Pathogens must precisely tailor their gene expression to cause infection. However, a signaling cascade from host signal to effector production has remained elusive for metazoan pathogens. Here, we show that plants contain molecular signals, termed effectostimulins, that activate the first identified regulator of plant-parasitic nematode effectors. SUGR-1 directly binds effector promoters, and is central to a transcriptional network that activates 58 effector genes. Importantly, we demonstrate that downregulation of