BcpLH organizes a specific subset of microRNAs to form a leafy head in Chinese cabbage (Brassica rapa ssp. pekinensis)

Abstract

<i>HYL1</i> (<i>HYPONASTIC LEAVES 1</i>) in <i>Arabidopsis thaliana</i> encodes a double-stranded RNA-binding protein needed for proper miRNA maturation, and its null mutant <i>hyl1</i> shows a typical leaf-incurvature phenotype. In Chinese cabbage, <i>BcpLH</i> (<i>Brassica rapa ssp. pekinensis LEAFY HEADS</i>), a close homolog of <i>HYL1</i>, is differentially expressed in juvenile leaves, which are flat, and in adult leaves, which display extreme incurvature. BcpLH lacks protein-protein interaction domains and is much shorter than HYL1. To test whether <i>BcpLH</i> is associated with defects in microRNA (miRNA) biogenesis and leaf flatness, we enhanced and repressed the activity of <i>BcpLH</i> by transgenics and investigated <i>BcpLH</i>-dependent miRNAs and plant morphology. BcpLH promoted miRNA biogenesis by the proper processing of primary miRNAs. <i>BcpLH</i> downregulation via antisense decreased a specific subset of miRNAs and increased the activities of their target genes, causing upward curvature of rosette leaves and early leaf incurvature, concurrent with the enlargement, earliness, and round-to-oval shape transition of leafy heads. Moreover, BcpLH-dependent miRNAs in Chinese cabbage are not the same as HYL1-dependent miRNAs in Arabidopsis. We suggest that <i>BcpLH</i> controls a specific subset of miRNAs in Chinese cabbage and coordinates the direction, extent, and timing of leaf curvature during head formation in <i>Brassica rapa</i>.