Guanine-nucleotide exchange factor (GEF) that acts as an activator of Rop (Rho of plants) GTPases by promoting the exchange of GDP for GTP. In association with ROPGEF1, acts as specific regulator of ARAC10/ROP11 function in ABA-mediated stomatal closure. Interacts with ARAC10/ROP11. Interacts with ARAC10/ROP11 on plasma membrane in guard cells. Expressed in root vascular tissue and trichoblast cell files. Expressed in root metaxylem cell files. Expressed in guard cells of cotyledons, rosette leaves, sepals, petal, stigmas and siliques. Expressed in root metaxylem cell files. The PRONE (plant-specific Rop nucleotide exchanger) domain is responsible for the GEF activity. No visible phenotype under normal growth conditions, but mutant plant roots have reduced density of secondary wall pits in metaxylem vessels. Plants overexpressing ROPGEF1 or ROPGEF4 are relatively insensitive to ABA-induced stomatal closure and become severely wilted during drought stress. A similar phenotype is observed in plants expressing a constitutively active ARAC10/ROP11 (PubMed:22500990).