CONICET | Buscador de Institutos y Recursos Humanos

Growth in the developing leaf is tightly coordinated, first through cell proliferation and then by cell expansion. Here, we describe the role of microRNA miR396 in the coordination of cell proliferation in Arabidopsis leaves. In Arabidopsis, miR396 regulates seven GROWTH-REGULATING FACTOR (GRF) transcription factors. Overexpression of miR396 downregulates GRF activity which in turn causes a reduction in cell proliferation, while high GRF levels has the opposite effect, increasing in the final number of cells in the leaf. Detailed expression studies have revealed an intricate regulation of the GRFs by miR396. We observed that the GRFs were mainly expressed at early stages of leaf development. In contrast, we found that miR396 steadily accumulated during the organ development, antagonizing the expression pattern of its targets. Interestingly, the increase of miR396 levels was in concert with the retreat of cell proliferation. Using GRF2-GUS reporter lines, we found that the post-transcriptional control of GRF2 by miR396 contributed significantly to shape its final expression pattern. Furthermore, we observed that in a single organ, miR396 was expressed at higher levels in the distal part of the leaf, causing a proximo-distal gradient of GRF2 expression. These observations prompted us to implicate the repression of the GRF expression by miR396 as a component of the mitotic arrest front during leaf development.

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