SimiR396d targets SiGRF1 to regulate drought tolerance and root growth in foxtail millet

MicroRNAs play critical roles in growth, development and abiotic stress responses. SimR396d is a miRNA whose expression level is much higher in foxtail millet roots than other tissues. Whether SimR396d is involved in foxtail millet root growth and response to abiotic stress is still unknown. Here, we demonstrate that SimiR396d modulates both drought response and root growth in foxtail millet. The expression of SimiR396d is induced by PEG treatment. Overexpression of SimiR396d enhances drought tolerance and root length, while knockdown SimiR396d expression using target mimics of SimiR396d (MIM396) resulted in reduced drought tolerance and shortened root length. Furthermore, we identified and confirmed a plant-specific transcription factor, growth-regulating factor 1 (SiGRF1), as a direct target of SimiR396d. Overexpression of SiGRF1 in foxtail millet resulted in suppressed root growth and reduced sensitivity to drought stress. Moreover, ethylene signaling is necessary for SimiR396d and SiGRF1 to participate in the regulation of plant root growth. These results revealed a pivotal role of SimiR396d in drought tolerance and root growth in foxtail millet. SimiR396d-SiGRF1 regulatory module provides a strategy to improve drought-stress resistance of crop.