The article reports on a study which examines the nuclear effector HaRxL44 of Hyaloperonospora arabidopsidis. An analysis of transgenic Arabidopsis thaliana lines in the MED19a subunit of the Mediator complex revealed that MED19a is a positive regulator of Arabidopsis thaliana immunity to Hyaloperonospora arabidopsidis infection. Findings also indicate that HaRxL44 effector targets MED19a to control the balance between jasmonic acid-ethylene and salicylic acid signalling.