miRNAs control HAM1 functions at the single-cell-layer level and are essential for normal embryogenesis in Arabidopsis.

KEY MESSAGE: miR171a controls HAM1 functions within the protodermal cells of the embryo, and these controls are essential for normal embryogenesis in Arabidopsis. Arabidopsis thaliana miR171a is known to bind to and cleave mRNAs of three HAIRY MERISTEM (HAM) genes that encode members of the GRAS family transcriptional regulators. The molecular functions of the HAM genes are still being elucidated in Arabidopsis. However, detailed expression patterns of miR171a and the effects of the failure of miR171a to suppress HAM genes were unknown till now. Here, we show the detailed expression patterns of miR171a and HAM1 using green fluorescent protein and confocal scanning microscopy. Our observations revealed that miR171a was expressed in the surface cell layer of the embryo and shoot apical meristem, and it controlled HAM1 functions. To determine the impact of the failure of miR171a to suppress of HAM1, we introduced seven synonymous mutations into the miR171a target site of the HAM1 gene (modified HAM1, mHAM1) and generated transgenic plants that had mHAM1 driven by HAM1 native promoter. The mHAM1 transgenic plants showed organogenic defects. These results indicate that the control of HAM1 functions at the single-cell-layer level by miR171a is essential for proper organ formation in Arabidopsis.