Contributes to the regulation of secretory and vacuolar transport pathways in the post-Golgi network, and to the maintenance of the Golgi apparatus and trans-Golgi network (TGN) morphologies (PubMed:22307646). Vesicle trafficking protein that functions in the secretory pathway and mediates liposome fusion (PubMed:24021022). Required for extracellular resistance responses to a fungal pathogen (PubMed:22307646). Also involved in the protection of chloroplasts from salicylic acid-dependent biotic stress (PubMed:22307646). Interacts with VTI12 and SYP61 to form a t-SNARE complex and with VPS45. SYP42 is found in a different region of the TGN than SYP41. Expressed at low levels in roots, stems, flowers and leaves. Gametophytic lethal in homozygote plants (PubMed:22307646). Slightly shorter roots (PubMed:22307646). The double mutant syp41 syp42 have short roots (PubMed:22307646). The double mutant syp42 syp43 exhibits severe pleiotropic defects, including short roots, a large number of lateral roots, semi dwarfism and early senescence; these phenotypes are associated with defective secretory and vacuolar transport pathways (PubMed:22307646). Double mutant plants syp42 syp43 have an increased sensitivity to the powdery fungus Golovinomyces orontii with leaf chlorosis in a pathogen-inducible salicylic acid (SA) biosynthesis-dependent manner, thus revealing a biotic stress-induced and SA-dependent chloroplast dysfunction (PubMed:22307646). Plants lacking the three genes SYP41 SYP42 and SYP43 are seedling lethals (PubMed:22307646). Belongs to the syntaxin family.