The Function And Molecular Mechanism Of Auxin In Regulating Stomatal Development

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, as well as global carbon and water circulation. Many factors are involved in regulating stomatal development, including EPF family peptides, TMM/ER family membrane protein/kinases, MAPK cascade kinases, and bHLH/MYB transcriptional factors. STOMAGEN, as a member of EPF family, is expressed in mesophyll instead of epidermis, and its encoding peptide then migrates to the epidermis where it is proposed to promote stomatal development by competitively inhibiting TMM/ER-mediated signaling. The phytohormone brassinosteroids and environment factors, such as light and carbon dioxides, also control stomatal production. Auxin, as the first identified phytohormone, participates in many aspects of plant growth and development, but whether auxin regulates stomatal development is unknown.This study establishes that auxin negatively regulates stomatal development through ARF5/MP(MONOPTEROS) repression of mobile peptide gene STOMAGEN expression in mesophyll cells, which is mediated by direct binding of ARF5 to auxin response elements in the STOMAGEN promoter.We observed significantly increased SMI(stomata plus meristemoids per total epidermal cells) and stomatal clusters in epidermis of iaaL, taa1 tar1 tar2, tir1 afb1 afb2 afb3, bdl, or arf5 mutant / transforming plants. ARF5 protein localizes in mesophyll cells, instead of the epidermis, consisted with STOMAGEN expression. Genetic analysis indicates that ARF5 regulates stomatal development, at least in part, through repressing STOMAGEN expression. From qRT-PCR analysis, histochemical staining, and Dual-LUC assay, we found that ARF5 regulates STOMAGEN expression depending on AuxREs in STOMAGEN promoter. ChIP and EMSA assays indicate that ARF5 directly associates with AuxREs in STOMAGEN promoter.This study advances our knowledge about the roles of auxin and the versatile regulator ARF5 in plant growth and development, while providing a paradigm of cross-talk between phytohormone auxin and peptide signaling in the regulation of stomatal production.