Auxin inhibits stomatal development through MONOPTEROS repression of a mobile peptide gene STOMAGEN in mesophyll.

Plants, as sessile organisms, must coordinate various physiological processes to adapt to ever-changing surrounding environments. Stomata, the epidermal pores facilitating gas and water exchange, play important roles in optimizing photosynthetic efficiency and adaptability. Stomatal development is under the control of an intrinsic program mediated by a secretory peptide gene family--namely, EPIDERMAL PATTERNING FACTOR, including positively acting STOMAGEN/EPFL9. The phytohormone brassinosteroids and environment factor light also control stomatal production. However, whether auxin regulates stomatal development and whether peptide signaling is coordinated with auxin signaling in the regulation of stomatal development remain largely unknown. Here we show that auxin negatively regulates stomatal development through MONOPTEROS (also known as ARF5) repression of the mobile peptide gene STOMAGEN in mesophyll. Through physiological, genetic, transgenic, biochemical, and molecular analyses, we demonstrate that auxin inhibits stomatal development through the nuclear receptor TIR1/AFB-mediated signaling, and that MONOPTEROS directly binds to the STOMAGEN promoter to suppress its expression in mesophyll and inhibit stomatal development. Our results provide a paradigm of cross-talk between phytohormone auxin and peptide signaling in the regulation of stomatal production.