiling of maize root reveals multifaceted mechanisms to cope with Cr (VI) stress

Chromium (Cr) contamination of soil and water poses serious threats to agricultural crop production. MicroRNAs (miRNAs) are conserved, non-coding small RNAs that play pivotal roles in plant growth, development and stress responses through fine-tuning of post-transcriptional gene expression. To better understand the molecular circuit of Cr-responsive miRNAs, two sRNA libraries were prepared from control and Cr (VI) [100 ppm] exposed maize roots. Using deep sequencing, we identified 80 known (1 up and 79 down) and 18 downregulated novel miRNAs from Cr (VI) challenged roots. Gene ontology (GO) analysis reveals that predicted target genes of Cr (VI) responsive miRNAs are potentially involved in diverse cellular and biological processes including plant growth and development (miR159c, miR164d, miR319b-3p and zma_25.145), redox homeostasis (miR528-5p, miR396a-5p and zma_9.132), heavy metal uptake and detoxification (miR159f-5p, 164e-5p, miR408a, miR444f and zma_2.127), signal transduction (miR159f, miR160a-5p, miR393a-5p, miR408-5p and zma_43.158), cell signalling (miR156j, 159c-5p, miR166c-5p and miR398b). Higher accumulation of Cr in maize roots might be due to upregulation of ABC transporter G family member 29 targeted by miR444f. Instead of isolated increase in SOD expression, significant decline in GSH:GSSH ratio and histochemical staining strongly suggest Cr (VI) stress mediated disruption of ROS scavenging machinery thus unbalancing normal cellular homeostasis. Moreover, miR159c-mediated enhanced expression of GAMYB might be a reason for impaired root growth under Cr (VI) stress. In a nutshell, the present microRNAomic study sheds light on the miRNA-target gene regulatory network involved in adaptive responses of maize seedlings to Cr (VI) stress.