MicroRNA Profiling of Root Meristematic Zone in Contrasting Genotypes Reveals Novel Insight into in Rice Response to Water Deficiency

Root system architecture (RSA) is critical for improving nutrient and water uptake and maintaining crop yield under both optimal and drought conditions. The meristematic zone of root tips is ideal for studying RSA, because of its high mitotic rate. miRNAs are important post-transcriptional regulators and play a crucial role in plant response to drought stress. To decipher the changes in miRNA expression patterns under drought stress, sequencing of small RNAs in the meristematic root tips of the highly tolerant genotypes Azucena and the susceptible IR64 was performed. Prediction of target genes of differentially expressed miRNAs (DEMs) in drought-stressed Azucena compared with normal conditions revealed that these genes mainly encode aquaporin, AP2, DELLA protein, ERF, and OsIAA18-auxin responsive. Most of these genes are involved in the regulation of response to stress, meristem growth, ethylene-activated signaling pathway, post-transcriptional regulation, and lipid metabolism. Whereas, the predicted target genes in IR64 under the same conditions were mainly involved in controlling carbohydrate biosynthesis, lignin catabolism, primary metabolic processes, cell death, and RNAi. This result shows that IR64 promotes thickening of cell walls to cope with stress, while lateral root system expansion and root length are used as a strategy to cope with drought stress in Azucena. Azucena-specific down-regulation of miR164-NAC5, miR1857-MADX-box5, miR1861-EXP1, and miR169-NF-YA may be the main reason for root elongation, root system development, and stress adaptation of this genotype. Moreover, miR528 and miR398, which serve as hub regulators, and target redox-related enzymes such as SOD and peroxidase (POX) to control cellular homeostasis in response to drought stress, were down-regulated in Azucena. Furthermore, our histochemical assay also confirmed the fact that the cell wall thickness of the root tips was increased in the drought-sensitive IR64 for insulation purposes to cope with stress. However, this may substantially reduce its ability to extend roots to obtain water from deeper soil layers. Our results shed light on drought-related miRNA regulatory networks as well as important miRNAs that have the potential to further improve drought tolerance in rice.