Negative regulator of cell death and defense responses. Repress a number of R genes and may have effects in promoting growth and development. May function in membrane trafficking and in fusion of vesicles with plasma membrane (By similarity). Interacts with BAP1 and BAP2. Expressed at an extremely low level. No visible phenotype; due to partial redundancy with BON1 and BON2. Bon2 and bon3 double mutant has no visible phenotype. bon1 and bon3 double mutant is seedling-lethal when grown at 22 degrees Celsius. Bon1, bon2 and bon3 triple mutant is seedling-lethal at any temperature. Belongs to the copine family.