MicroRNA775 Promotes Intrinsic Leaf Size and Reduces Cell Wall Pectin Level via a Target Galactosyltransferase in Arabidopsis

Plants possess unique primary cell walls made of complex polysaccharides that play critical roles in determining intrinsic cell and organ size. How genes responsible for synthesizing and modifying the polysaccharides are regulated by microRNAs (miRNAs) to control plant size remains largely unexplored. Here we identified 23 putative cell wall related miRNAs, termed CW-miRNAs, in Arabidopsis thaliana and characterized miR775 as an example. We showed that miR775 post-transcriptionally silences GALT9, which encodes an endomembrane-located galactosyltransferase belonging to the glycosyltransferase 31 family. Over-expression of miR775 and deletion of GALT9 significantly enlarged leaf-related organs, primarily owing to increases in cell size. Monosaccharide quantification, confocal Raman imaging, and immunolabelling combined with atomic force microscopy (AFM) revealed that the MIR775A-GALT9 circuit modulates pectin level and elastic modulus of the cell wall. We further showed that MIR775A is directly repressed by the transcription factor ELONGATED HYPOCOTYL 5 (HY5). Genetic analysis confirmed that HY5 is a negative regulator of leaf size and acts through the HY5-MIR775A-GALT9 repression cascade to control pectin level. These results demonstrate that miR775-regulated cell wall remodeling is an integral determinant for intrinsic leaf size in A. thaliana and highlight the need to study other CW-miRNAs for more insights into cell wall biology.