A double-negative feedback loop between miR319c and JAW-TCPs establishes growth pattern in incipient leaf primordia in Arabidopsis thaliana.

The microRNA miR319 and its target JAW-TCP transcription factors regulate the proliferation-to-differentiation transition of leaf pavement cells in diverse plant species. In young Arabidopsis leaf primordia, JAW-TCPs are detected towards the distal region whereas the major mRNA319-encoding gene MIR319C, is expressed at the base. Little is known about how this complementary expression pattern of MIR319C and JAW-TCPs is generated. Here, we show that MIR319C is initially expressed uniformly throughout the incipient primordia and is later abruptly down-regulated at the distal region, with concomitant distal appearance of JAW-TCPs, when leaves grow to ~100 μm long. Loss of JAW-TCPs causes distal extension of the MIR319C expression domain, whereas ectopic TCP activity restricts MIR319C more proximally. JAW-TCPs are recruited to and are capable of depositing histone H3K27me3 repressive marks on the MIR319C chromatin. JAW-TCPs fail to repress MIR319C in transgenic seedlings where the TCP-binding cis-elements on MIR319C are mutated, causing miR319 gain-of-function-like phenotype in the embryonic leaves. Based on these results, we propose a model for growth patterning in leaf primordia wherein MIR319C and JAW-TCPs repress each other and divide the uniformly growing primordia into distal differentiation zone and proximal proliferation domain. Author summary: Young leaves in Arabidopsis exhibit a characteristic polar growth pattern where cells at the base grow due to continued cell division, while the tip stops growing as the cells exit division and enter differentiation. The MIR319C and its target JAW-TCP genes are expressed in the proximal and the distal domain respectively and are important for maintaining the polar growth pattern. However, it is not known how the complementary expression pattern of MIR319C and JAW-TCP genes is generated when leaves are initiated. Here, we show that MIR319C is expressed ubiquitously during leaf inception and later downregulated at the distal region. This distal loss of MIR319C is marked by an onset of JAW-TCP expression in the same domain. JAW-TCP proteins directly bind to, and downregulate the transcription of, the MIR319C gene to restrict its expression domain towards the base. Disruption of JAW-TCP-binding sites on the MIR319C promoter leads to miR319-overexpression-like phenotype. Together with the knowledge of miR319c-mediated post-transcriptional regulation of JAW-TCPs, we have shown that JAW-TCPs and miR319c mutually repress each other and generate growth polarity in early leaf primordia. [ABSTRACT FROM AUTHOR]