ROS homeostasis mediated by MPK4 and SUMM2 determines synergid cell death

Sexual plant reproduction depends on the attraction of sperm-cell delivering pollen tubes (PT) by two synergids, followed by their programmed cell death (PCD) in Arabidopsis . Disruption of the mitogen-activated protein kinase 4 (MPK4) by pathogenic effectors activates the resistance protein (R) SUMM2-mediated immunity and cell death. Here we show that synergid preservation and reactive oxygen species (ROS) homeostasis are intimately linked and maintained by MPK4. In mpk4 , ROS levels are increased and synergids prematurely undergo PCD before PT-reception. However, ROS scavengers and the disruption of SUMM2, in mpk4 , restore ROS homeostasis, synergid maintenance and PT perception, demonstrating that the guardian of MPK4, SUMM2, triggers synergid-PCD. In mpk4/summ2 , PTs show a feronia -like overgrowth phenotype. Our results show that immunity-associated PCD and synergid cell death during plant reproduction are regulated by MPK4 underscoring an underlying molecular mechanism for the suppression of plant reproduction during systemic R-mediated immunity.